Selective inhibition of the citrate-to-isocitrate reaction of cytosolic aconitase by phosphomimetic mutation of serine-711

The active role of vitamin C in mammalian iron metabolism: much more than just enhanced iron absorption!

Ascorbate is a cofactor in numerous metabolic reactions. Humans cannot synthesize ascorbate owing to inactivation of the gene encoding the enzyme l-gulono-γ-lactone oxidase, which is essential for ascorbate synthesis. Accumulating evidence strongly suggests that in addition to the known ability of dietary ascorbate to enhance nonheme iron absorption in the gut, ascorbate within mammalian systems can regulate cellular iron uptake and metabolism. Ascorbate modulates iron metabolism by stimulating ferritin synthesis, inhibiting lysosomal ferritin degradation, and decreasing cellular iron efflux. Furthermore, ascorbate cycling across the plasma membrane is responsible for ascorbate-stimulated iron uptake from low-molecular-weight iron-citrate complexes, which are prominent in the plasma of individuals with iron-overload disorders. Importantly, this iron-uptake pathway is of particular relevance to astrocyte brain iron metabolism and tissue iron loading in disorders such as hereditary hemochromatosis and β-thalassemia. Recent evidence also indicates that ascorbate is a novel modulator of the classical transferrin-iron uptake pathway, which provides almost all iron for cellular demands and erythropoiesis under physiological conditions. Ascorbate acts to stimulate transferrin-dependent iron uptake by an intracellular reductive mechanism, strongly suggesting that it may act to stimulate iron mobilization from the endosome. The ability of ascorbate to regulate transferrin iron uptake could help explain the metabolic defect that contributes to ascorbate-deficiency-induced anemia.

Aconitase post-translational modification as a key in linkage between Krebs cycle, iron homeostasis, redox signaling, and metabolism of reactive oxygen species

Aconitase, an enzyme possessing an iron-sulfur cluster that is sensitive to oxidation, is involved in the regulation of cellular metabolism. There are two isoenzymes of aconitase (Aco)--mitochondrial (mAco) and cytosolic (cAco) ones. The primary role of mAdco is believed to be to control cellular ATP production via regulation of intermediate flux in the Krebs cycle. The cytosolic Aco in its reduced form operates as an enzyme, whereas in the oxidized form it is involved in the control of iron homeostasis as iron regulatory protein 1 (IRP1). Reactive oxygen species (ROS) play a central role in regulation of Aco functions. Catalytic Aco activity is regulated by reversible oxidation of [4Fe-4S]²⁺ cluster and cysteine residues, so redox-dependent posttranslational modifications (PTMs) have gained increasing consideration as regards possible regulatory effects. These include modifications of cysteine residues by oxidation, nitrosylation and thiolation, as well as Tyr nitration and oxidation of Lys residues to carbonyls. Redox-independent PTMs such as phosphorylation and transamination also have been described. In the presence of a sustained ROS flux, redox-dependent PTMs may lead to enzyme damage and cell stress by impaired energy and iron metabolism. Aconitase has been identified as a protein that undergoes oxidative modification and inactivation in aging and certain oxidative stress-related disorders. Here we describe possible mechanisms of involvement of the two aconitase isoforms, cAco and mAco, in the control of cell metabolism and iron homeostasis, balancing the regulatory, and damaging effects of ROS.

Structure-function study of deinococcal serine/threonine protein kinase implicates its kinase activity and DNA repair protein phosphorylation roles in radioresistance of Deinococcus radiodurans

