Translation of ferritin light and heavy subunit mRNAs is regulated by intracellular chelatable iron levels in rat hepatoma cells

The mRNA-binding protein which controls ferritin and transferrin receptor expression is conserved during evolution

A post-transcriptional regulatory protein, termed iron regulatory factor (IRF), that binds specifically to the iron-responsive elements of ferritin and transferrin receptor mRNA, has recently been identified in the cytoplasm of human and mouse cells. Activation of this factor by low intracellular iron levels leads to inhibition of ferritin translation and an increase of TR mRNA stability. To investigate whether these feedback regulatory mechanisms are conserved during evolution, we analysed cytoplasmic extracts from 12 different species for a specific IRE-binding activity. We found mRNA-binding proteins in chicken, frog, fish and fly, which are equivalent to human and mouse IRF in gel-retardation assays with radiolabeled RNA transcripts. Competition experiments, molecular weight determinations, and modulation of the mRNA-binding activity in response to intracellular iron levels or reduction by beta-mercaptoethanol indicate that IRF has similar structural and functional properties in these different species.

A high yield affinity purification method for specific RNA-binding proteins: isolation of the iron regulatory factor from human placenta

We describe a simple method for the affinity purification of specific RNA-binding proteins. DNA sequences corresponding to the protein-binding site of the RNA are subcloned into an in vitro transcription vector between the T7 viral promoter and a poly(A) track. A polyadenylated RNA transcript is bound to poly(U)-Sepharose and subsequently incubated with a cellular extract prepurified on heparin-agarose. Specifically adsorbed proteins are recovered in high yield and purity from the affinity matrix by high salt elution. Using this method we isolated the iron regulatory factor (IRF), a cytoplasmic protein which binds to specific palindromic elements in the 5' and 3' untranslated sequences of ferritin and transferrin receptor mRNA, respectively. Activation and binding of this regulatory factor correlates with increased transferrin receptor mRNA stability and inhibition of ferritin translation. The purified factor from human placenta migrates as a monomer in gel chromatography, but is present in equimolar amounts of two proteins with molecular weights of 95 and 100 kDa when analysed by SDS/PAGE. The two proteins are highly related as judged by the identity of their isoelectric points and their specificity to form RNA-protein complexes.

Derepression of ferritin messenger RNA translation by hemin in vitro

Incubation of a 90-kilodalton ferritin repressor protein (FRP), either free or complexed with an L-ferritin transcript, with hemin or Co3+-protoporphyrin IX prevented subsequent repression of ferritin synthesis in a wheat germ extract. Neither FeCl3 in combinations with H2O2, nor Fe3+ or Fe2+ chelated with EDTA, nor Zn2+-protoporphyrin IX, nor protoporphyrin IX caused significant inactivation of FRP. FRP that had been inactivated by hemin remained chemically intact, as revealed by SDS-polyacrylamide gel electrophoresis. Inclusion of chelators of iron or free radical scavengers did not alter the inactivation produced by hemin. These and other results indicate that hemin derepresses ferritin synthesis in vitro.

Inducible expression of encephalomyocarditis virus 3C protease activity in stably transformed mouse cell lines

An inducible expression vector system has been developed to facilitate the study of the effects of individual virus gene products on cell function. The vector utilizes the mouse metallothionein promoter carried on the bovine papillomavirus genome. Conditions which optimize the induced expression of open reading frames inserted downstream from the mouse metallothionein promoter have recently been described. In this communication we describe the use of this system to clone and express the encephalomyocarditis virus 3C protease in cultured mouse cells. Stably transformed cell lines could be induced to produce levels of 3C protease activity comparable to those observed during normal virus infection. In spite of this, no effects on cellular protein synthesis rate or membrane permeability were observed. It was also discovered that 3C protease as well as 3C protease-containing polyproteins are turned over. This was true not only in the induced cell clones, but also during the normal course of encephalomyocarditis virus infection, as well as in translation systems in vitro. This phenomenon was highly specific for this family of polypeptides, perhaps explaining their apparent lack of cytotoxic effects.

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.

