!" #$ % ! & ' ( " #$%& ! 3 ! ! "# # ' & $ % &! # &! )* * ' +,-,. #/01 "# # # ( ( ( 2 Abstract Cellular iron homeostasis is achieved by the coordinate and reciprocal post-transcriptional regulation of transferrin receptor 1 (TfR1) and ferritin expression, which are key molecules for iron acquisition and storage, respectively. The mechanism involves interactions between cytoplasmic iron regulatory proteins (IRP1 and IRP2) and iron responsive elements (IREs) located within non-coding sequences of TfR1 and ferritin mRNAs. In irondeficient cells, IRE/IRP interactions stabilize TfR1 mRNA and inhibit the translation of the mRNAs encoding H- and L-ferritin. The RNA-binding activities of IRP1 and IRP2 are induced in iron deficiency. In iron-replete cells, IRP1 assembles a cubane [4Fe-4S] iron sulfur cluster (ISC), which Correspondence/Reprint request: Dr. K. Pantopoulos, 3755 Côte-Ste-Catherine Road, Montreal, Quebec H3T 1E2, Canada E-mail: kostas.pantopoulos@mcgill.ca #% Carine Fillebeen et al. prevents the binding to IREs and converts the protein to a (cytosolic) c-aconitase. By contrast, IRP2 undergoes ubiquitination and degradation by the proteasome. IRPs respond not only to cellular iron levels but also to additional stimuli, such as oxygen levels, oxidative stress and nitric oxide. The targeted disruption of both IRP1 and IRP2 is associated with early embryonic lethality, underlying the physiological significance of the IRE/IRP regulatory system. IRP1(–/–) mice express increased ferritin levels in the kidney and brown fat without showing any pathological abnormalities. On the other hand, IRP2(–/–) mice misregulate iron metabolism in the duodenum, the CNS and the erythron and exhibit microcytic anemia. A gene targeting approach has also yielded IRP2(–/–) mice that progressively accumulate iron in the CNS and develop a severe neurodegenerative disease. Further development and characterization of animal models is expected to shed more light on the specific functions of each IRP in vivo. Abbreviations that are not listed in the general abbreviation list: aa, amino acids; ALAS2, aminolevulinate-δ-synthase 2; c-aconitase, cytosolic aconitase; CIA, cytosolic iron-sulfur cluster assembly; DMOG, dimethyl-oxalyl-glycine; DOX, doxorubicin; DOXol, doxorubicinol; HHCS, hereditary hyperferritinemia-cataract syndrome; HIF, hypoxia inducible factor; HOIL, heme-oxidized IRP2 ubiquitin ligase-1; ISCU, ironsulfur cluster scaffold homolog; LIP, labile iron pool; m-aconitase, mitochondrial aconitase; PMA, phorbol 12-myristate 13-acetate; pVHL, von Hippel-Lindau protein; SDH, succinate dehydrogenase; SNP, sodium nitroprusside; UTR untranslated region. 1. Introduction Iron is a crucial constituent for all eukaryotic cells and organisms. It is required as a cofactor in essential biochemical activities, such as oxygen transport, energy metabolism and DNA synthesis [1]. On the other hand, when present in excess iron becomes toxic, mainly due to its ability to catalyze the formation of free radicals by Fenton chemistry [2]. These reactive species oxidize membrane lipids, proteins and nucleic acids, ultimately leading to cell damage and disease. Thus, cells and organisms have evolved elaborate mechanisms to satisfy their metabolic needs for iron and at the same time to minimize the risk of iron-induced toxicity. In mammals, maintenance of cellular iron homeostasis requires a coordinate control in the expression of transferrin receptor 1 (TfR1) and ferritin (a multi-subunit protein composed of H- and L-chains), which mediate iron uptake and storage, respectively [3]. While the expression of TfR1 and ferritin genes is regulated by multiple transcriptional and post-transcriptional mechanisms [4,5], the coordinate control in their expression in response to iron levels is accomplished by the IRE/IRP regulatory system. It is well documented that this post-transcriptional regulatory circuit, which involves interactions between mRNA iron responsive elements (IREs) and iron regulatory proteins (IRPs), plays a major role in cellular iron homeostasis with regard to cellular iron acquisition and storage via TfR1 and ferritin, respectively [6]. Furthermore, there is an increasing body of evidence that the IRE/IRP system may also have an important function in the context of iron utilization and efflux in specialized cells, by controlling the expression of the erythroid-specific isoform of aminolevulinate synthase (ALAS2) and ferroportin [6]. The former catalyzes the first step for heme biosynthesis in erythroid cells [7] and the latter is a transmembrane transporter of Fe(II) IRE/IRP regulatory system # ions, the only molecule with the capacity to release iron from the basolateral membrane of enterocytes and from macrophages [8]. For more details on ferroportin see Chapter 3: »The Roles of ferroportin and hfe in iron export and iron homeostasis«. 