The DR2518 (RqkA) a eukaryotic type serine/threonine protein kinase in Deinococcus radiodurans was characterized for its role in bacterial response to oxidative stress and DNA damage. The K42A, S162A, T169A and S171A mutation in RqkA differentially affected its kinase activity and functional complementation for γ radiation resistance in Δdr2518 mutant. For example, K42A mutant was completely inactive and showed no complementation while S171A, T169A and T169A/S171A mutants were less active and complemented proportionally to different levels as compared to wild type. Amongst, different DNA binding proteins that purified RqkA could phosphorylate, PprA a DNA repair protein, phosphorylation had improved its affinity to DNA by 4 fold and could enhance its supportive role in intermolecular ligation by T4 DNA ligase. RqkA phosphorylates PprA at threonine 72 (T72), serine 112 (S112) and threonine 144 (T144) in vitro with the majority of it goes to T72 site. Unlike wild type PprA and single mutants of T72, S112 and T144 residues, the T72AS112A double and T72AS112AT144A triple mutant derivatives of PprA did not phosphorylate in vivo and also failed to complement PprA loss in D. radiodurans. Deletion of rqkA in pprA::cat background enhanced radiosensitivity of pprA mutant, which became nearly similar to ΔrqkA resistance to γ radiation. These results suggested that K42 of RqkA is essential for catalytic functions and the kinase activity of RqkA as well as phosphorylation of PprA have roles in γ radiation resistance of D. radiodurans.

Nucleotide-specific recognition of iron-responsive elements by iron regulatory protein 1

IRP1 [iron regulatory protein (IRP) 1] is a bifunctional protein with mutually exclusive end-states. In one mode of operation, IRP1 binds iron-responsive element (IRE) stem-loops in messenger RNAs encoding proteins of iron metabolism to control their rate of translation. In its other mode, IRP1 serves as cytoplasmic aconitase to correlate iron availability with the energy and oxidative stress status of the cell. IRP1/IRE binding occurs through two separate interfaces, which together contribute about two-dozen hydrogen bonds. Five amino acids make base-specific contacts and are expected to contribute significantly to binding affinity and specificity of this protein:RNA interaction. In this mutagenesis study, each of the five base-specific amino acids was changed to alter binding at each site. Analysis of IRE binding affinity and translational repression activity of the resulting IRP1 mutants showed that four of the five contact points contribute uniquely to the overall binding affinity of the IRP1:IRE interaction, while one site was found to be unimportant. The stronger-than-expected effect on binding affinity of mutations at Lys379 and Ser681, residues that make contact with the conserved nucleotides G16 and C8, respectively, identified them as particularly critical for providing specificity and stability to IRP1:IRE complex formation. We also show that even though the base-specific RNA-binding residues are not part of the aconitase active site, their substitutions can affect the aconitase activity of holo-IRP1, positively or negatively.

Mammalian iron homeostasis in health and disease: uptake, storage, transport, and molecular mechanisms of action

Iron is a crucial factor for life. However, it also has the potential to cause the formation of noxious free radicals. These double-edged sword characteristics demand a tight regulation of cellular iron metabolism. In this review, we discuss the various pathways of cellular iron uptake, cellular iron storage, and transport. Recent advances in understanding the reduction and uptake of non-transferrin-bound iron are discussed. We also discuss the recent progress in the understanding of transcriptional and translational regulation by iron. Furthermore, we discuss recent advances in the understanding of the regulation of cellular and systemic iron homeostasis and several key diseases resulting from iron deficiency and overload. We also discuss the knockout mice available for studying iron metabolism and the related human conditions.

Phosphorylation of podocalyxin (Ser415) Prevents RhoA and ezrin activation and disrupts its interaction with the actin cytoskeleton

Podocalyxin (PC) is a polysialylated, anti-adhesin that is essential for maintaining foot process architecture and the integrity of the glomerular filtration barrier. We showed previously that PC is firmly attached to the actin cytoskeleton through ezrin, that in puromycin aminonucleoside (PAN)-mediated nephrosis the PC-ezrin-actin complex is disrupted, and that PC is uncoupled from actin. However, the precise mechanisms involved remained unknown. Here we show that detachment of PC from actin is regulated by phosphorylation of PC. PC is hyperphosphorylated at serines in PAN- and protamine sulfate (PS)-treated rat glomeruli. We determined that PC is a substrate of PKC and that the site of phosphorylation is Ser415, located within the juxtamembrane, ezrin-binding domain of the cytoplasmic tail of PC. Mutation of Ser415 to the phosphomimetic residues Glu (S415E) or Asp (S415D) interfered with direct binding of the PC cytoplasmic tail to ezrin in vitro. Moreover, stable expression of a phosphomimetic (S415E) PC mutant but not the WT or the phosphorylation-deficient (S415A) PC mutant, disrupted PC-ezrin-actin interaction, failed to activate RhoA, and the cytoskeletal linker, ezrin, remained inactive. Our data indicate that phosphorylation of PC at Ser415 prevents attachment of PC and ezrin to actin and highlights the strategic position of Ser415 and direct binding of PC to ezrin in regulating podocyte foot process architecture.

