The effect of iron on the synthesis and amount of ferritin in red blood cells during ontogeny

Iron responsive mRNAs: a family of Fe2+ sensitive riboregulators

Messenger RNAs (mRNAs) are emerging as prime targets for small-molecule drugs. They afford an opportunity to assert control over an enormous range of biological processes: mRNAs regulate protein synthesis rates, have specific 3-D regulatory structures, and, in nucleated cells, are separated from DNA in space and time. All of the many steps between DNA copying (transcription) and ribosome binding (translation) represent potential control points. Messenger RNAs can fold into complex, 3-D shapes, such as tRNAs and rRNAs, providing an added dimension to the 2-D RNA structure (base pairing) targeted in many mRNA interference approaches. In this Account, we describe the structural and functional properties of the IRE (iron-responsive element) family, one of the few 3-D mRNA regulatory elements with known 3-D structure. This family of related base sequences regulates the mRNAs that encode proteins for iron metabolism. We begin by considering the IRE-RNA structure, which consists of a short (~30-nucleotide) RNA helix. Nature tuned the structure by combining a conserved AGU pseudotriloop, a closing C-G base pair, and a bulge C with various RNA helix base pairs. The result is a set of IRE-mRNAs with individual iron responses. The physiological iron signal is hexahydrated ferrous ion; in vivo iron responses vary over 10-fold depending on the individual IRE-RNA structure. We then discuss the interaction between the IRE-RNA structure and the proteins associated with it. IRE-RNA structures, which are usually noncoding, tightly bind specific proteins called IRPs. These repressor proteins are bound to IRE-RNA through C-bulge and AGU contacts that flip out a loop AG and a bulge C, bending the RNA helix. After binding, the exposed RNA surface then invites further interactions, such as with iron and other proteins. Binding of the IRE-RNA and the IRP also changes the IRP conformation. IRP binding stabilities vary 10-fold within the IRE family, reflecting individual IRE-RNA paired and unpaired bases. This variation contributes to the graded (hierarchical) iron responses in vivo. We also consider the mechanisms of IRE-mRNA control. The binding of Fe(2+) to IRE-RNA facilitates IRP release and the binding of eukaryotic initiation factors (eIFs), which are proteins that assemble mRNA, ribosomes, and tRNA for translation. IRE-RNAs are riboregulators for the inorganic metabolic signal, Fe(2+); they control protein synthesis rates by changing the distribution of the iron metabolic mRNAs between complexes with enhancing eIFs and inhibitory IRPs. The regulation of mRNA in the cytoplasm of eukaryotic cells is a burgeoning frontier in biomedicine. The evolutionarily refined IRE-RNAs, although absent in plants and bacteria, constitute a model system for 3-D mRNAs in all organisms. IRE-mRNAs have yielded "proof of principle" data for small-molecule targeting of mRNA structures, demonstrating tremendous potential for chemical manipulation of mRNA and protein synthesis in living systems.

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)

Secreted ferritin: mosquito defense against iron overload?

The yellow fever mosquito, Aedes aegypti, must blood feed in order to complete her life cycle. The blood meal provides a high level of iron that is required for egg development. We are interested in developing control strategies that interfere with this process. We show that A. aegypti larval cells synthesize and secrete ferritin in response to iron exposure. Cytoplasmic ferritin is maximal at low levels of iron, consists of both the light chain (LCH) and heavy chain (HCH) subunits and reflects cytoplasmic iron levels. Secreted ferritin increases in direct linear relationship to iron dose and consists primarily of HCH subunits. Although the messages for both subunits increase with iron treatment, our data indicate that mosquito HCH synthesis could be partially controlled at the translational level as well. Importantly, we show that exposure of mosquito cells to iron at low concentrations increases cytoplasmic iron, while higher iron levels results in a decline in cytoplasmic iron levels indicating that excess iron is removed from mosquito cells. Our work indicates that HCH synthesis and ferritin secretion are key factors in the response of mosquito cells to iron exposure and could be the primary mechanisms that allow these insects to defend against an intracellular iron overload.