2. The IRE/IRP regulatory system Historically, the foundation for the discovery of the IRE/IRP regulatory system was laid in the 60’s and 70’s by studies showing that iron-dependent ferritin expression is regulated at the level of translation and not transcription [9,10]. A molecular mechanism was first offered in the late 80’s with the identification of an IRE as a cis-acting element within the 5 -untranslated region (UTR) of ferritin mRNA [11] and of an IRE-binding activity as a cytoplasmic trans-acting factor [12]. This activity accounts for a specific inhibition of H- and L-ferritin mRNA translation in iron-starved cells. Soon thereafter, multiple IREs were described in the 3 -UTR of TfR1 mRNA [13,14]. Here, IRE-binding stabilizes TfR1 mRNA against nucleolytic degradation [15,16]. These seminal discoveries were followed by the cloning of the cDNAs corresponding to two IREbinding proteins [17], which are now known as iron regulatory proteins IRP1 and IRP2 (IRP1 can also be found in older literature as IRE-BP, IRF or IRP). The concept of the IRE/IRP regulatory system is summarized in Figure 1: in irondeficient cells IRP1 and IRP2 are activated for high affinity (Kd ≈ 10–12 M) IRE-binding; the IRE/IRP interactions stabilize TfR1 mRNA and inhibit translation of (H- and L-) ferritin mRNAs (reviewed in [18-21]). As a result, when iron is scarce, cells activate TfR1 expression to acquire more transferrin-bound iron from the circulation, while they shut-off the machinery for iron storage. By contrast, in iron-replete cells, IRP1 and IRP2 are inactivated for IRE-binding, which permits TfR1 mRNA degradation and ferritin mRNA translation. Consequently, when iron availability exceeds cellular needs, the downregulation of TfR1 ultimately prevents the uptake of transferrin-bound iron, while Figure 1. The IRE/IRP regulatory system. Iron deficiency promotes IRE/IRP interactions. These result in translational inhibition of the mRNAs encoding H- and L-ferritin and in stabilization of TfR1 mRNA. Opposite responses occur when iron levels increase. Carine Fillebeen et al. # excess intracellular iron is stored and detoxified within newly synthesized ferritin. Notably, the mRNAs encoding TfR2 and mitochondrial ferritin do not possess any IRE; the former is a TfR1 homolog primarily expressed in hepatocytes [22] (see Chapter 7), and the latter is a ferritin isoform mostly expressed in testis and erythroid cells and targeted to mitochondria by a leader sequence [23]. 3. Iron responsive elements The canonical IRE motif consists of 25- to 30-nucleotides forming a stable stem-loop structure with a double-stranded 5-base-pair long helix and the conserved 5 CAGUGH-3 (H means A, C or U) loop [24] (Figure 2). The underlined C and G form a A B Figure 2. IRE motifs. (A) The consensus IRE motif. It consists of a hexanucleotide loop with the sequence 5 -CAGUGH-3 (H could be A, C, or U) and a stem, interrupted by a bulge with an unpaired C residue. Base pairing between C1 and G5 is functionally important. The ferritin-bulge consists of an asymmetric tetranucleotide (right). (B) The segment of the 3 -UTR of TfR1 mRNA containing 5 IRE motifs. The indicated IREs B, C and D are necessary for regulation. IRE/IRP regulatory system # base pair of functional significance [25,26]. The moderately stable stem (∆G ≈ –7 kcal/mol) is interrupted in the middle by a small bulge, which may consist of an asymmetric tetranucleotide (in ferritin IRE), or is restricted to a single unpaired C residue (in TfR1 IREs). This may provide some selectivity in the binding of IRP1 or IRP2, resulting in the fine-tuning of mRNA regulation [27,28]. The mRNAs encoding H- and L-ferritin contain a single IRE motif at their 5 -UTR (Figure 2A), located close (ca. 60 nucleotides) to the cap structure (an inverted 7methylguanosine). The cap-proximal segment of eukaryotic mRNAs is critical for translation initiation because it provides a binding site for the 43S pre-initiation complex, composed of the small ribosomal subunit and translation initiation factors. Once it associates with the mRNA, the 43S complex scans the entire 5 -UTR to identify the initiation codon [29]. The subsequent joining of the large ribosomal subunit signals the onset of polypeptide synthesis. The IRE/IRP interaction sterically inhibits the stable association of the small ribosomal subunit with the initiation factor eIF4F and the assembly of 43S pre-initiation complex [30,31]. Moving the IRE to more distal positions does not affect IRP binding, but nevertheless, attenuates the capacity of the IRE/IRP interaction to inhibit translation [32,33] by ribosomal pausing and scanning arrest [34]. TfR1 mRNA accommodates five IRE motifs in its 3 -UTR [13] (Figure 2B), even though an alternate structure with overall similar thermodynamic stability has also been proposed [14]. Deletion analysis revealed that a minimal 252-nt segment containing only three out of the five predicted IREs (B, C and D) was sufficient for mRNA stabilization upon IRE-binding [16]. The formation of these three IRE structures has been documented by enzymatic and chemical probing [35,36]. The binding of IRP1 protected conserved residues in all three IREs and induced a conformational change [36]. It should also be noted that the 3 -UTR of TfR1 mRNA contains AU-rich elements (ARE) adjacent to the IREs, which are known instability determinants [37], but their functional significance has not been addressed. The mechanism for TfR1 mRNA degradation remains elusive. It appears that the initial event is an endonucleolytic cleavage, which does not require deadenylation [38]. The transcription inhibitor actinomycin D and the protein synthesis inhibitor cycloheximide prevent iron-mediated destabilization of TfR1 mRNA, suggesting the involvement of labile trans-acting factors in the pathway [39-41]. 