ISCA2 inhibition decreases HIF and induces ferroptosis in clear cell renal carcinoma

Clear cell renal cell carcinoma (ccRCC), the most common form of kidney cancer, is typically initiated by inactivation of the von Hippel Lindau (VHL) gene, which results in the constitutive activation of the hypoxia inducible factors, HIF-1α and HIF-2α. Using a high throughput screen, we identify novel compounds that decrease HIF-1/2α levels and induce ferroptosis by targeting Iron Sulfur Cluster Assembly 2 (ISCA2), a component of the late mitochondrial Iron Sulfur Cluster (L-ISC) assembly complex. ISCA2 inhibition either pharmacologically or using siRNA decreases HIF-2α protein levels by blocking iron-responsive element (IRE)-dependent translation, and at higher concentrations, also decreases HIF-1α translation through unknown mechanisms. Additionally, ISCA2 inhibition triggers the iron starvation response, resulting in iron/metals overload and death via ferroptosis. ISCA2 levels are decreased in ccRCC compared to normal kidney, and decreased ISCA2 levels are associated with pVHL loss and with sensitivity to ferroptosis induced by ISCA2 inhibition. Strikingly, pharmacological inhibition of ISCA2 using an orally available ISCA2 inhibitor significantly reduced ccRCC xenograft growth in vivo, decreased HIF-α levels and increased lipid peroxidation, suggesting increased ferroptosis in vivo. Thus, the targeting of ISCA2 may be a promising therapeutic strategy to inhibit HIF-1/2α and to induce ferroptosis in pVHL deficient cells.

An Erythropoietin-Independent Mechanism of Erythrocytic Precursor Proliferation Underlies Hypoxia Tolerance in Sea Nomads

The Bajau Sea Nomads were recently demonstrated to have evolved larger spleens as an adaptation to millennia of a marine foraging lifestyle. The large-spleen phenotype appears to derive from increases in thyroid hormone (TH) production as a result of reduced expression of phosphodiesterase 10A (PDE10A), though the exact mechanism remains unknown. Through pharmacological inhibition of PDE10A using the selective inhibitor MP-10 in mice, we were able to mimic the Bajau adaptation and show that treated mice had significantly larger spleens than control animals. This difference appears connected to an excess of early stage erythrocytes and an apparent increase in red blood cell (RBC) precursor proliferation in response to increased TH. However, we determined that the stimulation of RBC production in the mouse model via TH is Erythropoietin (EPO)-independent, unlike in the altitude (chronic hypoxemia) response. We confirmed this using human GWAS data; although the Bajau PDE10A variants are significantly associated with increased TH levels and RBC count, they are not associated with EPO levels, nor are other strongly thyroid-associated SNPs. We therefore suggest that an EPO-independent mechanism of stimulating RBC precursor proliferation via TH upregulation underlies the increase in spleen size observed in Sea Nomad populations.

Iron in infection and immunity in C. elegans

Iron is essential for survival of most organisms, but is toxic in excess. All organisms have thus developed mechanisms to sense, acquire and sequester iron. In C. elegans, iron uptake and sequestration are regulated by HIF-1. We previously showed that hif-1 loss of function mutants are developmentally delayed when grown under iron limitation. Here we identify nhr-14, encoding a nuclear receptor homologous to vertebrate HNF4, in a genetic screen conducted for mutations that rescue the developmental delay of hif-1 mutants under iron limitation. NHR-14 is highly expressed in intestinal cell nuclei and in cells in the head, and its subcellular localization and expression are not regulated by iron. Loss of nhr-14 leads to the upregulation of the intestinal metal transporter SMF-3 that increases iron uptake in hif-1 mutants, rescuing the low iron-dependent developmental delay. Loss of nhr-14 also promotes the nuclear localization of the zinc-finger transcription factor PQM-1, which activates smf-3 through the interaction with GATA-like DAF-16-associated elements (DAEs) in the smf-3 promoter. In addition to smf-3, RNA-seq analysis revealed upregulation of innate immune response genes as well as DAF-16/FoxO-suppressed Class 2 genes. Consistent with the RNA-seq results, nhr-14 mutants showed enhanced resistance to the human pathogen Pseudomonas aeruginosa that depends in part on iron uptake as well as the upregulation of innate immune response genes. Our data provide insight into how C. elegans utilizes nuclear receptors to regulate innate immunity and iron availability, and show iron sequestration as an important component of the innate immune response.

FBXL5 Regulates IRP2 Stability in Iron Homeostasis via an Oxygen-Responsive [2Fe2S] Cluster

Cellular iron homeostasis is dominated by FBXL5-mediated degradation of iron regulatory protein 2 (IRP2), which is dependent on both iron and oxygen. However, how the physical interaction between FBXL5 and IRP2 is regulated remains elusive. Here, we show that the C-terminal substrate-binding domain of FBXL5 harbors a [2Fe2S] cluster in the oxidized state. A cryoelectron microscopy (cryo-EM) structure of the IRP2-FBXL5-SKP1 complex reveals that the cluster organizes the FBXL5 C-terminal loop responsible for recruiting IRP2. Interestingly, IRP2 binding to FBXL5 hinges on the oxidized state of the [2Fe2S] cluster maintained by ambient oxygen, which could explain hypoxia-induced IRP2 stabilization. Steric incompatibility also allows FBXL5 to physically dislodge IRP2 from iron-responsive element RNA to facilitate its turnover. Taken together, our studies have identified an iron-sulfur cluster within FBXL5, which promotes IRP2 polyubiquitination and degradation in response to both iron and oxygen concentrations.

Irp2 regulates insulin production through iron-mediated Cdkal1-catalyzed tRNA modification

Regulation of cellular iron homeostasis is crucial as both iron excess and deficiency cause hematological and neurodegenerative diseases. Here we show that mice lacking iron-regulatory protein 2 (Irp2), a regulator of cellular iron homeostasis, develop diabetes. Irp2 post-transcriptionally regulates the iron-uptake protein transferrin receptor 1 (TfR1) and the iron-storage protein ferritin, and dysregulation of these proteins due to Irp2 loss causes functional iron deficiency in β cells. This impairs Fe-S cluster biosynthesis, reducing the function of Cdkal1, an Fe-S cluster enzyme that catalyzes methylthiolation of t⁶A37 in tRNA^Lys_UUU to ms²t⁶A37. As a consequence, lysine codons in proinsulin are misread and proinsulin processing is impaired, reducing insulin content and secretion. Iron normalizes ms²t⁶A37 and proinsulin lysine incorporation, restoring insulin content and secretion in Irp2^-/- β cells. These studies reveal a previously unidentified link between insulin processing and cellular iron deficiency that may have relevance to type 2 diabetes in humans.