Regulation of cellular iron metabolism

Iron is an essential but potentially hazardous biometal. Mammalian cells require sufficient amounts of iron to satisfy metabolic needs or to accomplish specialized functions. Iron is delivered to tissues by circulating transferrin, a transporter that captures iron released into the plasma mainly from intestinal enterocytes or reticuloendothelial macrophages. The binding of iron-laden transferrin to the cell-surface transferrin receptor 1 results in endocytosis and uptake of the metal cargo. Internalized iron is transported to mitochondria for the synthesis of haem or iron–sulfur clusters, which are integral parts of several metalloproteins, and excess iron is stored and detoxified in cytosolic ferritin. Iron metabolism is controlled at different levels and by diverse mechanisms. The present review summarizes basic concepts of iron transport, use and storage and focuses on the IRE (iron-responsive element)/IRP (iron-regulatory protein) system, a well known post-transcriptional regulatory circuit that not only maintains iron homoeostasis in various cell types, but also contributes to systemic iron balance.

Interaction of iron regulatory protein-1 (IRP-1) with ATP/ADP maintains a non-IRE-binding state

In its aconitase-inactive form, IRP-1 (iron regulatory protein-1)/cytosolic aconitase binds to the IRE (iron-responsive element) of several mRNAs to effect post-transcriptional regulation. We have shown previously that IRP-1 has ATPase activity and that binding of ATP suppresses the IRP-1/IRE interaction. In the present study, we characterize the binding activity further. Binding is observed with both [alpha-32P]ATP and [alpha-32P]ADP, but not with [gamma-32P]ATP. Recombinant IRP-1 binds approximately two molecules of ATP, and positive co-operativity is observed with a Hill coefficient of 1.67+/-0.36 (EC50=44 microM) commencing at 1 microM ATP. Similar characteristics are observed with both apoprotein and the aconitase form. On binding, ATP is hydrolysed to ADP, and similar binding parameters and co-operativity are seen with ADP, suggesting that ATP hydrolysis is not rate limiting in product formation. The non-hydrolysable analogue AMP-PNP (adenosine 5'-[beta,gamma-imido]triphosphate) does not induce co-operativity. Upon incubation of IRP-1 with increasing concentrations of ATP or ADP, the protein migrates more slowly on agarose gel electrophoresis, and there is a shift in the CD spectrum. In this new state, adenosine nucleotide binding is competed for by other nucleotides (CTP, GTP and AMP-PNP), although ATP and ADP, but not the other nucleotides, partially stabilize the protein against spontaneous loss of aconitase activity when incubated at 37 degrees C. A mutant IRP-1(C437S) lacking aconitase activity shows only one ATP-binding site and lacks co-operativity. It has increased IRE-binding capacity and lower ATPase activity (Km=75+/-17 nmol/min per mg of protein) compared with the wild-type protein (Km=147+/-48 nmol/min per mg of protein). Under normal cellular conditions, it is predicted that ATP/ADP will maintain IRP-1 in a non-IRE-binding state.