4. Additional IRE-containing mRNAs The IRE sequences of ferritin and TfR1 mRNAs define two distinct groups, which are highly conserved among different species [24,42]. Phylogenetically conserved IRE motifs have also been identified in additional mRNAs, primarily encoding proteins of iron or energy metabolism, such as ALAS2 [43,44], (mitochondrial) m-aconitase (an iron-sulfur cluster (ISC) enzyme of the citric acid cycle) [45,46] and the iron transporters ferroportin [47,48] (also called IREG1, MTP1 or SLC40A1) and DMT1 [49] (also called DCT1, Nramp2 or SLC11A2). Interestingly, in Drosophila melanogaster (and possibly also in other insects) m-aconitase mRNA contains no IRE; instead, an IRE motif is present in the mRNA encoding the iron-protein (Ip) subunit of mitochondrial succinate dehydrogenase (SDH), another metalloenzyme of the citric acid cycle [45,50]. It has been proposed that ferritin IRE represents the ancestral version of the IRE motif, which was subsequently adopted during evolution by other genes in higher organisms [42]. #) Carine Fillebeen et al. The mRNAs encoding ALAS2 and m-aconitase (or insect SDH) contain a single translation-type IRE motif in their 5 -UTR. This operates pretty much as in ferritin mRNA but offers a somehow smaller range of regulation [45,50-52], possibly due to variations in the IRE sequence and structure [27]. The IRP-mediated inhibition of ALAS2 mRNA translation is expected to limit the production and accumulation of toxic protoporphyrin IX in iron-deficient cells. The inhibition of m-aconitase or insect SDH synthesis by IRE/IRP interactions very likely serves to coordinate the expression of the iron-containing polypeptides with iron availability. Moreover, the regulation of enzymes of the citric acid cycle by the IRE/IRP system provides a link between iron and energy homeostasis. The IRE in ferroportin mRNA is also located at the 5 -UTR [47,48]. Even though ferroportin expression is upregulated in the duodenum of iron-deficient mice [47,48], an opposite mode of expression is apparent in the liver [48]. Consistently with the liver expression data, in vitro experiments with indicator constructs suggest that ferroportin IRE is a functional translational control element [48,53]. Future work is expected to delineate the functional significance of ferroportin IRE in the duodenum. Interestingly, a radiation-induced deletion of 58 bp within mouse ferroportin IRE resulted in increased ferroportin expression in the duodenum and liver during early postnatal development [54,55]. Surprisingly, this deletion associates with erythropoietin-dependent polycythemia in the heterozygous state, while homozygous animals develop hypochromic microcytic anemia [54]. The above findings highlight the complexity of ferroportin regulation by the IRE/IRP system. Furthermore, it is pertinent to note that the expression of ferroportin is also regulated by IRE-independent mechanisms, as the protein undergoes lysosomal degradation upon binding of the iron-regulatory peptide hepcidin [56]. DMT1 mRNA is expressed in four alternatively spliced variants, two of which contain a single IRE in their 3 -UTR [57]. This IRE appears to control the stability of the transcripts and probably accounts for the upregulation of DMT1 expression in the duodenum in response to iron-deficiency [49,58]. However, the underlying mechanism is far from being understood and the IRE most likely operates only in a tissue specific manner [59], in conjunction with additional regulatory elements in exon 1A [57]. Even though there are conceptual similarities in the IRE/IRP mediated regulation of TfR1 and DMT1 mRNA regulation, it is worth pointing out that a single IRE does not suffice to stabilize TfR1 mRNA [16]. This suggests that the mechanisms for IRE-mediated regulation of DMT1 and TfR1 mRNAs are different. TfR1 mRNA remains the only one containing multiple IREs within its 3 -UTR, which in turn constitute the only well documented example of stability-type IRE structure. Nevertheless, the relatively less well-characterized IRE within DMT1 mRNA poses, thus far, another unique case for localization of an apparently functional IRE at the 3 -UTR. A conserved IRE-like motif at the 3 -UTR of the mRNA encoding glycolate oxidase is not functional [60]. More recently, single IRE motifs were identified in the 3 -UTR of the mRNAs encoding myotonic dystrophy kinase-related Cdc42-binding kinase α (MRCKα) [61] and human cell division cycle 14A (Cdc14A) [61,62], and evidence was provided that these novel IREs function as stability elements. These data link iron metabolism with cytoskeletal remodeling and the cell cycle. In addition, recent experiments revealed the presence of a functional IRE within the 5 -UTR of the mRNA encoding hypoxia inducible factor 2α IRE/IRP regulatory system #$ (HIF-2α), which is thought to regulate iron availability and utilization for erythropoiesis under hypoxic conditions [63]. As a final point, atypical functional IRE motifs have been described within the 5 UTR of the mRNAs encoding the Alzheimer’s amyloid precursor protein and the 75 kDa subunit of mitochondrial complex I protein [64,65], but the biological significance of these findings remains to be further explored. Even though the IRE/IRP regulatory system is restricted to higher eukaryotes, IRE-like sequences have also been identified within some bacterial mRNAs, which are reasonable IRP substrates in vitro [66,67]. However, only limited evidence for an in vivo function of such sequences has been provided as yet [68] (see below). 