NHR-14 loss of function couples intestinal iron uptake with innate immunity in C. elegans through PQM-1 signaling

Iron is essential for survival of most organisms. All organisms have thus developed mechanisms to sense, acquire and sequester iron. In C. elegans, iron uptake and sequestration are regulated by HIF-1. We previously showed that hif-1 mutants are developmentally delayed when grown under iron limitation. Here we identify nhr-14, encoding a nuclear receptor, in a screen conducted for mutations that rescue the developmental delay of hif-1 mutants under iron limitation. nhr-14 loss upregulates the intestinal metal transporter SMF-3 to increase iron uptake in hif-1 mutants. nhr-14 mutants display increased expression of innate immune genes and DAF-16/FoxO-Class II genes, and enhanced resistance to Pseudomonas aeruginosa. These responses are dependent on the transcription factor PQM-1, which localizes to intestinal cell nuclei in nhr-14 mutants. Our data reveal how C. elegans utilizes nuclear receptors to regulate innate immunity and iron availability, and show iron sequestration as a component of the innate immune response.

Regulation of the iron regulatory proteins by reactive nitrogen and oxygen species

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) are RNA binding proteins that posttranscriptionally regulate the expression of mRNAs coding for proteins involved in the maintenance of iron and energy homeostasis. The RNA binding activities of the IRPs are regulated by changes in cellular iron. Thus, the IRPs are considered iron sensors and the principle regulators of cellular iron homeostasis. The mechanisms governing iron regulation of the IRPs are well described. Recently, however, much attention has focused on the regulation of IRPs by reactive nitrogen and oxygen species (RNS, ROS). Here we focus on summarizing the iron-regulated RNA binding activities of the IRPs, as well as the recent findings of IRP regulation by RNS and ROS. The recent observations that changes in oxygen tension regulate both IRP1 and IRP2 RNA binding activities will be addressed in light of ROS regulation of the IRPs.

Abnormal brain iron metabolism in Irp2 deficient mice is associated with mild neurological and behavioral impairments

Iron Regulatory Protein 2 (Irp2, Ireb2) is a central regulator of cellular iron homeostasis in vertebrates. Two global knockout mouse models have been generated to explore the role of Irp2 in regulating iron metabolism. While both mouse models show that loss of Irp2 results in microcytic anemia and altered body iron distribution, discrepant results have drawn into question the role of Irp2 in regulating brain iron metabolism. One model shows that aged Irp2 deficient mice develop adult-onset progressive neurodegeneration that is associated with axonal degeneration and loss of Purkinje cells in the central nervous system. These mice show iron deposition in white matter tracts and oligodendrocyte soma throughout the brain. A contrasting model of global Irp2 deficiency shows no overt or pathological signs of neurodegeneration or brain iron accumulation, and display only mild motor coordination and balance deficits when challenged by specific tests. Explanations for conflicting findings in the severity of the clinical phenotype, brain iron accumulation and neuronal degeneration remain unclear. Here, we describe an additional mouse model of global Irp2 deficiency. Our aged Irp2^−/− mice show marked iron deposition in white matter and in oligodendrocytes while iron content is significantly reduced in neurons. Ferritin and transferrin receptor 1 (TfR1, Tfrc), expression are increased and decreased, respectively, in the brain from Irp2^−/− mice. These mice show impairments in locomotion, exploration, motor coordination/balance and nociception when assessed by neurological and behavioral tests, but lack overt signs of neurodegenerative disease. Ultrastructural studies of specific brain regions show no evidence of neurodegeneration. Our data suggest that Irp2 deficiency dysregulates brain iron metabolism causing cellular dysfunction that ultimately leads to mild neurological, behavioral and nociceptive impairments.

Mechanisms of iron metabolism in Caenorhabditis elegans

Iron is involved in many biological processes essential for sustaining life. In excess, iron is toxic due to its ability to catalyze the formation of free radicals that damage macromolecules. Organisms have developed specialized mechanisms to tightly regulate iron uptake, storage and efflux. Over the past decades, vertebrate model organisms have led to the identification of key genes and pathways that regulate systemic and cellular iron metabolism. This review provides an overview of iron metabolism in the roundworm Caenorhabditis elegans and highlights recent studies on the role of hypoxia and insulin signaling in the regulation of iron metabolism. Given that iron, hypoxia and insulin signaling pathways are evolutionarily conserved, C. elegans provides a genetic model organism that promises to provide new insights into mechanisms regulating mammalian iron metabolism.

Mammalian iron metabolism and its control by iron regulatory proteins

Cellular iron homeostasis is maintained by iron regulatory proteins 1 and 2 (IRP1 and IRP2). IRPs bind to iron-responsive elements (IREs) located in the untranslated regions of mRNAs encoding protein involved in iron uptake, storage, utilization and export. Over the past decade, significant progress has been made in understanding how IRPs are regulated by iron-dependent and iron-independent mechanisms and the pathological consequences of IRP2 deficiency in mice. The identification of novel IREs involved in diverse cellular pathways has revealed that the IRP-IRE network extends to processes other than iron homeostasis. A mechanistic understanding of IRP regulation will likely yield important insights into the basis of disorders of iron metabolism. This article is part of a Special Issue entitled: Cell Biology of Metals.

HIF-1 regulates iron homeostasis in Caenorhabditis elegans by activation and inhibition of genes involved in iron uptake and storage

Caenorhabditis elegans ftn-1 and ftn-2, which encode the iron-storage protein ferritin, are transcriptionally inhibited during iron deficiency in intestine. Intestinal specific transcription is dependent on binding of ELT-2 to GATA binding sites in an iron-dependent enhancer (IDE) located in ftn-1 and ftn-2 promoters, but the mechanism for iron regulation is unknown. Here, we identify HIF-1 (hypoxia-inducible factor -1) as a negative regulator of ferritin transcription. HIF-1 binds to hypoxia-response elements (HREs) in the IDE in vitro and in vivo. Depletion of hif-1 by RNA interference blocks transcriptional inhibition of ftn-1 and ftn-2 reporters, and ftn-1 and ftn-2 mRNAs are not regulated in a hif-1 null strain during iron deficiency. An IDE is also present in smf-3 encoding a protein homologous to mammalian divalent metal transporter-1. Unlike the ftn-1 IDE, the smf-3 IDE is required for HIF-1–dependent transcriptional activation of smf-3 during iron deficiency. We show that hif-1 null worms grown under iron limiting conditions are developmentally delayed and that depletion of FTN-1 and FTN-2 rescues this phenotype. These data show that HIF-1 regulates intestinal iron homeostasis during iron deficiency by activating and inhibiting genes involved in iron uptake and storage.

Due to its presence in proteins involved in hemoglobin synthesis, DNA synthesis, and mitochondrial respiration, eukaryotic cells require iron for survival. Excess iron can lead to oxidative damage, while iron deficiency reduces cell growth and causes cell death. Dysregulation of iron homeostasis in humans caused by iron deficiency or excess leads to anemia, diabetes, and neurodegenerative disorders. All organisms have thus developed mechanisms to sense, acquire, and store iron. We use Caenorhabditis elegans as a model organism to study mechanisms of iron regulation. Our previous studies show that the iron-storage protein ferritin (FTN-1, FTN-2) is transcriptionally inhibited in intestine during iron deficiency, but the mechanisms regulating iron regulation are not known. Here, we find that hypoxia-inducible factor 1 (HIF-1) transcriptionally inhibits ftn-1 and ftn-2 during iron deficiency. We also show that HIF-1 activates the iron uptake gene smf-3. Transcriptional activation and inhibition by HIF-1 is dependent on an iron enhancer in the promoters of these genes. HIF-1 is a known transcriptional activator, but its role in transcriptional inhibition is not well understood. Our data show that HIF-1 regulates iron homeostasis by activating and inhibiting iron uptake and storage genes, and they provide insight into HIF-1 transcriptional inhibition.