Hepatic heme oxygenase is inducible in neonatal rats during the early postnatal period

Heme oxygenase (HO) is the rate-limiting enzyme in the catabolism of heme to bilirubin. Cobalt chloride (CoCl2) and many other agents that generate oxidant stresses induce the HO-1 isoform. Furthermore, HO-1 has been shown to protect against oxidant stress in vitro and in vivo by mechanisms involving increased ferritin synthesis. However, little is known about the inducibility of hepatic HO-1 during the very early postnatal period, and whether HO-1 induction is associated with increased ferritin synthesis in neonates. Therefore, we studied hepatic HO-1 mRNA, HO-1 protein concentration, total HO activity, and ferritin protein levels in neonatal rats. Neonatal rats 0-5 d of age were injected with 250 mumol/kg body weight of CoCl2. 6H2O in saline or with an equal volume of saline in age-matched controls. Liver samples were collected 4 h after injection for HO-1 mRNA analysis and 20 h after injection for analysis of HO-1 protein concentration, total HO activity, and ferritin protein levels. In CoCl2-treated rats, hepatic HO-1 mRNA was 3-10 times the levels in control rats (p < 0.05), HO-1 protein concentration was 2-5 times the levels in control rats (p < 0.05), and total HO activity was higher by 20-80% than in control rats (p < 0.05). There were no differences in hepatic ferritin protein levels between CoCl2-treated neonatal rats and controls; however, in CoCl2-treated adult rats, hepatic ferritin protein levels were 1.6 times the levels in controls (p < 0.05). Thus, neonatal rats can up-regulate hepatic HO-1 mRNA, HO-1 protein concentration, and total HO activity in response to CoCl2; however, no upregulation of hepatic ferritin protein levels was observed in neonatal rats after CoCl2 administration or subsequent HO-1 induction. We speculate that neonatal rats induce hepatic HO-1 and up-regulate ferritin by different mechanisms than do adult rats.

Ferritin gene organization: differences between plants and animals suggest possible kingdom-specific selective constraints

Ferritin, a protein widespread in nature, concentrates iron approximately 10(11)-10(12)-fold above the solubility within a spherical shell of 24 subunits; it derives in plants and animals from a common ancestor (based on sequence) but displays a cytoplasmic location in animals compared to the plastid in contemporary plants. Ferritin gene regulation in plants and animals is altered by development, hormones, and excess iron; iron signals target DNA in plants but mRNA in animals. Evolution has thus conserved the two end points of ferritin gene expression, the physiological signals and the protein structure, while allowing some divergence of the genetic mechanisms. Comparison of ferritin gene organization in plants and animals, made possible by the cloning of a dicot (soybean) ferritin gene presented here and the recent cloning of two monocot (maize) ferritin genes, shows evolutionary divergence in ferritin gene organization between plants and animals but conservation among plants or among animals; divergence in the genetic mechanism for iron regulation is reflected by the absence in all three plant genes of the IRE, a highly conserved, noncoding sequence in vertebrate animal ferritin mRNA. In plant ferritin genes, the number of introns (n = 7) is higher than in animals (n = 3). Second, no intron positions are conserved when ferritin genes of plants and animals are compared, although all ferritin gene introns are in the coding region; within kingdoms, the intron positions in ferritin genes are conserved. Finally, secondary protein structure has no apparent relationship to intron/exon boundaries in plant ferritin genes, whereas in animal ferritin genes the correspondence is high. The structural differences in introns/exons among phylogenetically related ferritin coding sequences and the high conservation of the gene structure within plant or animal kingdoms of the gene structure within plant or animal kingdoms suggest that kingdom-specific functional constraints may exist to maintain a particular intron/exon pattern within ferritin genes. In the case of plants, where ferritin gene intron placement is unrelated to triplet codons or protein structure, and where ferritin is targeted to the plastid, the selection pressure on gene organization may relate to RNA function and plastid/nuclear signaling.

Superficial siderosis of the central nervous system: a case with an unruptured intracranial aneurysm

We present a case of superficial siderosis (SS) of the central nervous system (CNS) with an unruptured intracranial aneurysm to illustrate that the commonly encountered unexplainable progressive sensorineural hearing loss (SNHL) can be an important sign for the early awareness of this rare disorder. The literature on SS is reviewed and the pathogenesis of SS is discussed.

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.