5. Iron regulatory proteins Both iron regulatory proteins, IRP1 and IRP2, are members of the ISC isomerase family and display homology to m-aconitase [69-71]. This enzyme catalyzes the stereospecific conversion of citrate to iso-citrate in the citric acid cycle. Structural studies showed that m-aconitase is composed of three compact domains, which are linked to a fourth domain via a flexible hinge. A cleft formed between domains 1–3 and domain 4 accommodates a cubane [4Fe-4S] ISC [72]. Three iron atoms of the ISC are coordinated by cysteine residues in the polypeptide backbone, while the fourth iron (Fea) binds to the substrate and is indispensable for catalysis. IRP1 retains all active site residues and iron-binding cysteines of m-aconitase [6971]; thus, it could be easily predicted that it assembles an aconitase-type ISC and displays enzymatic activity. This has been confirmed experimentally [73-75] and moreover, the data further demonstrated that IRP1 is identical to the earlier described and partially characterized c-aconitase [76]. Interestingly, m- and c-aconitases share similar catalytic efficiencies [77,78]. However, in contrast to m-aconitase, IRP1 assembles its [4Fe-4S] ISC only in iron-replete cells. A decrease in cellular iron levels leads to removal of the [4Fe-4S] ISC and the resulting apo-IRP1 acquires IRE-binding activity [77,79]. Thus, IRP1 is a bifunctional protein, controlled by an unusual ISC switch (Figure 3). IRP2 shares 57% sequence identity and 75% similarity with IRP1 [71]. It also contains a unique cysteine- and proline-rich stretch of 73 amino acids towards its Nterminus, which is encoded by a single exon [80]. Despite its overall significant homology to IRP1 and m-aconitase, IRP2 does not retain crucial aconitase active site residues [71] and consequently, it only functions as an IRE-binding protein [81]. In ironreplete cells, IRP2 undergoes degradation by the proteasome pathway following ubiquitination [82] (Figure 4). The IRE-binding activity of IRP1 and IRP2 would not have been easily predicted from sequence analysis. First, the polypeptides contain no known RNA-binding motifs. In addition, there was no evidence that m-aconitases can bind to IREs [73]. Nevertheless, some bacterial aconitases were later found to possess RNA-binding activity, at least in vitro. For example, Bacillus subtilis aconitase binds to mammalian IRE or bacterial IRE-like sequences [66], while Escherichia coli aconitases AcnA and AcnB bind to structural elements within the 3 -UTR of their own mRNAs; the latter interaction may regulate the stability of these transcripts [68]. # Carine Fillebeen et al. Figure 3. IRP1 regulation by an ISC switch. In iron-loaded cells it assembles a cubane [4Fe4S] ISC that converts it to a c-aconitase. The ISC is lost in response to iron deficiency, NO or H2O2. Figure 4. IRP2 regulation. IRP2 is regulated at the level of protein stability: in iron loaded cells it undergoes proteasomal degradation by a pathway sensitive to dimethyl-oxalyl-glycine (DMOG), an inhibitor of 2-oxoglutarate-dependent oxygenases. 6. Structural features of IRP1 Initially, the available mammalian m-aconitase structure was used to model IRP1 [83,84]. The main outcome of this approach was that the substrate-binding cleft of the enzyme should open to accommodate the RNA that is larger than aconitase substrates (citrate and isocitrate). The switch between the closed and open conformations was proposed to involve movement around the linker joining domain 3 and domain 4 in the m-aconitase structure. This model was experimentally probed with human IRP1 by a combination of biophysical methods, such as circular dichroism, sedimentation velocity, light scattering and small angle neutron scattering experiments [85,86]. Measurements of the radii of gyration indicated that IRP1 has a relatively relaxed conformation in the absence of substrates that becomes more compact upon IRE-binding or after assembling the [4Fe-4S] ISC, with the building of secondary structure elements [85]. The aconitase form was found to display a smaller radius of gyration than the IRE-binding form in agreement with the smaller size of the [4Fe-4S] ISC compared to the IRE. In addition, a slightly larger proportion of secondary structure elements was present in the aconitase form than in the IRE-binding form, suggesting that the enzyme active site is more tightly IRE/IRP regulatory system # folded than the RNA