Control of iron homeostasis by an iron-regulated ubiquitin ligase

Eukaryotic cells require iron for survival and have developed regulatory mechanisms for maintaining appropriate intracellular iron concentrations. The degradation of iron regulatory protein 2 (IRP2) in iron-replete cells is a key event in this pathway, but the E3 ubiquitin ligase responsible for its proteolysis has remained elusive. We found that a SKP1-CUL1-FBXL5 ubiquitin ligase protein complex associates with and promotes the iron-dependent ubiquitination and degradation of IRP2. The F-box substrate adaptor protein FBXL5 was degraded upon iron and oxygen depletion in a process that required an iron-binding hemerythrin-like domain in its N terminus. Thus, iron homeostasis is regulated by a proteolytic pathway that couples IRP2 degradation to intracellular iron levels through the stability and activity of FBXL5.

Iron-independent phosphorylation of iron regulatory protein 2 regulates ferritin during the cell cycle

Iron regulatory protein 2 (IRP2) is a key iron sensor that post-transcriptionally regulates mammalian iron homeostasis by binding to iron-responsive elements (IREs) in mRNAs that encode proteins involved in iron metabolism (e.g. ferritin and transferrin receptor 1). During iron deficiency, IRP2 binds IREs to regulate mRNA translation or stability, whereas during iron sufficiency IRP2 is degraded by the proteasome. Here, we identify an iron-independent IRP2 phosphorylation site that is regulated by the cell cycle. IRP2 Ser-157 is phosphorylated by Cdk1/cyclin B1 during G(2)/M and is dephosphorylated during mitotic exit by the phosphatase Cdc14A. Ser-157 phosphorylation during G(2)/M reduces IRP2 RNA-binding activity and increases ferritin synthesis, whereas Ser-157 dephosphorylation during mitotic exit restores IRP2 RNA-binding activity and represses ferritin synthesis. These data show that reversible phosphorylation of IRP2 during G(2)/M has a role in modulating the iron-independent expression of ferritin and other IRE-containing mRNAs during the cell cycle.

An iron enhancer element in the FTN-1 gene directs iron-dependent expression in Caenorhabditis elegans intestine

Ferritin is a ubiquitous protein that sequesters iron and protects cells from iron toxicity. Caenorhabditis elegans express two ferritins, FTN-1 and FTN-2, which are transcriptionally regulated by iron. To identify the cis-acting sequences and proteins required for iron-dependent regulation of ftn-1 and ftn-2 expression, we generated transcriptional GFP reporters corresponding to 5 '-upstream sequences of the ftn-1 and ftn-2 genes. We identified a conserved 63-bp sequence, the iron-dependent element (IDE), that is required for iron-dependent regulation of a ftn-1 GFP reporter in intestine. The IDE contains two GATA-binding motifs and three octameric direct repeats. Site-directed mutagenesis of the GATA sequences, singly or in combination, reduces ftn-1 GFP reporter expression in the intestine. In vitro DNA mobility shift assays show that the intestine-specific GATA protein ELT-2 binds to both GATA sequences. Inhibition of ELT-2 function by RNA interference blocks ftn-1 GFP reporter expression in vivo. Insertion of the IDE into the promoter region of a heterologous reporter activates iron-dependent transcription in intestine. These data demonstrate that the activation of ftn-1 and ftn-2 transcription by iron requires ELT-2 and that the IDE functions as an iron-dependent enhancer in intestine.

HOIL-1 is not required for iron-mediated IRP2 degradation in HEK293 cells

Iron regulatory protein 2 (IRP2) binds to iron-responsive elements (IREs) to regulate the translation and stability of mRNAs encoding several proteins involved in mammalian iron homeostasis. Increases in cellular iron stimulate the polyubiquitylation and proteasomal degradation of IRP2. One study has suggested that haem-oxidized IRP2 ubiquitin ligase-1 (HOIL-1) binds to a unique 73-amino acid (aa) domain in IRP2 in an iron-dependent manner to regulate IRP2 polyubiquitylation and degradation. Other studies have questioned the role of the 73-aa domain in iron-dependent IRP2 degradation. We investigated the potential role of HOIL-1 in the iron-mediated degradation of IRP2 in human embryonic kidney 293 (HEK293) cells. We found that transiently expressed HOIL-1 and IRP2 interact via the 73-aa domain, but this interaction is not iron-dependent, nor does it enhance the rate of IRP2 degradation by iron. In addition, stable expression of HOIL-1 does not alter the iron-dependent degradation or RNA-binding activity of endogenous IRP2. Reduction of endogenous HOIL-1 by siRNA has no affect on the iron-mediated degradation of endogenous IRP2. These data demonstrate that HOIL-1 is not required for iron-dependent degradation of IRP2 in HEK293 cells, and suggest that a HOIL-1 independent mechanism is used for IRP2 degradation in most cell types.

Alteration of iron regulatory proteins (IRP1 and IRP2) and ferritin in the brains of scrapie-infected mice

Considerable evidence suggests that oxidative stress may be involved in the pathogenesis of Transmissible Spongiform Encephalopathies (TSEs). To investigate the involvement of iron metabolism in TSEs, we examined the expression levels of iron regulatory proteins (IRPs), ferritins, and binding activities of IRPs to iron-responsive element (IRE) in scrapie-infected mice. We found that the IRPs-IRE-binding activities and ferritins were increased in the astrocytes of hippocampus and cerebral cortex in the brains of scrapie-infected mice. These results suggest that alteration of iron metabolism contributes to development of neurodegeneration and that some protective mechanisms against iron-induced oxidative damage may occur during the pathogenesis of TSEs.

Molecular control of vertebrate iron homeostasis by iron regulatory proteins

Both deficiencies and excesses of iron represent major public health problems throughout the world. Understanding the cellular and organismal processes controlling iron homeostasis is critical for identifying iron-related diseases and in advancing the clinical treatments for such disorders of iron metabolism. Iron regulatory proteins (IRPs) 1 and 2 are key regulators of vertebrate iron metabolism. These RNA binding proteins post-transcriptionally control the stability or translation of mRNAs encoding proteins involved in iron homeostasis thereby controlling the uptake, utilization, storage or export of iron. Recent evidence provides insight into how IRPs selectively control the translation or stability of target mRNAs, how IRP RNA binding activity is controlled by iron-dependent and iron-independent effectors, and the pathological consequences of dysregulation of the IRP system.