Selective translational control of the Alzheimer amyloid precursor protein transcript by iron regulatory protein-1

Iron influx increases the translation of the Alzheimer amyloid precursor protein (APP) via an iron-responsive element (IRE) RNA stem loop in its 5'-untranslated region. Equal modulated interaction of the iron regulatory proteins (IRP1 and IRP2) with canonical IREs controls iron-dependent translation of the ferritin subunits. However, our immunoprecipitation RT-PCR and RNA binding experiments demonstrated that IRP1, but not IRP2, selectively bound the APP IRE in human neural cells. This selective IRP1 interaction pattern was evident in human brain and blood tissue from normal and Alzheimer disease patients. We computer-predicted an optimal novel RNA stem loop structure for the human, rhesus monkey, and mouse APP IREs with reference to the canonical ferritin IREs but also the IREs encoded by erythroid heme biosynthetic aminolevulinate synthase and Hif-2α mRNAs, which preferentially bind IRP1. Selective 2'-hydroxyl acylation analyzed by primer extension analysis was consistent with a 13-base single-stranded terminal loop and a conserved GC-rich stem. Biotinylated RNA probes deleted of the conserved CAGA motif in the terminal loop did not bind to IRP1 relative to wild type probes and could no longer base pair to form a predicted AGA triloop. An AGU pseudo-triloop is key for IRP1 binding to the canonical ferritin IREs. RNA probes encoding the APP IRE stem loop exhibited the same high affinity binding to rhIRP1 as occurs for the H-ferritin IRE (35 pm). Intracellular iron chelation increased binding of IRP1 to the APP IRE, decreasing intracellular APP expression in SH-SY5Y cells. Functionally, shRNA knockdown of IRP1 caused increased expression of neural APP consistent with IRP1-APP IRE-driven translation.

Iron control of erythroid development by a novel aconitase-associated regulatory pathway

Human red cell differentiation requires the action of erythropoietin on committed progenitor cells. In iron deficiency, committed erythroid progenitors lose responsiveness to erythropoietin, resulting in hypoplastic anemia. To address the basis for iron regulation of erythropoiesis, we established primary hematopoietic cultures with transferrin saturation levels that restricted erythropoiesis but permitted granulopoiesis and megakaryopoiesis. Experiments in this system identified as a critical regulatory element the aconitases, multifunctional iron-sulfur cluster proteins that metabolize citrate to isocitrate. Iron restriction suppressed mitochondrial and cytosolic aconitase activity in erythroid but not granulocytic or megakaryocytic progenitors. An active site aconitase inhibitor, fluorocitrate, blocked erythroid differentiation in a manner similar to iron deprivation. Exogenous isocitrate abrogated the erythroid iron restriction response in vitro and reversed anemia progression in iron-deprived mice. The mechanism for aconitase regulation of erythropoiesis most probably involves both production of metabolic intermediates and modulation of erythropoietin signaling. One relevant signaling pathway appeared to involve protein kinase Calpha/beta, or possibly protein kinase Cdelta, whose activities were regulated by iron, isocitrate, and erythropoietin.

IRP1 Ser-711 is a phosphorylation site, critical for regulation of RNA-binding and aconitase activities

In iron-starved cells, IRP1 (iron regulatory protein 1) binds to mRNA iron-responsive elements and controls their translation or stability. In response to increased iron levels, RNA-binding is inhibited on assembly of a cubane [4Fe-4S] cluster, which renders IRP1 to a cytosolic aconitase. Phosphorylation at conserved serine residues may also regulate the activities of IRP1. We demonstrate that Ser-711 is a phosphorylation site in HEK-293 cells (human embryonic kidney 293 cells) treated with PMA, and we study the effects of the S711E (Ser-711-->Glu) mutation on IRP1 functions. A highly purified preparation of recombinant IRP1(S711E) displays negligible IRE-binding and aconitase activities. It appears that the first step in the aconitase reaction (conversion of citrate into the intermediate cis-aconitate) is more severely affected, as recombinant IRP1(S711E) retains approx. 45% of its capacity to catalyse the conversion of cis-aconitate into the end-product isocitrate. When expressed in mammalian cells, IRP1(S711E) completely fails to bind to RNA and to generate isocitrate from citrate. We demonstrate that the apparent inactivation of IRP1(S711E) is not related to mutation-associated protein misfolding or to alterations in its stability. Sequence analysis of IRP1 from all species currently deposited in protein databases shows that Ser-711 and flanking sequences are highly conserved in the evolutionary scale. Our results suggest that Ser-711 is a critical residue for the control of IRP1 activities.