interacting site. However, these studies in solution do not provide the resolution suited to describe conformational details at the atomic scale. For this purpose, the c-aconitase form of IRP1 obtained from heterologous production in bacteria was crystallized. Diffraction data from different crystals were recorded up to a nominal resolution of 1.85 Å and a structural model was built [87]. As expected from sequence alignments, the overall folding of IRP1 resembles that of maconitase. It is a compact protein with four globular domains, the last two in the sequence being joined by a surface linker (Figure 5). However, the structural alignment between human IRP1 and m-aconitase differs from published sequence alignments [71]. IRP1, a protein of 889 amino acids, is larger than m-aconitase (780 amino acids), but the fragments of IRP1 that do not superimpose with the structure of m-aconitase are short, one being 31 amino acids long and the others shorter than 23 residues. All of these fragments lie on the surface of the protein and they are involved in the building of secondary structure elements. As a result, the surface topology of IRP1 differs from that of m-aconitase. These differences are likely to reflect the different interactions the two proteins may be exposed to in their respective cellular compartments. Figure 5. Schematic drawing of human IRP1 structure. The aconitase structural model from Dupuy et al. [87] is drawn with each globular domain represented in a different color and the linker between domains 3 and 4 in pink. The [4Fe−4S] ISC is shown as magenta and orange spheres and a few arginine residues that participate in IRE-binding (and aconitase substrate binding for some of them) are represented as green dots. The aconitase substrate-binding cleft is at the bottom in this view. The presence of a zinc atom in some crystals of IRP1 was unexpected since no zinc was added at any stage during purification and crystallization. Zinc has been shown to strongly inhibit m-aconitase [88], but it is unlikely that the zinc atom evidenced in the IRP1 structure plays a similar role. Indeed, zinc has no significant impact on the aconitase activity of IRP1, but it strongly decreases the solubility of the IRE-binding form [89]. It is, thus, probably significant that the zinc atom in IRP1 structure is found at the surface of the protein in a region where two IRP1 molecules are close in the crystal: the cation may compete with solvent to interact with exposed hydrophilic residues (histidines and aspartate), but it may also play a role, yet to be defined, in a possible interaction of IRP1 with other cellular partners. #* Carine Fillebeen et al. At the resolution achieved with both substrate-bound m- and c-aconitases, the structures of the [4Fe-4S] ISCs in these proteins are not significantly different. Moreover, the environment of the ISC building the aconitase active site is very well conserved between the two proteins. In line with these observations, the kinetic parameters of the two enzymes are remarkably similar [78]. Previous UV cross-linking and protein footprinting experiments identified IRP1 sequences involved in the interaction with RNA. These include an area at the N-terminus around amino acids 120–130, another area encompassing amino acids 480–623 and R721 [90-93]. Some of the amino acids at the N-terminal RNA-binding site are also involved in citrate / isocitrate binding [94-96]. A series of arginine residues (Figure 5) are located in the major cleft that leads from the surface of the protein to the buried [4Fe-4S] ISC and that was proposed to allow access of the substrates to the active site of m-aconitase [97]. The crystal structure of IRP1 complexed with ferritin IRE was recently solved [98] and uncovered the details of the protein reorganization upon loss of its [4Fe-4S] ISC. This involves a rotation of domain 4 by 32°, but also an unpredicted extensive rearrangement of domain 3 by 52°, which creates a hydrophilic cavity and allows access to the IRE. The binding of the IRE requires two crucial segments at the interface of domains 2 and 3, the 430 region (residues 436-442) and the 530 loop (residues 534-544). T438 and N439 make direct contacts with the RNA. The terminal loop residues A15, G16 and U17 of the IRE (designated as A2, G3 and U4 in Fig. 2) interact with S371, K379 and R269, respectively. A second binding site is formed around the C8 bulge residue (the unpaired C in Fig. 2), which occupies a pocket within domain 4, sandwiched between R713 and R780. The RNA/protein complex is stabilized by additional bonds, ionic interactions and van der Waal’s contacts. The crystallographic structures of human IRP1 provide a framework to analyze the wealth of information accumulated over the last years on this exceptional protein that balances between two mechanistically unrelated activities by assembly and destruction of a [4Fe-4S] ISC. 7. Mechanisms for the regulation of IRP1 7.1. The [4Fe-4S] ISC of IRP1 as a regulatory site The [4Fe-4S] ISC is a hallmark of IRP1 structure and a major determinant for its enzymatic (aconitase) or gene regulatory function (as an IRE-binding protein). The modulation of