Influence of gestational age and fetal iron status on IRP activity and iron transporter protein expression in third-trimester human placenta

Placental iron transport during the last trimester of pregnancy determines the iron endowment of the neonate. Iron transport is a function of the major iron transport proteins: transferrin receptor-1 (TfR-1) and ferroportin-1 (FPN-1). The mRNAs for TfR-1 and, potentially, FPN-1 are posttranscriptionally regulated by iron regulatory protein (IRP)-1 and IRP-2. We assessed the effect of gestational age and fetal iron status on IRP-1- and IRP-2-binding activity and on the localization and protein expression of TfR-1 and FPN-1 protein at 24-40 wk of gestation in 21 placentas obtained from iron-sufficient nonanemic mothers. Gestational age had no effect on cord serum ferritin concentration, IRP-2 RNA-binding activity, transporter protein location, and TfR-1 or FPN-1 protein expression. IRP-1 activity remained constant until full term, when it decreased (P = 0.01). Placental ferritin (r = 0.76, P < 0.001) and FPN-1 (r = 0.44, P < 0.05) expression increased with gestational age. Fetal iron status, as indexed by cord serum ferritin concentration, was inversely related to placental IRP-1 (r = -0.66, P < 0.001) and IRP-2 (r = -0.42, P = 0.05) activities. Placental ferritin protein expression correlated better with IRP-1 (r = -0.45, P = 0.04) than with IRP-2 (r = -0.35, P = 0.10) activity. Placental TfR-1 and FPN-1 protein expression was independent of fetal or placental iron status and IRP activities. Iron status had no effect on transport protein localization. We conclude that, toward the end of the third trimester of iron-sufficient human pregnancy, the placenta accumulates ferritin and potentially increases placental-fetal iron delivery through increased FPN-1 expression. IRP-1 may have a more dominant role than IRP-2 activity in regulating ferritin expression.

Effects of iron regulatory protein regulation on iron homeostasis during hypoxia

Iron regulatory proteins (IRP1 and IRP2) are RNA-binding proteins that affect the translation and stabilization of specific mRNAs by binding to stem-loop structures known as iron responsive elements (IREs). IREs are found in the 5'-untranslated region (UTR) of ferritin (Ft) and mitochondrial aconitase (m-Aco) mRNAs, and in the 3'-UTR of transferrin receptor (TfR) and divalent metal transporter-1 (DMT1) mRNAs. Our previous studies show that besides iron, IRPs are regulated by hypoxia. Here we describe the consequences of IRP regulation and show that iron homeostasis is regulated in 2 phases during hypoxia: an early phase where IRP1 RNA-binding activity decreases and iron uptake and Ft synthesis increase, and a late phase where IRP2 RNA-binding activity increases and iron uptake and Ft synthesis decrease. The increase in iron uptake is independent of DMT1 and TfR, suggesting an unknown transporter. Unlike Ft, m-Aco is not regulated during hypoxia. During the late phase of hypoxia, IRP2 RNA-binding activity increases, becoming the dominant regulator responsible for decreasing Ft synthesis. During reoxygenation (ReO2), Ft protein increases concomitant with a decrease in IRP2 RNA-binding activity. The data suggest that the differential regulation of IRPs during hypoxia may be important for cellular adaptation to low oxygen tension.

Oxygen and iron regulation of iron regulatory protein 2

Iron regulatory protein 2 (IRP2) is a central regulator of cellular iron homeostasis due to its regulation of specific mRNAs encoding proteins of iron uptake and storage. Iron regulates IRP2 by mediating its rapid proteasomal degradation, where hypoxia and the hypoxia mimetics CoCl2 and desferrioxamine (DFO) stabilize it. Previous studies showed that iron-mediated degradation of IRP2 requires the presence of critical cysteines that reside within a 73-amino acid unique region. Here we show that a mutant IRP2 protein lacking this 73-amino acid region degraded at a rate similar to that of wild-type IRP2. In addition, DFO and hypoxia blocked the degradation of both the wild-type and mutant IRP2 proteins. Recently, members of the 2-oxoglutarate (2-OG)-dependent dioxygenase family have been shown to hydroxylate hypoxia-inducible factor-1 alpha (HIF-1 alpha), a modification required for its ubiquitination and proteasomal degradation. Since 2-OG-dependent dioxygenases require iron and oxygen, in addition to 2-OG, for substrate hydroxylation, we hypothesized that this activity may be involved in the regulation of IRP2 stability. To test this we used the 2-OG-dependent dioxygenase inhibitor dimethyloxalylglycine (DMOG) and showed that it blocked iron-mediated IRP2 degradation. In addition, hypoxia, DFO and DMOG blocked IRP2 ubiquitination. These data indicate that the region of IRP2 that is involved in IRP2 iron-mediated degradation lies outside of the 73-amino acid unique region and suggest a model whereby 2-OG-dependent dioxygenase activity may be involved in the oxygen and iron regulation of IRP2 protein stability.

Iron deficiency alters iron regulatory protein and iron transport protein expression in the perinatal rat brain

Iron plays an important role in numerous vital enzyme systems in the perinatal brain. The membrane proteins that mediate iron transport [transferrin receptor (TfR) and divalent metal transporter 1 (DMT-1)] and the iron regulatory proteins (IRP-1 and IRP-2) that stabilize their mRNAs undergo regional developmental changes in the iron-sufficient rat brain between postnatal day (P) 5 and 15. Perinatal iron deficiency (ID) affects developing brain regions nonhomogeneously, suggesting potential differences in regional iron transporter and regulatory protein expression. The objective of the study was to determine the effect of perinatal ID on regional expression of IRP-1, IRP-2, TfR, and DMT-1 in the developing rat brain. Gestationally iron-deficient Sprague Dawley rat pups were compared with iron-sufficient control pups at P10. Serial 12-mu coronal sections of fixed frozen brain from pups on P10 were assessed by light microscopy for IRP-1, IRP-2, DMT-1, and TfR localization. ID did not change the percentage of cells with positive staining for the four proteins in the choroid epithelium, ependyma, vascular endothelium, or neurons of the striatum. ID increased the percentage of neurons expressing the four proteins in the hippocampus and the cerebral cortex. Increased numbers of TfR- and DMT-1-positive cells were always associated with increased IRP-positive cells. The P10 rat responds to perinatal ID by selectively increasing the number of neurons expressing IRP-regulated transporters in brain regions that are rapidly developing, without any change at transport surfaces or in regions that are quiescent. Brain iron distribution during ID seems to be locally rather than globally regulated.