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.

Protein kinase C regulates expression and function of inhibitory killer cell Ig-like receptors in NK cells

The inhibitory killer cell Ig-like receptors (KIR) negatively regulate NK cell cytotoxicity by activating the Src homology 2 domain-containing protein tyrosine phosphatases 1 and 2 following ligation with MHC class I molecules expressed on normal cells. This requires tyrosine phosphorylation of KIR on ITIMs in the cytoplasmic domain. Surprisingly, we have found that KIR3DL1 is strongly and constitutively phosphorylated on serine and weakly on threonine residues. In this study, we have mapped constitutive phosphorylation sites for casein kinases, protein kinase C, and an unidentified kinase on the KIR cytoplasmic domain. Three of these phosphorylation sites are highly conserved in human inhibitory KIR. Functional studies of the wild-type receptor and serine/threonine mutants indicated that phosphorylation of Ser(394) by protein kinase C slightly suppresses KIR3DL1 inhibitory function, and reduces receptor internalization and turnover. Our results provide evidence that serine/threonine phosphorylation is an important regulatory mechanism of KIR function.

Activation of aconitase in mouse fast-twitch skeletal muscle during contraction-mediated oxidative stress

Aconitase is a mitochondrial enzyme that converts citrate to isocitrate in the tricarboxylic acid cycle and is inactivated by reactive oxygen species (ROS). We investigated the effect of exercise/contraction, which is associated with elevated ROS production, on aconitase activity in skeletal muscle. Humans cycled at 75% of maximal workload, followed by six 60-s bouts at 125% of maximum workload. Biopsies were taken from the thigh muscle at rest and after the submaximal and supramaximal workloads. Isolated mouse extensor digitorum longus (EDL; fast twitch) and soleus (slow twitch) muscles were stimulated to perform repeated contractions for 10 min. Muscles were analyzed for enzyme activities and glutathione status. Exercise did not affect aconitase activity in human muscle despite increased oxidative stress, as judged by elevated levels of oxidized glutathione. Similarly, repeated contractions did not alter aconitase activity in soleus muscle. In contrast, repeated contractions significantly increased aconitase activity in EDL muscle by ∼50%, despite increased ROS production. This increase was not associated with a change in the amount of immunoreactive aconitase (Western blot) but was markedly inhibited by cyclosporin A, an inhibitor of the protein phosphatase calcineurin. Immunoprecipitation experiments demonstrated that aconitase was phosphorylated on serine residues. Aconitase in cell-free extracts was inactivated by the addition of the ROS hydrogen peroxide. In conclusion, the results suggest that aconitase activity can be regulated by at least two mechanisms: oxidation/reduction and phosphorylation/dephosphorylation. During contraction, a ROS-mediated inactivation of aconitase can be overcome, possibly by dephosphorylation of the enzyme. The dual-control system may be important in maintaining aerobic ATP production during muscle contraction.

The role of iron regulatory proteins in mammalian iron homeostasis and disease

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) are mammalian proteins that register cytosolic iron concentrations and post-transcriptionally regulate expression of iron metabolism genes to optimize cellular iron availability. In iron-deficient cells, IRPs bind to iron-responsive elements (IREs) found in the mRNAs of ferritin, the transferrin receptor and other iron metabolism transcripts, thereby enhancing iron uptake and decreasing iron sequestration. IRP1 registers cytosolic iron status mainly through an iron-sulfur switch mechanism, alternating between an active cytosolic aconitase form with an iron-sulfur cluster ligated to its active site and an apoprotein form that binds IREs. Although IRP2 is homologous to IRP1, IRP2 activity is regulated primarily by iron-dependent degradation through the ubiquitin-proteasomal system in iron-replete cells. Targeted deletions of IRP1 and IRP2 in animals have demonstrated that IRP2 is the chief physiologic iron sensor. The physiological role of the IRP-IRE system is illustrated by (i) hereditary hyperferritinemia cataract syndrome, a human disease in which ferritin L-chain IRE mutations interfere with IRP binding and appropriate translational repression, and (ii) a syndrome of progressive neurodegenerative disease and anemia that develops in adult mice lacking IRP2. The early death of mouse embryos that lack both IRP1 and IRP2 suggests a central role for IRP-mediated regulation in cellular viability.