IRP1 activities largely relies on the full assembly and destruction of this [4Fe-4S] ISC via mechanisms that remain incompletely characterized and have no precedent in any other protein. In vitro, the [4Fe-4S] ISC of IRP1 can be reconstituted by incubation with ferrous salts, sulfide and reducing agents [74,75,99-101]. This treatment efficiently converts the IRE-binding protein to aconitase. Conversely, a treatment of holo-IRP1 with ferricyanide destroys its [4Fe-4S] ISC and results in restoration of IREbinding and loss of aconitase activity [79]. Full IRE-binding activity can also be recovered by treatment of holo-IRP1 with 2% 2-mercaptoethanol [102], which displaces the [4Fe-4S] ISC from the polypeptide backbone. Not much is known on how the [4Fe-4S] ISC of IRP1 is assembled and disassembled in vivo. The mechanisms for the de novo ISC assembly and repair of mainly mitochondrial iron-sulfur proteins are beginning to being understood. Genetic experiments in bacteria and in yeast led to the identification of essential factors involved in this process, such as the cysteine desulfurase Nfs1 (sulfur donor), the iron-binding protein frataxin (possible IRE/IRP regulatory system ## iron donor) and the scaffold protein Isu1/Isu2 (or ISCU), which are part of an ISC assembly machinery [103-105]. The biogenesis of ISCs for extra-mitochondrial proteins, such as cytosolic IRP1, appears more complex. It has been proposed that the ISCs are synthesized in the mitochondrion (by the mitochondrial ISC assembly machinery) and exported to other cellular compartments (with the aid of additional auxiliary proteins) [106]. An alternative scenario postulates that ISCs can be assembled de novo in the cytosol by cytosolic homologs of members of the ISC assembly machinery [107]. The biogenesis of ISCs is presented in detail in Chapter 14. Deficiency in the mitochondrial proteins frataxin [108-110], glutaredoxin 5 [111], ISCU [112], Nfs1 [113-115] or Abcb7 [116] leads to activation of IRP1 for IRE-binding, due to an apparent defect in the assembly of its [4Fe-4S] ISC, and the repair of this ISC requires ATP production [117]. These results highlight the significance of the mitochondrion in the biosynthesis of cytosolic ISCs. Recent data have also implicated the cytosolic homologs of ISCU and cysteine desulfurase in the pathway for IRP1 [4Fe-4S] ISC assembly [112,118]. Additional candidates for having an active role in this pathway include the proteins Cfd1 [119], Nar1 [120], Nbp35 [121] and Cia1 [122], which are members of a cytosolic iron-sulfur cluster assembly (CIA) machinery [106]. Since the assembly of the [4Fe-4S] ISC of IRP1 in vivo is considered as a response to increased cellular iron availability, it will be important to clarify if any mitochondrial or cytosolic factors serves as iron sensors. Cellular iron deficiency triggers the disassembly of the [4Fe-4S] ISC of IRP1. There is no evidence that this is associated with any post-translational modification of the protein. In kinetic terms, this appears to be a relatively slow process, since the activation of IRP1 for IRE-binding in cells exposed to the iron chelator desferrioxamine is completed within 8–12 h [123]. Earlier experiments showed that the activation of IREbinding does not require de novo protein synthesis [124,125]. Along these lines, IRP1 has a long half-life (~24 h), which, under normal circumstances, is not affected by iron manipulations. Nevertheless, under conditions where the ISC assembly pathway is impaired, IRP1 is destabilized by iron [126,127]. The half-life of the [4Fe-4S] ISC may not necessarily be identical to that of the polypeptide. It is conceivable that in the intracellular milieu, both holo- and apo-IRP1 coexist in a dynamic equilibrium and the ISC is prone to ongoing assembly-disassembly. In iron-replete cells, the abundance of iron (and a suitable sulfur source) would favor assembly of the ISC. By contrast, in iron-starved cells, the reduction of intracellular iron below a critical threshold, would lead to a slow shift of the equilibrium towards apoIRP1. While the biogenesis of ISCs clearly involves a large set of protein co-factors [106,107], no proteins with ISC destabilizing activities have been identified thus far. Thus, it can be hypothesized that the disassembly of ISCs when iron or sulfide are limiting may be thermodynamically-driven and spontaneous. Recent data suggest that the switch from holo- to apo-IRP1 in response to iron deficiency is favored in culture conditions when cells typically grow in the presence of 21% oxygen, but appears inefficient at lower oxygen concentrations (3–6%), which possibly recapitulate physiological conditions in tissues [128]. 