Cytosolic aconitase and ferritin are regulated by iron in Caenorhabditis elegans

Iron regulatory protein-1 (IRP-1) is a cytosolic RNA-binding protein that is a regulator of iron homeostasis in mammalian cells. IRP-1 binds to RNA structures, known as iron-responsive elements, located in the untranslated regions of specific mRNAs, and it regulates the translation or stability of these mRNAs. Iron regulates IRP-1 activity by converting it from an RNA-binding apoprotein into a [4Fe-4S] cluster protein exhibiting aconitase activity. IRP-1 is widely found in prokaryotes and eukaryotes. Here, we report the biochemical characterization and regulation of an IRP-1 homolog in Caenorhabditis elegans (GEI-22/ACO-1). GEI-22/ACO-1 is expressed in the cytosol of cells of the hypodermis and the intestine. Like mammalian IRP-1/aconitases, GEI-22/ACO-1 exhibits aconitase activity and is post-translationally regulated by iron. Although GEI-22/ACO-1 shares striking resemblance to mammalian IRP-1, it fails to bind RNA. This is consistent with the lack of iron-responsive elements in the C. elegans ferritin genes, ftn-1 and ftn-2. While mammalian ferritin H and L mRNAs are translationally regulated by iron, the amounts of C. elegans ftn-1 and ftn-2 mRNAs are increased by iron and decreased by iron chelation. Excess iron did not significantly alter worm development but did shorten their life span. These studies indicated that iron homeostasis in C. elegans shares some similarities with those of vertebrates.

Developmental changes in the expression of iron regulatory proteins and iron transport proteins in the perinatal rat brain

The perinatal brain requires a tightly regulated iron transport system. Iron regulatory proteins (IRPs) 1 and 2 are cytosolic proteins that regulate the stability of mRNA for the two major cellular iron transporters, transferrin receptor (TfR) and divalent metal transporter-1 (DMT-1). We studied the localization of IRPs, their change in expression during perinatal development, and their relationship to TfR and DMT-1 in rat brain between postnatal days (PND) 5 and 15. Twelve-micron frozen coronal sections of fixed brain tissue were obtained from iron-sufficient Sprague-Dawley rat pups on PND 5, 10, and 15, and were visualized at 20 to 1,000x light microscopy for diaminobenzidine activity after incubation with specific primary IRP-1, IRP-2, DMT-1, and TfR antibodies and a universal biotinylated secondary and tertiary antibody system. IRP and transport protein expression increased in parallel over time. IRP1, IRP2, and DMT-1 were partially expressed in the choroid plexus epithelial cells at PND 5 and 10, and fully expressed at PND 15. The cerebral blood vessels and ependymal cells strongly expressed IRP1, IRP2, and DMT-1 as early as PND 5. Substantive TfR staining was not seen in the choroid plexus or ependyma until PND 15. Glial and neuronal expression of IRP1, IRP2, DMT-1, and TfR in cortex, hippocampal subareas and striatum increased over time, but showed variability in cell number and intensity of expression based on brain region, cell type, and age. These developmental changes in IRP and transporter expression suggest potentially different time periods of brain structure vulnerability to iron deficiency or iron overload.

Immunolocalization of iron regulatory protein expression in the murine central nervous system

We examined the expression of the iron regulatory proteins 1 and 2 (IRP1 and IRP2) in the brains of adult (4-6 months) CBA/J mice. Anti-IRP1 immunoreactivity was localized to cell bodies, including putative neurons and oligodendrocytes. In contrast, anti-IRP2 staining was prevalent throughout the neuropil of regions of the brain consistent with the central autonomic network (CAN) and mossy fibers emanating from hippocampal dentate granule cells. Essentially no staining for IRP2 was observed in the cerebellum in contrast to strong IRP1 immunoreactivity in Purkinje cells. Notably, cells within one vestibular nucleus exhibited staining by both IRP1 and IRP2. Our results suggest distinct roles for IRP1 and IRP2 in the regulation of iron homeostasis in the mammalian nervous system where IRP1 may provide a maintenance function in contrast to IRP2 that could participate in modulating proper CAN functions, including cardiopulmonary, gustatory as well as fine motor control.

Regulation of mammalian iron homeostasis

Iron homeostasis is regulated with respect to uptake, storage and utilization. Newer work is presented that defines proteins responsible for iron transport, sequestration and sensing, and that addresses their regulation at the cellular and organismal levels by ambient iron concentrations, demand for erythropoiesis, body iron burden, and redox stimuli.

Iron metabolism

The understanding of iron metabolism at the molecular level has been enormously expanded in recent years by new findings about the functioning of transferrin, the transferrin receptor and ferritin. Other recent developments include the discovery of the hemochromatosis gene HFE, identification of previously unknown proteins involved in iron transport, divalent metal transporter 1 and stimulator of Fe transport, and expanded insights into the regulation and expression of proteins involved in iron metabolism. Interactions among principal participants in iron transport have been uncovered, although the complexity of such interactions is still incompletely understood. Correlated efforts involving techniques and concepts of crystallography, spectroscopy and molecular biology applied to cellular processes have been, and should continue to be, particularly revealing.

Hypoxia post-translationally activates iron-regulatory protein 2

Iron-regulatory proteins 1 and 2 (IRP1 and IRP2) are RNA-binding proteins that post-transcriptionally regulate the expression of mRNAs that code for proteins involved in the maintenance of iron and energy homeostasis. Here we show that hypoxia differentially regulates the RNA binding activities of IRP1 and IRP2 in human 293 and in mouse Hepa-1 cells. In contrast to IRP1, where hypoxic exposure decreases IRP1 RNA binding activity, hypoxia increases IRP2 RNA binding activity. The hypoxic increase in IRP2 RNA binding activity results from increased IRP2 protein levels. Cobalt, which mimics hypoxia by activation of hypoxia-inducible factor 1 (HIF-1), also increases IRP2 protein levels; however, cobalt-induced IRP2 lacks RNA binding activity. Addition of a reductant to cobalt-treated extracts restored IRP2 RNA binding activity. Hypoxic activation of IRP2 is not because of an increase in transcriptional activation by HIF-1, because IRP2 accumulates in Hepa-1 cells lacking a functional HIF-1beta subunit, nor is it because of an increase in IRP2 mRNA stability. Rather, our data indicate that hypoxia increases IRP2 levels by a post-translational mechanism involving protein stability. Differential regulation of IRP1 and IRP2 during hypoxia may regulate specific IRP target mRNAs whose expression is required for hypoxic adaptation. Furthermore, these data imply mechanistic parallels between the hypoxia-induced post-transcriptional regulation of IRP2 and HIF-1alpha.

Increased placental iron regulatory protein-1 expression in diabetic pregnancies complicated by fetal iron deficiency

Placental transferrin receptor (TfR) protein expression is increased in diabetic pregnancies that are complicated by low fetal iron stores, suggesting regulation of placental iron transport by fetoplacental iron status. In cell culture, iron homeostasis is regulated by coordinate stabilization of TfR mRNA and translation inactivation of ferritin mRNA by iron regulatory proteins (IRP-1 and -2) which bind to iron-responsive elements (IREs) on the respective mRNAs. Concentrations of IRP-1, IRP-2 and TfR mRNA were measured in 10 placentae obtained from diabetic and non-diabetic human pregnancies with a wide range of fetoplacental iron status. IRP-1 activity was present in human placenta and correlated closely with TfR mRNA concentration (r=0.82; P=0.007). IRP-2 activity and protein were not detected. In a second experiment, placentae were collected from 12 diabetic pregnancies, six with low fetal cord serum ferritin and placental non-heme iron concentrations, and six with normal iron status. IRP-1 activity and TfR Bmax for diferric transferrin were greater in the iron-deficient group (P<0.05). IRP-1 activity correlated inversely with cord serum ferritin (r=0.75; P<0.01) and placental non-heme iron (r=0.61; P=0.05) concentration. Placental IRP-1 activity is directly related to TfR mRNA concentration and is more highly expressed in iron-deficient placentae. The study provides direct in vivo evidence for IRP regulation of TfR expression in the human placenta.