Thyroid hormone receptor induction by triiodothyronine in tadpole erythrocytes in vivo and in vitro and the effect of cycloheximide and actinomycin-D

Tadpole erythrocyte nuclei contain specific T3 binding sites which increase in number during spontaneous or T3-induced metamorphosis. In the present studies the increase in the number of T3 binding sites after a T3 injections appeared to be completely prevented by cycloheximide and actinomycin D, inhibitors of protein synthesis and RNA synthesis, respectively. However, in some experiments the effect was not statistically significant. The increase in sites was prevented only if the inhibitors were administered at 0 or 24 hr after T3 injection, but not at 48 or 72 hr after T3. When tadpole erythrocytes were incubated with T3 in vitro in M199 culture medium, the number of nuclear T3 binding sites increased within 48 hr. This increase was highly sensitive to inhibition by cycloheximide (maximal at 1 x 10(-6) M) or actinomycin D (maximal at 0.02 micrograms/ml). These inhibitor concentrations only slightly reduced the incorporation of labeled precursors. The T3 concentration required to induce a half-maximal increase in binding sites in vitro was about the same as the T3 concentration at which half the binding sites were occupied. The T3 binding sites had a high affinity for thyroid hormone analogs which stimulate metamorphosis. These results support the designation of the T3 binding sites as T3 receptors. They show that the tadpole erythrocytes respond to T3 with an increase in the number of T3 binding sites without the involvement of other tissues. It is proposed that this is a receptor induction involving synthesis of RNA and protein.

The iron regulatory region of ferritin mRNA is also a positive control element for iron-independent translation

The iron regulatory element (IRE) in the 5'-untranslated region of ferritin mRNA interacts with a specific regulator protein (P-90, IRE-BP, or FRP) to block translation. High cellular iron changes the IRE/P-90 interaction to relax the translational block and allow polyribosome formation. We now show that the IRE and base-paired flanking regions also enhance translation in the absence of P-90, explaining the high translational efficiency of deregulated ferritin mRNA observed previously. The effect of the IRE on translational efficiency was examined by comparing four sets of mRNAs: (1) +/- IRE in animal (frog) ferritin, regulated translationally by iron in vivo; (2) +/- animal IRE fused with plant (soybean) ferritin, regulated transcriptionally by iron in vivo; (3) repositioned IRE in animal ferritin; (4) mutated IRE in animal ferritin with G16A substitution, which decreases P-90 binding (negative control). The IRE region increased translational efficiency of both the animal ferritin and the heterologous IRE/soybean ferritin fusion mRNAs; the effect was observed in cell-free translation systems from either plants (wheat germ) or animals (rabbit reticulocyte). Repositioning the IRE further from the 5' cap eliminated positive control of translation. The single base mutation had no effect, indicating that positive and negative translational control involves different sections of the IRE region. Thus, the IRE region in ferritin mRNA encodes both positive translational control and, when combined with the regulator protein P-90, negative translational control.

Ferritin mRNA probed, near the iron regulatory region, with protein and chemical (1,10-phenanthroline-Cu) nucleases. A possible role for base-paired flanking regions

Iron stimulates ferritin synthesis in whole cells and animals, by increasing the entry of ferritin mRNA into polyribosomes. Dissection of the regulation at the molecular level has identified a 28-nucleotide, conserved, regulatory sequence (IRE = iron regulatory element) in the 5' non-coding region of ferritin mRNAs, plus trans-acting factor(s), one of which is a 90-kDa protein. The site of iron action is not entirely characterized but may involve heme; sequences in the 3' non-coding region of ferritin mRNA can modulate regulation. Ferritin mRNA is the first eukaryotic mRNA for which a conserved regulatory sequence and regulator protein have been identified. The same RNA-protein motif is used, through iron-dependent degradation of transferrin receptor mRNA, to decrease synthesis of the receptor and cellular iron uptake. The regulatory structure of the transferrin receptor mRNA is composed, in part, of five copies of the IRE in the 3' non-coding region. IRE structure, probed by cleavage with RNases T1, V1, 1,10-phenanthroline-Cu or modification with dimethyl sulfate, is a hairpin loop with conformational variations dependent on magnesium; a base-paired region flanking the IRE is also structurally sensitive to magnesium. Similar results were obtained with a synthetic 55-mer containing the IRE and with a full-length in vitro transcript with a G----A substitution in the loop.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonally induced elevations of alpha- and beta-casein mRNA levels are blocked by cyclic adenine nucleotide and prostaglandins