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.

The multiple activities of CtBP/BARS proteins: the Golgi view

The C terminal-binding protein (CtBP) family functions in the nucleus as co-repressors of transcription and has a crucial role in differentiation, apoptosis, oncogenesis and development. Recently, the products of the CtBP1 gene have been implicated in important cytoplasmic functions, including membrane fission in intracellular trafficking, the partitioning of the Golgi complex during mitosis and the organization of ribbon synapses. This has led to a redefinition of the CtBPs as multifunctional proteins. Shuttling of CtBPs between the nucleus and the cytoplasm can be finely regulated by post-translational modifications. In addition, the structural homology with the dehydrogenase family of proteins and the ability of CtBPs to bind NAD(+) and acyl-CoAs have offered clues to the molecular mechanisms that enable these proteins to have different functions. Here, we discuss the cytoplasmic roles of the CtBPs and the possible mechanisms that enable them to switch between cell compartments and multiple functions.

Kinetics of regulatory serine variants of tyrosine hydroxylase with cyclic AMP-dependent protein kinase and extracellular signal-regulated protein kinase 2

Rat tyrosine hydroxylase is phosphorylated at four serine residues, at positions 8, 19, 31, and 40 in its amino terminal regulatory domain by multiple protein kinases. Cyclic AMP-dependent protein kinase phosphorylates S40, which results in alleviation of inhibition by dopamine. Extracellular signal-regulated protein kinase 2 phosphorylates S8 and S31. Site-directed serine-to-glutamate mutations were introduced into tyrosine hydroxylase to mimic prior phosphorylation of the regulatory serines; these proteins were used as substrates for cAMP-dependent kinase and extracellular signal-regulated kinase 2. The activity of cAMP-dependent kinase was unaffected by the substitution of serines 8, 19 or 31 with glutamate and the activity of extracellular signal-regulated kinase 2 was unaffected by substitution of serines 19 or 40 with glutamate. Cyclic AMP-dependent kinase was less active in phosphorylating S40 if dopamine was bound to tyrosine hydroxylase, but extracellular signal-regulated kinase 2 phosphorylation at S31 was unaffected by the presence of dopamine.

The mitochondrial ATP-binding cassette transporter Abcb7 is essential in mice and participates in cytosolic iron–sulfur cluster biogenesis

Proteins with iron-sulfur (Fe-S) clusters participate in multiple metabolic pathways throughout the cell. The mitochondrial ABC half-transporter Abcb7, which is mutated in X-linked sideroblastic anemia with ataxia in humans, is a functional ortholog of yeast Atm1p and is predicted to export a mitochondrially derived metabolite required for cytosolic Fe-S cluster assembly. Using an inducible Cre/loxP system to delete exons 9 and 10 of the Abcb7 gene, we examined the phenotype of mice deficient in Abcb7. We found that Abcb7 was essential in extra-embryonic tissues early in gestation and that the mutant allele exhibits an X-linked parent-of-origin lethality effect. Furthermore, using X-chromosome inactivation assays and tissue-specific deletions, Abcb7 was found to be essential for the development and function of numerous other cell types and tissues. A notable exception to this was liver, where loss of Abcb7 impaired cytosolic Fe-S cluster assembly but was not lethal. In this situation, control of iron regulatory protein 1, a key cytosolic modulator of iron metabolism, which is responsive to the availability of cytosolic Fe-S clusters, was impaired and contributed to the dysregulation of hepatocyte iron metabolism. Altogether, these studies demonstrate the essential nature of Abcb7 in mammals and further substantiate a central role for mitochondria in the biogenesis of cytosolic Fe-S proteins.