Loops and bulge/loops in iron-responsive element isoforms influence iron regulatory protein binding. Fine-tuning of mRNA regulation?

A family of noncoding mRNA sequences, iron-responsive elements (IREs), coordinately regulate several mRNAs through binding a family of mRNA-specific proteins, iron regulatory proteins (IRPs). IREs are hairpins with a constant terminal loop and base-paired stems interrupted by an internal loop/bulge (in ferritin mRNA) or a C-bulge (in m-aconitase, erythroid aminolevulinate synthase, and transferrin receptor mRNAs). IRP2 binding requires the conserved C-G base pair in the terminal loop, whereas IRP1 binding occurs with the C-G or engineered U-A. Here we show the contribution of the IRE internal loop/bulge to IRP2 binding by comparing natural and engineered IRE variants. Conversion of the internal loop/bulge in the ferritin-IRE to a C-bulge, by deletion of U, decreased IRP2 binding by >95%, whereas IRP1 binding changed only 13%. Moreover, IRP2 binding to natural IREs with the C-bulge was similar to the DeltaU6 ferritin-IRE: >90% lower than the ferritin-IRE. The results predict mRNA-specific variation in IRE-dependent regulation in vivo and may relate to previously observed differences in iron-induced ferritin and m-aconitase synthesis in liver and cultured cells. Variations in IRE structure and cellular IRP1/IRP2 ratios can provide a range of finely tuned, mRNA-specific responses to the same (iron) signal.

Regulation of iron regulatory protein 1 during hypoxia and hypoxia/reoxygenation

Given the important relationship between O2 and iron (Fenton chemistry) a study was undertaken to characterize the effects of hypoxia, as well as subsequent reoxygenation, on the iron-regulatory proteins 1 and 2 (IRP1 and IRP2) in a rat hepatoma cell line. IRP1 and IRP2 are cytosolic RNA-binding proteins that bind RNA stem-loops located in the 5'- or 3'-untranslated regions of specific mRNAs encoding proteins that are involved in iron homeostasis. In cells exposed to hypoxia, IRP1 RNA binding was decreased approximately 2. 8-fold after a 6-h exposure to 3% O2. Hypoxic inactivation of IRP1 was abolished when cells were pretreated with the iron chelator desferrioxamine, indicating a role for iron in inactivation. IRP1 inactivation was reversible since re-exposure of hypoxically-treated cells to 21% O2 increased RNA binding activity approximately 7-fold after 21 h with an increase in activity seen as early as 1-h post-reoxygenation. IRP1 protein levels were unaffected during hypoxia as well as during reoxygenation. Whereas the protein synthesis inhibitor cycloheximide did not block IRP1 inactivation during hypoxia, it completely blocked IRP1 reactivation during subsequent reoxygenation. Reactivation of IRP1 during reoxygenation was also partially blocked by the phosphatase inhibitor okadaic acid. Finally, reactivated IRP1 was found to be resistant to inactivation by exogenous iron known to down-regulate its activity during normoxia. These data demonstrate that IRP1 RNA binding activity is post-translationally regulated during hypoxia and hypoxia/reoxygenation. Regulation of IRP1 by changing oxygen tension may provide a novel mechanism for post-transcriptionally regulating gene expression under these stresses.

Expression and biochemical characterization of iron regulatory proteins 1 and 2 in Saccharomyces cerevisiae

Iron-regulatory proteins (IRPs) 1 and 2 are cytosolic RNA-binding proteins that bind to specific stem-loop structures, termed iron-responsive elements (IREs) that are located in the untranslated regions of specific mRNAs encoding proteins involved in iron metabolism. The binding of IRPs to IREs regulates either translation or stabilization of mRNA. Although IRP1 and IRP2 are similar proteins in that they are ubiquitously expressed and are negatively regulated by iron, they are regulated by iron by different mechanisms. IRP1, the well-characterized IRP in cells, is a dual-function protein exhibiting either aconitase activity when cellular iron is abundant or RNA-binding activity when cellular iron is scarce. In contrast, IRP2 lacks detectable aconitase activity and functions exclusively as an RNA-binding protein. To study and compare the biochemical characteristics of IRP1 and IRP2, we expressed wild-type and mutant rat IRP1 and IRP2 in the yeast Saccharomyces cerevisiae. IRP1 and IRP2 expressed in yeast bind the IRE RNA with high affinity, resulting in the inhibition of translation of an IRE-reporter mRNA. Mutant IRP2s lacking a 73 amino acid domain unique to IRP2 and a mutant IRP1 containing an insertion of this domain bound RNA, but lacked detectable aconitase activity, suggesting that the presence of this domain prevents aconitase activity. Like IRP1, the RNA-binding activity of IRP2 was sensitive to inactivation by N-ethylmaleimide (NEM) or 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), indicating IRP2 contains a cysteine(s) that is (are) necessary for RNA binding. However, unlike IRP1, where reconstitution of the 4Fe-4S cluster resulted in a loss in RNA-binding activity, the RNA-binding activity of IRP2 was unaffected using the same iron treatment. These data suggested that IRP2 does not contain a 4Fe-4S cluster similar to the cluster in IRP1, indicating that they sense iron by different mechanisms.

Differential regulation of IRP1 and IRP2 by nitric oxide in rat hepatoma cells

Iron-regulatory proteins (IRP1 and IRP2) are RNA-binding proteins that bind to stem-loop structures known as iron-responsive elements (IREs). IREs are located in the 5'- or 3'-untranslated regions (UTRs) of specific mRNAs that encode proteins involved in iron homeostasis. The binding of IRPs to 5' IREs represses translation of the mRNA, whereas the binding of IRPs to 3' IREs stabilizes the mRNA. IRP1 and IRP2 binding activities are regulated by intracellular iron levels. In addition, nitric oxide (NO.) increases the affinity of IRP1 for IREs. The role of NO. in the regulation of IRP1 and IRP2 in rat hepatoma cells was investigated by using the NO.-generating compound S-nitroso-N-acetylpenicillamine (SNAP), or by stimulating cells with multiple cytokines and lipopolysaccharide (LPS) to induce NO. production. Mitochondrial and IRP1 aconitase activities were decreased in cells producing NO(.). NO. increased IRE binding activity of IRP1, but had no effect on IRE binding activity of IRP2. The increase in IRE binding activity of IRP1 was coincident with the translational repression of ferritin synthesis. Transferrin receptor (TfR) mRNA levels were increased in cells treated with NO.-generating compounds, but not in cytokine- and LPS-treated cells. Our data indicate that IRP1 and IRP2 are differentially regulated by NO. in rat hepatoma cells, suggesting a role for IRP1 in the regulation of iron homeostasis in vivo during hepatic inflammation.