Post-transcriptional control of sucrose synthase expression in anaerobic seedlings of maize

We have examined post-transcriptional control of expression of the anaerobically induced sucrose synthase 1 (SS1) isozyme mRNA encoded by the shrunken (Sh) gene of maize (Zea mays L.). The SS1 transcript level is increased in maize seedling roots during anaerobiosis without a concomitant increase in the SS1 protein level. We show that the anaerobic SS1 RNA was loaded onto polyribosomes and that SS1 proteins produced by in vitro translation of polyribosomal RNA from anaerobic roots and immature kernels were indistinguishable based on abundance and apparent molecular weight. [(35)S]Methionine uptake in control and anaerobically stressed seedling roots indicated a detectable, but only slight, increase in radiolabel in the SS1 polypeptide as compared to the sucrose synthase 2 isozyme, SS2. However, this slight increase in [(35)S]methionine uptake did not contribute to a detectable increase in the steady state level of SS1 protein relative to SS2 protein. Chase experiments with unlabeled methionine indicated that SS1 protein was relatively stable in the anaerobic environment. From these results we conclude that SS1 protein was not rapidly turned over in the anaerobic environment and that expression of anaerobically induced SS1 transcripts was blocked at some step beyond polyribosomal loading.

Iron induction of ferritin synthesis in soybean cell suspensions

In animal cells specialized for iron storage, iron-induced accumulation of ferritin is known to result from a shift of stored mRNA from the ribonucleoprotein fraction to polysomes. Previous reports with bean leaves suggested that in plants iron induction of ferritin synthesis would result from a regulation at the transcriptional level (F van der Mark, F Bienfait, H van der Ende [1983] Biochem Biophys Res Commun 115:463-469). Soybean (Glycine max, cv Mandarin) cell suspension cultures have been used here to support these findings. Ferritin induction is obtained by addition of Fe-citrate to the culture medium. A good correlation is found between cellular iron content and the amount of ferritin accumulation. This protein accumulation corresponds to an increase of in vitro translatable ferritin mRNA. Addition of 4 micrograms actinomycin D per milliliter to the cultures inhibits completely in vivo RNA synthesis, whereas protein synthesis was poorly affected, at least for 24 hours. During the same time, this concentration of actinomycin D strongly inhibits the iron-induced synthesis of ferritin. These results show that in soybean cell cultures, the mechanism of regulation of ferritin synthesis in response to iron does not result from recruitment of preexisting mRNA. They confirm that in plant systems, ferritin synthesis results from increased transcription of the corresponding genes.

A cytosolic protein binds to structural elements within the iron regulatory region of the transferrin receptor mRNA

The level of mRNA encoding the transferrin receptor (TfR) is regulated by iron, and this regulation is mediated by a portion of the 3' untranslated region (UTR) of the TfR transcript. This portion of 3' UTR of the human TfR mRNA contains five RNA elements that have structural similarity to the iron-responsive element (IRE) found as a single copy in the 5' UTR of the mRNA for ferritin, whose translation is regulated by iron. Moreover, five very similar elements are also contained in the 3' UTR of the chicken TfR mRNA. Cytosolic extracts of human cell lines are shown by a gel shift assay involving RNase T1 protection to contain an IRE-binding protein capable of specific interaction with the human TfR 3' UTR. When the protecting protein is removed, the protected RNA can be digested with RNase T1 to yield oligoribonucleotide fragments characteristic of two of the IREs contained in the TfR 3' UTR. As judged by cross-competition experiments, the same IRE-binding protein interacts with the ferritin IRE. The apparent affinity of RNA sequence elements for the IRE-binding protein is shown to depend upon the sequence of the RNA. A comprehensive secondary structure for the regulatory region of the TfR mRNA is proposed based on the experimentally demonstrated presence of at least two IRE-like structural elements.

T-cell mitogenesis stimulates the synthesis of a mRNA species coding for a 43-kDa peptide reactive with CM-H-9, a monoclonal antibody specific for placental isoferritin

In studying the changes that occur in concanavalin A-activated T lymphocytes, an mRNA species was discovered by hybridization of poly(A)+ mRNA with a human ferritin heavy chain cDNA probe. This ferritin mRNA, termed superheavy chain mRNA, differed from the known human ferritin heavy chain mRNAs by its larger size and degree of homology. The superheavy chain mRNA was isolated by sucrose-gradient centrifugation and translated in vitro in a cell-free system. The products obtained included two peptides (superheavy) of 43kDa that reacted with CM-H-9, a monoclonal antibody specific for placental isoferritin. De novo synthesis in intact transformed T cells revealed the synthesis of the superheavy chain peptides that were immunoprecipitated by anti-ferritin monoclonal antibody CM-G-8 and by placental isoferritin specific monoclonal antibody CM-H-9. The above results indicated that blast transformation of human T cells stimulated the appearance of a unique mRNA species that coded for a superheavy chain peptide associated with placental isoferritin, which was not detected in resting T cells.