Normal mammary gland development during pregnancy follows a coordinated program of morphological development (formation of lobuloalveoli) and biochemical differentiation (casein production). In culture, whole mammary glands of Balb/c mice can be similarly induced by application of a mixture of insulin, prolactin, aldosterone and hydrocortisone (IPAH) for 7 days. Our previous reports have shown that lobuloalveolar development, induced by IPAH, is competitively inhibited by the simultaneous presence of dibutyryl cyclic AMP (Bt2cAMP), prostaglandins (PGs) E1, E2, and B1, and papaverine (pap). However, if this mixture is not added until day 4, lobuloalveolar development is relatively unaffected but casein synthesis is repressed. This report explores the mechanism by which cyclic adenine nucleotides and prostaglandins interfere with the normal developmental pathway. The accumulation of alpha- and beta-casein mRNAs induced by prolactin, hydrocortisone and aldosterone is blocked by the combination of Bt2cAMP, PGs E1, E2, and B1, and pap added to the medium for the final 3 days (days 4-7). Under these conditions the glands retain their lobuloalveoli, and little squamous metaplasia can be discerned. Furthermore, de novo synthesis of both caseins is selectively inhibited, despite the continued presence of casein mRNAs in the glands and normal protein synthesis. In contrast, the synthesis of keratin is stimulated. Incomplete mixtures of Bt2cAMP and pap or the combination of PGs E1, E2, and B1, are only partly effective in preventing the accumulation of casein mRNAs. All three mixtures bring about similar effects on both alpha- and beta-casein mRNAs.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential inhibition of protein synthesis: a possible biochemical mechanism of thalidomide teratogenesis

A theory concerning the chemical and biochemical mechanisms of thalidomide teratogenesis is presented. A considerable body of evidence suggests that the glutarimide ring of thalidomide may exert its biological activity because of its resemblance to the imide pyrimidines thymine and uracil. In addition to the glutarimide ring, thalidomide contains a moderately reactive phthalimide moiety, which allows the spontaneous formation of various glutarimide derivatives in fetal tissues. A model is proposed in which the phthalimide group reacts with small nucleophiles, most likely the polyamines, to produce a derivative(s) having a similar biochemical potential to that of cycloheximide, a glutarimide which is a powerful inhibitor of the elongation phase of protein synthesis. Interference in the elongation phase results in the selective inhibition of the translation of messages which have a high translational efficiency. Evidence is reviewed concerning the differential inhibition or protein synthesis by cycloheximide and the effects of this inhibition on various biochemical and biological processes which are critical during development and differentiation. A similar biochemical activity by the putative thalidomide derivative(s) could explain its extreme teratogenic potential. A number of parallels between the biological effects of thalidomide and cycloheximide are discussed which support the idea that a similar biochemical activity is involved. The theory readily explains many of the observed biological effects of thalidomide including the large difference between fetal and adult toxicity. In addition, evidence is reviewed which suggests that the teratogenic properties of a number of drugs which are structurally related to thalidomide may have a common chemical basis due to the similarity of their imide core structures to thymine and uracil.

Erythrocyte and plasma ferritin in normal subjects, blood donors and iron deficiency anemia patients

Ferritin concentration has been determined with an immunoradiometric assay in plasma and washed sedimented erythrocytes after hypotonic lysis. There was a gradual decrease of plasma ferritin in the sequence normal males, normal females, blood donors and patients with iron deficiency anemia. Erythrocyte ferritin remained unchanged in normal males and females and in blood donors, but dropped significantly in the anemic patients. Correspondingly, the ratio of erythrocyte to plasma ferritin rose from less than 2 in healthy males up to 8 in persons with iron deficiency. Little, if any effect on plasma and erythrocyte ferritin was observed in 12 male and female volunteers when taking iron for 4 weeks. In 2 patients with iron deficiency anemia the blood counts were normalized within 2-3 months during oral iron substitution, accompanied by a drastic increase of the erythrocyte ferritin concentration to values far above normal. In contrast, the plasma ferritin concentration remained below normal. Thus, in iron deficiency erythrocyte ferritin is synthesized with priority in the presence of iron and, in addition to plasma ferritin, appears to be a useful parameter of the iron status.