Iron-responsive degradation of iron-regulatory protein 1 does not require the Fe-S cluster

The generally accepted role of iron-regulatory protein 1 (IRP1) in orchestrating the fate of iron-regulated mRNAs depends on the interconversion of its cytosolic aconitase and RNA-binding forms through assembly/disassembly of its Fe-S cluster, without altering protein abundance. Here, we show that IRP1 protein abundance can be iron-regulated. Modulation of IRP1 abundance by iron did not require assembly of the Fe-S cluster, since a mutant with all cluster-ligating cysteines mutated to serine underwent iron-induced protein degradation. Phosphorylation of IRP1 at S138 favored the RNA-binding form and promoted iron-dependent degradation. However, phosphorylation at S138 was not required for degradation. Further, degradation of an S138 phosphomimetic mutant was not blocked by mutation of cluster-ligating cysteines. These findings were confirmed in mouse models with genetic defects in cytosolic Fe-S cluster assembly/disassembly. IRP1 RNA-binding activity was primarily regulated by IRP1 degradation in these animals. Our results reveal a mechanism for regulating IRP1 action relevant to the control of iron homeostasis during cell proliferation, inflammation, and in response to diseases altering cytosolic Fe-S cluster assembly or disassembly.

Citrate enhances in vitro metastatic behaviours of PC-3M human prostate cancer cells: Status of endogenous citrate and dependence on aconitase and fatty acid synthase

Prostate is a unique organ that produces and releases large amounts of citrate. This is reduced significantly in cancer and it is possible that citrate is (re)taken up and used as a metabolite to enhance cellular activity. The main purpose of this study was to determine how cytosolic citrate might affect in vitro metastatic cell behaviours (lateral motility, endocytosis and adhesion). Normal (PNT2-C2) and metastatic (PC-3M) human prostate cancer cells were used in a comparative approach. As regards intermediary metabolic enzymes, aconitase and fatty acid synthase, already implicated in prostate cancer, were evaluated. The level of intracellular citrate was significantly higher in PNT2-C2 cells under both control conditions and following preincubation in extracellular citrate. Supply of exogenous citrate enhanced endocytosis, lateral motility, decreased cell adhesion of PC-3M cells but failed to produce any effect on normal cells. Real-time PCR measurements showed that the mRNA levels of mitochondrial and cytosolic aconitases and fatty acid synthase were significantly higher in PC-3M cells. Correspondingly, aconitase activity was also higher in PC-3M cells. Using cerulenin (an inhibitor of fatty acid synthase), oxalomalate and fluorocitrate (inhibiting aconitases), we investigated the dependence of citrate-induced down-regulation of cellular adhesion on aconitase and fatty acid synthase activities. It was concluded: (1) that strongly metastatic PC-3M cells stored less/utilised more cytosolic citrate than the normal PNT2-C2 cells and (2) that cancer cells could metabolise cytoplasmic citrate via aconitase and fatty acid synthase to enhance their metastatic behaviour.

Iron regulatory protein 1 as a sensor of reactive oxygen species

Iron regulatory proteins (IRP1 and 2) function as translational regulators that coordinate the cellular iron metabolism of eukaryotes by binding to the mRNA of target genes such as the transferrin receptor or ferritin. In addition to IRP2, IRP1 serves as sensor of reactive oxygen species (ROS). As iron and oxygen are essential but potentially toxic constituents of most organisms, ROS-mediated modulation of IRP1 activity may be an important regulatory element in dissecting iron homeostasis and oxidative stress. The responses of IRP1 towards reactive oxygen species are compartment-specific and rather complex: H2O2 activates IRP1 via a signaling cascade that leads to upregulation of the transferrin receptor and cellular iron accumulation. Contrary, superoxide inactivates IRP1 by a direct chemical attack being limited to the intracellular compartment. In particular, activation of IRP1 by H2O2 has established a new regulatory link between inflammation and iron metabolism with new clinical implications. This mechanism seems to contribute to the anemia of chronic disease and inflammation-mediated iron accumulation in tissues. In addition, the cytotoxic side effects of redox-cycling anticancer drugs such as doxorubicin may involve H2O2-mediated IRP1 activation. These molecular insights open up new therapeutic strategies for the clinical management of chronic inflammation and drug-mediated cardiotoxicity.