7.2. Regulation of IRP1 by iron-independent signals The [4Fe-4S] ISC of IRP1 is not only sensitive to iron levels, but also to other stimuli [129-133]. In vitro, most reactive oxygen and nitrogen species readily destroy the %% Carine Fillebeen et al. [4Fe-4S] ISC and diminish the aconitase activity of IRP1, but fail to convert it to an IREbinding protein [78,134,135]. Likewise, a treatment of B6 fibroblasts with menadione, a redox cycling drug, decreases both aconitase and IRE-binding activities of IRP1 [136]. Exposure of various cell lines to nitric oxide (NO) [137,138] or hydrogen peroxide (H2O2) [139,140] is associated with induction of IRE-binding activity, following complete removal of the [4Fe-4S] ISC. The mechanisms by which NO and H2O2 modulate IRP1 are distinct. It has been proposed that apart from attacking the [4Fe-4S] ISC of IRP1 [134,141], NO may also decrease intracellular iron availability by chelating iron in potentially metabolically inactive dinitrosyl-iron-dithiol complexes [129,132]. NO may also promote iron release from cells [142,143]. In fact, it was recently shown that NO-mediated iron release requires formation of a glutathione conjugate, which is exported by the multi-drug resistance-associated protein 1 (MRP1) [144]. A model of NO-induced iron deficiency could accommodate the relatively slow (> 4 h) induction of IRP1 by this stimulus, with kinetics comparable to those elicited by desferrioxamine [123] in many cell types. Accordingly, external exposure of cells to high NO concentrations or strong endogenous NO production (for example in activated macrophages), trigger IRP1 activation by a rate that may correlate NO fluxes. By contrast, a transient exposure of cultured cells to micromolar concentrations of H2O2 elicits a biphasic induction of IRP1 within 30–60 min [145]. Similar results have been observed in the perfused rat liver [146]. In vitro, the stress activation of IRP1 can only be reconstituted in the presence of the particulate fraction of permeabilized cells [147]. Taken together, the H2O2-mediated induction of IRP1 is very likely a result of an illdefined signaling pathway. Interestingly, this can be antagonized by hypochlorous acid (HOCl), a potent oxidant generated by the myeloperoxidase reaction [148]. This reaction may also contribute to tyrosine nitration of IRP1 following NO exposure, leading to its inactivation [149]. 7.3. Regulation of IRP1 by phosphorylation Earlier work revealed that IRP1 is amenable to phosphorylation by protein kinase C (PKC) in cells treated with phorbol 12-myristate 13-acetate (PMA), possibly at conserved S138 and S711 [150]. Phosphorylation by PKC is more efficient in the absence of the [4Fe-4S] ISC or bound RNA [151]. Phosphomimetic mutations at S138, which is located close to the substrate-binding cleft of IRP1, negatively affect the stability of the [4Fe-4S] ISC [152,153] and, furthermore, result in accelerated degradation of the apo-protein by the proteasome [126]. Similar results were obtained with endogenous wild type IRP1, following exposure of HEK cells to PMA [127]. Thus, phosphorylation at S138 appears to provide a checkpoint for the assembly of IRP1 [4Fe-4S] ISC and, moreover, orchestrate another mode of IRP1 regulation at the level of protein stability. The mechanism by which phosphorylation at S138 destabilizes the [4Fe-4S] ISC is not defined, but is conceivable that the introduction of a negative charge could interfere with the access of CIA factors to apo-IRP1. It should be noted that defects in ISC assembly / repair also sensitize apo-IRP1 for iron-dependent proteasomal degradation, independently of phosphorylation [114,115,127,154]. These results emphasize the crucial function of the [4Fe-4S] ISC in maintaining the stability of IRP1. Furthermore, they provide an alternative backup mechanism to prevent accumulation of excess and potentially deleterious apo-IRP1 by proteasomal degradation. IRE/IRP regulatory system % Experiments with phosphorylation deficient mutants demonstrated that S711 is target of IRP1 phosphorylation in response to PMA [101,155]. This residue is located within domain 4 of IRP1 and very likely plays an important role in the control of its activities. Thus, phosphomimetic mutations at S711 significantly impair the ability of IRP1 to catalyze the conversion of citrate to the intermediate cis-aconitate (first step in the aconitase reaction) and also interfere with IRE-binding [101,155]. Phosphorylation may control IRP1 function in a cell- and tissue-specific manner. For example, in pituitary thyrotrophs, thyrotropin releasing hormone (TRH) and epidermal growth factor (EGF) stimulate the IRE-binding activity of IRP1 (and IRP2) via PKC and mitogen-activated protein kinases (MAPK); however, the same stimuli elicit opposite responses in pituitary lactotrophs [156]. These data also link the IRE/IRP system to signal transduction pathways mediated by hormones and growth factors, which ultimately regulate TfR1 and ferritin expression [5]. Additional examples include the decrease of IRE-binding activity in thyroid hormone-treated HepG2 cells [157], and the induction of IRE-binding in murine erythroleukemia (MEL) cells following administration of erythropoietin [158]. Phosphorylation may also control the subcellular distribution of IRP1 (and IRP2) between the cytosol and Golgi and ER membranes [159]. 