Multiple post-transcriptional regulatory mechanisms in ferritin gene expression

We have investigated the mechanisms involved in the regulation of ferritin biosynthesis in K562 human erythroleukemia cells during prolonged exposure to iron. We show that, upon addition of hemin (an efficient iron donor) to the cell culture, the rate of ferritin biosynthesis reaches a maximum after a few hours and then decreases. During a 24-hr incubation with the iron donor the concentrations of total ferritin heavy (H) and light (L) subunit mRNAs rise 2- to 5-fold and 2- to 3-fold, respectively, over the control values, while the amount of the protein increases 10- to 30-fold. The hemin-induced increment in ferritin subunit mRNA is not prevented by deferoxamine, suggesting that it is not directly mediated by chelatable iron. In vitro nuclear transcription analyses performed on nuclei isolated from control cells and cells grown in the presence of hemin indicate that the rates of synthesis of H- and L-subunit mRNAs remain constant. We conclude that iron-induced ferritin biosynthesis is governed by multiple post-transcriptional regulatory mechanisms. We propose that exposure of cells to iron leads to stabilization of ferritin mRNAs, in addition to activation and translation of stored H- and L-subunit mRNAs.

Chelation of transferrin iron by desferrioxamine in K562 cells. The partition of iron between ferrioxamine and ferritin

In this study we have determined whether desferrioxamine can chelate iron delivered to human leukaemic cells by the transferrin endocytic cycle. The cellular uptake of desferrioxamine was investigated by an indirect method in which the conversion of repeated pulses of [59Fe]transferrin to [59Fe]ferrioxamine was determined at two concentrations of the drug. Maximum generation of [59Fe]ferrioxamine occurred in cells exposed to either 100 microM- or 500 microM-desferrioxamine after 40-60 min. Thereafter (up to 180 min) [59Fe]ferrioxamine levels remained steady with 20% of a 59Fe pulse partitioning to chelator at 100 microM and 50% at 500 microM. Of the cellular [59Fe]ferrioxamine loss 50% occurred within 90-120 min. In cells preloaded with desferrioxamine for 1 or 4 h the partitioning of iron during a 3 h incubation with [59Fe]transferrin was dependent upon the extracellular concentration of the chelator. Above 1 mM more than 80% of entering iron was converted to ferrioxamine and less than 5% partitioned to ferritin. Below this concentration (50-500 microM) a proportion of the iron became ferritin associated (7-41%). There was a linear increase in the total amount of intracellular [59Fe]ferrioxamine in accordance with cellular iron uptake showing that transferrin continued to cycle in the presence of high concentrations of desferrioxamine. The uptake of iron and generation of ferrioxamine were markedly reduced by 5 mM-methylamine, which prevented endosome acidification and uncoupling of iron from endocytosed transferrin.

Binding of a cytosolic protein to the iron-responsive element of human ferritin messenger RNA

The human ferritin H chain messenger RNA contains a specific iron-responsive element (IRE) in its 5' untranslated region, which mediates regulation by iron of ferritin translation. An RNA gel retardation assay was used to demonstrate the affinity of a specific cytosolic binding protein for the IRE. A single-base deletion in the IRE eliminated both the interaction of the cytoplasmic protein with the IRE and translational regulation. Thus, the regulatory potential of the IRE correlates with its capacity to specifically interact with proteins. Titration curves of binding activity after treatment of cells with an iron chelator suggest that the factor acts as a repressor of ferritin translation.

Iron-responsive elements: regulatory RNA sequences that control mRNA levels and translation

The biosynthetic rates for both the transferrin receptor (TfR) and ferritin are regulated by iron. An iron-responsive element (IRE) in the 5' untranslated portion of the ferritin messenger RNA (mRNA) mediates iron-dependent control of its translation. In this report the 3' untranslated region of the mRNA for the human TfR was shown to be necessary and sufficient for iron-dependent control of mRNA levels. Deletion studies identified a 678-nucleotide fragment of the TfR complementary DNA that is critical for this iron regulation. Five potential stem-loops that resemble the ferritin IRE are contained within the region critical for TfR regulation. Each of two of the five TfR elements was independently inserted into the 5' untranslated region of an indicator gene transcript. In this location they conferred iron regulation of translation. Thus, an mRNA element has been implicated in the mediation of distinct regulatory phenomena dependent on the context of the element within the transcript.