Iron regulates the intracellular degradation of iron regulatory protein 2 by the proteasome

Iron regulatory proteins (IRP1 and IRP2) are RNA-binding proteins that bind to specific structures, termed iron-responsive elements (IREs), that are located in the 5'- or 3'-untranslated regions of mRNAs that encode proteins involved in iron homeostasis. IRP1 and IRP2 RNA binding activities are regulated by iron; IRP1 and IRP2 bind IREs with high affinity in iron-depleted cells and with low affinity in iron-repleted cells. The decrease in IRP1 RNA binding activity occurs by a switch between apoprotein and 4Fe-4S forms, without changes in IRP1 levels, whereas the decrease in IRP2 RNA binding activity reflects a reduction in IRP2 levels. To determine the mechanism by which iron decreases IRP2 levels, we studied IRP2 regulation by iron in rat hepatoma and human HeLa cells. The iron-dependent decrease in IRP2 levels was not due to a decrease in the amount of IRP2 mRNA or to a decrease in the rate of IRP2 synthesis. Pulse-chase experiments demonstrated that iron resulted in a 3-fold increase in the degradation rate of IRP2. IRP2 degradation depends on protein synthesis, but not transcription, suggesting a requirement for a labile protein. IRP2 degradation is not prevented by lysosomal inhibitors or calpain II inhibitors, but is prevented by inhibitors that block proteasome function. These data suggest the involvement of the proteasome in iron-mediated IRP2 proteolysis.

Characterization and expression of iron regulatory protein 2 (IRP2). Presence of multiple IRP2 transcripts regulated by intracellular iron levels

Iron regulatory proteins (IRP1 and IRP2) are RNA-binding proteins that bind to stem-loop structures, termed iron-responsive elements (IREs), present in either the 5'- or 3'-untranslated regions of specific mRNAs. The binding of IRPs to 5'-IREs inhibits translation of mRNA, whereas the binding of IRPs to 3'-IREs stabilizes mRNA. To study the structure and regulation of IRP2, we isolated cDNAs for rat and human IRP2. The derived amino acid sequence of rat IPR2 is 93% identical with that of human IRP2 and is present in lower eukaryotes, indicating that IRP2 is highly conserved. IRP1 and IRP2 share 61% overall amino acid identity. IRP2 is ubiquitously expressed in rat tissues, the highest amounts present in skeletal muscle and heart. IRP2 is encoded by multiple mRNAs of 6.4, 4.0, and 3.7 kilobases. The 3'-untranslated region of rat IRP2 contains multiple polyadenylation signals, two of which could account for the 4.0-kb and 3.7-kb mRNAs. The 3.7-kb mRNA is increased in iron-depleted cells and occurs with a reciprocal decrease in the 6.4-kb transcript. These data suggest that the 3.7-kb mRNA is produced by alternative poly(A) site utilization in iron-depleted cells.

Iron regulates cytoplasmic levels of a novel iron-responsive element-binding protein without aconitase activity

Iron-responsive element-binding proteins (IRE-BPs) are cytosolic proteins that bind to a conserved RNA stem-loop, termed the iron-responsive element (IRE), that is located in the 5'- or 3'-untranslated regions of mRNAs involved in iron metabolism. Binding of the IRE-BP to 5'-IREs represses translation, whereas binding to 3'-IREs stabilizes the mRNA. The previously identified IRE-BP (BP1) contains a 4Fe-4S cluster and has sequence homology to mitochondrial aconitase. The 4Fe-4S cluster is important for iron-dependent regulation: BP1 containing iron has low affinity for the IRE and contains aconitase activity, whereas BP1 lacking iron has high affinity for the IRE, but lacks aconitase activity. A second IRE-BP (BP2) has been identified in rat tissues and cells and exhibits many of the hallmarks of an IRE-BP, including binding to the IRE and functioning as a translational repressor of IRE-containing RNAs. BP1 and BP2 RNA binding activities are decreased in extracts from cells treated with iron, indicating that BP1 and BP2 are negatively regulated by iron. Although BP1 and BP2 share similar characteristics, they differ in two significant ways. Unlike BP1 levels, which do not change when RNA binding activity decreases in response to iron, BP2 decreases to undetectable levels in extracts from cells treated with iron; and unlike BP1, BP2 does not have aconitase activity. These data indicate that BP1 and BP2 are distinct proteins that have similar specificity for IRE binding and that function similarly in translation, but are regulated by iron via different mechanisms.

The iron-responsive element binding protein. Purification, cloning, and regulation in rat liver

The iron-responsive element binding protein (IRE-BP) is a cytosolic protein that binds a highly conserved sequence in the untranslated regions of mRNAs involved in iron metabolism including ferritin, transferrin receptor, and erythroid 5-aminolevulinate acid synthase. This conserved sequence is termed the iron-responsive element and is necessary for the post-transcriptional regulation of these mRNAs by iron. The rat liver IRE-BP was purified to homogeneity by chromatographic methods and partial amino acid sequence was obtained. A cDNA was isolated from a rat liver cDNA library and sequenced. The amino acid sequence deduced from the cDNA sequence corresponds to a protein of 889 amino acids with a predicted molecular weight of 97.946. The NH2-terminal sequence obtained by Edman degradation matched the deduced amino acid sequence obtained from the cDNA, confirming the translational start site. Rat liver IRE-BP shares 95% identity with human IRE-BP and 98% identity with mouse IRE-BP indicating that the IRE-BPs have remained highly conserved during evolution. The 5'-untranslated region is at least 236 nucleotides and contains interesting structural features including two direct repeats, an inverted repeat, and three small open reading frames. The rat IRE-BP mRNA is approximately 3600 nucleotides and is expressed in a variety of rat tissues including liver, spleen, and gut. Over the course of 16 h following an intraperitoneal injection of iron in rats. IRE-BP RNA binding activity decreases to 50% of control levels. The decrease in IRE-BP RNA binding activity in extracts from iron-treated rats is reversible by pretreatment of the extracts with reducing agents. The steady-state levels of IRE-BP mRNA remain constant during iron treatment. These data suggest that the decrease in IRE-BP RNA binding activity by iron in rat liver is due to post-translational changes in the RNA binding affinity of the IRE-BP and not due a decrease in the transcription of the IRE-BP gene or to the destabilization of the IRE-BP mRNA.