Cytoplasmic protein binds in vitro to a highly conserved sequence in the 5' untranslated region of ferritin heavy- and light-subunit mRNAs

The mRNAs for the heavy and light subunits of the iron-storage protein ferritin occur in cells largely as inactive ribonucleoprotein particles, which are recruited for translation when iron enters the cell. Cytoplasmic extracts from rat tissues and hepatoma cells were shown by an electrophoretic separation procedure to form RNA-protein complexes involving a highly conserved sequence in the 5' untranslated region of both ferritin heavy- and light-subunit mRNAs. The pattern of complex formation was affected by pretreatment of rats or cells with iron. Crosslinking by UV irradiation showed that the complexes contained an 87-kDa protein interacting with the conserved sequence of the ferritin mRNA. We propose that intracellular iron levels regulate ferritin synthesis by causing changes in specific protein binding to the conserved sequence in the ferritin heavy- and light-subunit mRNAs.

Identification of the iron-responsive element for the translational regulation of human ferritin mRNA

Regulated translation of messenger RNA offers an important mechanism for the control of gene expression. The biosynthesis of the intracellular iron storage protein ferritin is translationally regulated by iron. A cis-acting element that is both necessary and sufficient for this translational regulation is present within the 5' nontranslated leader region of the human ferritin H-chain messenger RNA. In this report the iron-responsive element (IRE) was identified by deletional analysis. Moreover, a synthetic oligodeoxynucleotide was shown to be able to transfer iron regulation to a construct that would otherwise not be able to respond to iron. The IRE has been highly conserved and predates the evolutionary segregation between amphibians, birds, and man. The IRE may prove to be useful for the design of translationally regulated expression systems.

Iron regulates ferritin mRNA translation through a segment of its 5' untranslated region

In previous studies, we showed that acute administration of iron to intact rats or to rat hepatoma cells in culture induces synthesis of the iron-storage protein ferritin by activating translation of inactive cytoplasmic ferritin mRNAs for both the heavy (H) and the light (L) subunits. In the course of activation, these ferritin mRNAs are recruited onto polysomes. To elucidate the structural features of these mRNAs involved in the translational response to iron, a chimera was constructed from the 5' and 3' untranslated regions (UTRs) of ferritin L subunit mRNA fused to the reading frame of the mRNA of bacterial chloramphenicol acetyltransferase (CAT). This chimera and deletion constructs derived from it were introduced into a rat hepatoma cell line by retrovirus-mediated gene transfer. The complete chimera showed increased CAT activity in response to iron enrichment of the medium, whereas deletion of the first 67 nucleotides of the 5' UTR, which contain a highly conserved sequence, caused loss of regulation by iron. Whereas cis-acting sequences located in the 5' flanking regions of many genes have been repeatedly implicated in modulating their transcriptional expression, we report here a specific regulatory translational sequence found within the 5' UTR of a eukaryotic mRNA.

Conservation of ferritin heavy subunit gene structure: implications for the regulation of ferritin gene expression

Ferritin stores iron within a protein shell consisting of 24 subunits of two types, heavy (H) and light (L). According to Southern blotting, the rat genome contains four copies homologous to the H-subunit cDNA (H cDNA). To determine whether only one of these is expressed, H cDNAs isolated from rat liver and heart mRNAs were compared and found to share identical nucleotide sequences. Next, genomic clones for three of the four rat H-subunit loci were isolated. Two were classical processed pseudogenes, whereas the third contained an expressed gene. RNase intron mapping of this expressed gene generated the same exon protection pattern when total RNA from rat liver or heart was used, indicating that this gene accounts for most or all of the H-subunit mRNAs (H mRNAs) in these tissues. Comparison of the expressed rat H-subunit gene (H gene) structure with published sequences for other species displays considerable conservation. The coding sequence of the rat H gene predicts 95% similarity to the human amino acid sequence, thus being more highly conserved than the L-subunit sequence of these species. Near the cap region of the 5' untranslated region, the rat H mRNA displays a 28-nucleotide sequence that is almost totally conserved in the corresponding region of the human, bullfrog, and chicken H mRNA and is also faithfully represented in the rat and human L-subunit mRNAs (L mRNAs), thus making this sequence a prime candidate for involvement in the known translational regulation of both subunits by iron. In the 5' flanking region, partially conserved sequences common to H gene and L-subunit gene (L gene) of the rat may be involved in transcriptional regulation by iron, whereas those conserved only in the H gene of man and the rat imply that other factors may independently control H-subunit regulation.