Structural requirements of iron-responsive elements for binding of the protein involved in both transferrin receptor and ferritin mRNA post-transcriptional regulation

The synthesis of both transferrin receptor (TfR) and ferritin is regulated post-transcriptionally by iron. This is mediated by iron responsive elements (IREs) in the 5'- and 3'-untranslated regions, respectively, of TfR and ferritin mRNAs. Although these IREs have different sequences, they both form a characteristic stem-loop. We used competition assays and partial peptide mapping of UV-crosslinked ferritin and TfR IRE-protein complexes to show that the cytosolic protein binding to the ferritin 5'-IRE, the iron-responsive element binding protein (IRE-BP), also binds to TfR 3'-IREs. To identify the structural requirements necessary for RNA-protein binding, ferritin IRE RNAs were synthesized which contained altered secondary structures and base substitutions. Affinities of these RNAs for IRE-BP were assayed in RNA-protein binding gels. Substitutions disrupting base-pairing of the stem prevented IRE-BP binding. Substitutions which restored base-pairing also restored IRE-BP binding. We conclude that the IRE-BP binds to both ferritin and TfR IREs and recognizes a particular IRE conformation.

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.

Characterization and evolution of the expressed rat ferritin light subunit gene and its pseudogene family. Conservation of sequences within noncoding regions of ferritin genes

The iron storage protein ferritin consists of two types of subunits of different molecular weight, heavy (H) and light (L). The rat genome contains approximately 20 copies of the ferritin L-subunit gene, of which we have sequenced seven. One is an expressed ferritin gene containing three introns located between the alpha-helical domains of the L-subunit protein. The remaining six have the characteristics of processed pseudogenes. Sequence divergence suggest that these pseudogenes arose approximately 3-12 X 10(6) years ago, well within the 30 X 10(6) years of divergence of rat and mouse. By using intron probes derived from the expressed ferritin L-gene, a homologous second copy has been identified in some Fischer rats. Comparison of the 5'-untranslated region of the rat L-gene with the published sequences of this region of the human L (Santoro, C., Marone, M., Ferrone, M., Costanzo, F., Colombo, M., Minganti, C., Cortese, R., and Silengo, L. (1986) Nucleic Acids Res. 14, 2863-2876) and H (Costanzo, F., Colombo, M., Staempfli, S., Santoro, C., Marone, M., Frank, R., Delius, H., and Cortese, R. (1986) Nucleic Acids Res. 14, 721-735) genes and of a bullfrog cDNA (Didsbury, J. R., Theil, E. C., Kaufman, R. E., and Dickey, L. F. (1986) J. Biol. Chem. 261, 949-955) show a strongly conserved 28-base pair sequence, suggesting a translational regulatory function. The 5' flanking region of the rat L-gene contains sequences homologous to those in the flanking areas of the human L- and H-genes. The implications of these conserved sequences for control of ferritin expression are discussed.

Structure of human ferritin light subunit messenger RNA: comparison with heavy subunit message and functional implications

Ferritin has a protein shell of 5 X 10(6) Da consisting of 24 subunits of two types, a heavier (H) chain of 21,000 Da and a lighter (L) chain of 19,000 Da. A cDNA clone of the messenger for the L subunit has been isolated from a human monocyte-like leukemia cell line. The clone contains an open reading frame of 522 nucleotides coding for an amino acid sequence matching 97% of the published sequence of human liver ferritin L subunit determined by sequenator, but it corresponds to only 55% of the reported amino acid sequence of a human liver H-subunit clone. Nevertheless, computer analysis of the subunit conformations predicted from the open reading frames of the L and H clones shows that most of the amino acid differences are conservative and would allow both subunits to form the five alpha-helices and beta-turns established by x-ray crystallography for horse spleen ferritin subunits. This suggests that L and H subunits are structurally interchangeable in forming an apoferritin shell. The 5' untranslated region of our human ferritin L clone has considerable homology with that of the rat liver ferritin L clone in the region immediately upstream from the initiator codon, notably showing an identical sequence of 10 nucleotides at the same position in both subunit clones that may participate in regulating the known activation of ferritin mRNA after iron administration. Extensive homology, including several blocks of nucleotides, was identified between the 3' untranslated regions of the human and rat L clones. The common structural features of the H and L subunits lead us to conclude that they have diverged from a single ancestral gene.

Conservation in rat liver of light and heavy subunit sequences of mammalian ferritin. Presence of unique octopeptide in the light subunit

Ferritin, an iron-storage protein found in all life forms examined, is composed of varying proportions of two subunits of different molecular weight, heavy (H) and light (L). Using cDNA clones, we have determined the nucleotide sequence corresponding to the mRNA of the L-subunit of rat liver ferritin. The coding region of 546 nucleotides (182 amino acids) is flanked by 5'- and 3' -untranslated regions of approximately 130 and 150 nucleotides, respectively. The rat liver L-subunit amino acid sequence derived from the reading frame of the cDNA showed 88% and 82% homology, respectively, with the amino acid sequences of horse spleen ferritin (Heusterspreute, M., and Crichton, R. R. (1981) FEBS Lett. 129, 322-327), and human spleen ferritin (Wustefeld, C., and Crichton, R. R. (1982) FEBS Lett. 150, 43-48), thus demonstrating evolutionary conservation of the L-subunit sequence. However, a major difference between the rat and the horse and human sequences is the insertion of an octopeptide near the COOH-terminus of the rat protein resulting in a slightly longer peptide chain in this species. The reading frame and parts of the derived amino acid sequence including the octopeptide sequence were confirmed by direct amino acid sequencing of cyanogen bromide peptides from rat liver ferritin. Minor fragments of rat liver ferritin, presumably derived from the H-subunit, were also isolated after cyanogen bromide treatment. On sequencing, these H-peptides showed limited homology with regions of the L-sequence but extensive homology with published H-sequences from human liver and spleen. The H-subunit sequence did not contain the octopeptide found as part of the L-subunit sequence.

Isolation of cDNA clones for the light subunit of rat liver ferritin: evidence that the light subunit is encoded by a multigene family

The shell of the iron-storage protein ferritin consists of two types of subunit (heavy, Mr = 21,000; light, Mr = 19,000). To study the structure and expression of the ferritin subunit genes, recombinant plasmids containing ferritin cDNA have been isolated. A cDNA library was constructed in the vector pBR322 from rat liver polysomal mRNA and screened by using I125-labeled antibody to rat liver ferritin. Six positive clones were identified and were shown to contain cDNA inserts ranging in length from 800 to 950 base pairs. When these cDNA clones were used for hybrid selection of rat liver mRNA and the selected mRNAs were translated in vitro, the products from each clone migrated on denaturing gels in a position similar to that of the light subunit of ferritin. No evidence of translation of the heavy subunit was obtained, indicating that the two subunits are encoded by separate mRNAs. RNA blot analysis gave a length of 1,100 nucleotides for the light-subunit mRNA. One of the cDNA inserts was fractionated into four fragments by using the restriction enzyme Sau3A. When the fragments were hybridized with Southern blots of rat spleen DNA, each fragment yielded similar patterns of hybridization to that obtained with the intact cDNA. Therefore, all regions of the cDNA sequence contain homologous sequences to similar genomic restriction fragments. This is consistent with the existence of a family of genes that encode the light subunit of rat ferritin.