8. Mechanisms for the regulation of IRP2 8.1. Regulation of IRP2 by iron and oxygen Even though IRP2 shares significant sequence identity and similarity with IRP1, it is regulated by fundamentally different mechanisms. IRP2 abundance is dramatically altered in response to iron levels and oxygen supply. The protein is synthesized de novo in iron-starved cells [160] and remains stable under these conditions or in hypoxia [161]. In iron-replete and normoxic cells, IRP2 undergoes ubiquitination and degradation by the proteasome [81,82,162]. Based on transfection experiments with IRP2 mutants, it has been proposed that the mechanism for iron-dependent degradation of IRP2 requires elements of a unique insertion of 73 amino acids (aa) embedded within its domain 1 [162]. Furthermore, this 73-aa-region was proposed to define an iron-dependent degradation domain and to function as an iron sensor. According to an early model, the recognition of IRP2 by the proteasome involved site-specific oxidation of three critical cysteine residues (C168, C174 and C178), possibly as a result of the direct binding of iron [163]. Further evidence was provided by in vitro studies with a recombinant 73-aa-peptide, which showed irondependent generation of oxidized cysteine species, mainly dehydrocysteine, from a single cysteine residue [164]. However, data reported in [162] with IRP2 constructs bearing mutations in C168, C174 and C178 could not be reproduced [165,166], challenging the validity of this model. Further studies demonstrated that the deletion of the entire 73-aaregion did not stabilize IRP2 in iron-replete cells [166,167]. As the degradation of IRP2 exhibits a dose-dependent saturable pattern, exogenous IRP2 expressed at high levels is able to titrate out limiting factors of the degradation machinery [166]. Thus, the overall experimental design and the inclusion of appropriate controls are very crucial to avoid ambiguous interpretation of transfection experiments. A two-hybrid screen with the 73-aa-region as a bait resulted in the identification of a protein with a RING finger domain and E3 ubiquitin ligase activity, which was termed HOIL-1 (for heme-oxidized IRP2 ubiquitin ligase 1) and proposed to be the long-sought % Carine Fillebeen et al. ubiquitin ligase for the iron-dependent degradation of IRP2 [168]. According to a revised model, the 73-aa-region recruits HOIL-1 upon binding of heme at C201 and H204 [169]. However, preliminary data have raised the possibility that the binding of HOIL-1 to the 73-aa-region of IRP2 is not iron-dependent and that this ubiquitin ligase does not play any specific role in IRP2 degradation [170]. The involvement of heme in IRP2 degradation has also been proposed earlier [171] and several laboratories have reported that succinylacetone, an inhibitor of heme synthesis, efficiently protects IRP2 against iron-dependent degradation [165,171-173]. While the presence of a heme binding site within the 73-aa-region of IRP2 has been confirmed by in vitro experiments [169,174], the exact role of heme on IRP2 degradation in vivo is less clear. Considering that this region is dispensable for the iron-dependent destabilization of IRP2 [166,167], it is tempting to speculate that heme and non-heme iron may activate distinct degradation pathways, which possibly involve different domains of IRP2. Pharmacological evidence suggests that a pathway for iron-mediated degradation of IRP2 requires the activity of 2-oxoglutarate-dependent oxygenases [166,167]. These enzymes utilize iron, oxygen and ascorbate as cofactors [175,176] and are crucial for the degradation of the hypoxia inducible factor 1α (HIF-1α) [177-179]. The substrate analogue dimethyl-oxalyl-glycine (DMOG), a well-established inhibitor of HIF-1α degradation, efficiently antagonizes the iron-dependent degradation of IRP2 in previously iron-depleted cells [166,167]. Interestingly, DMOG fails to inhibit IRP2 degradation by iron in cells not pre-treated with desferrioxamine, [166], suggesting that additional pathways predominate when a threshold of intracellular iron content is restored. Ciselements within IRP2 and components of the pathway, including the 2-oxoglutaratedependent enzymes, remain to be defined. The analogies between IRP2 and HIF-1α degradation have raised the possibility that pVHL (von Hippel-Lindau protein), the ubiquitin ligase involved in HIF-1α ubiquitination [180], may also operate for the ubiquitination of IRP2. Even though pVHL was shown to interact with IRP2 and promote its degradation in co-transfection experiments, these effects were not iron-dependent and endogenous IRP2 can be efficiently degraded in pVHL-deficient cells [181]. Thus, pVHL is not a necessary component for the IRP2 degradation machinery. 8.2. Regulation of IRP2 by iron-independent signals IRP2 not only responds to alterations in iron and oxygen levels, but it is also regulated by NO. However, conflicting data have been reported over the past years on whether NO activates or inhibits IRP2. For example, the IRE-binding activity of IRP2 was induced in J774 macrophages treated with γ-interferon / lipopolysaccharide (γIFN/LPS) to stimulate the inducible NO-synthase II (NOS II) [137], in B6.NOS cells stably transfected with a NOS II cDNA [182] and in Ltk– fibroblasts exposed to the NOdonor S-nitroso-N-acetylpenicillamine (SNAP) [123]. However, in other studies employing γ-interferon / lipopolysaccharide-stimulated J774 [183] or RAW [184,185] macrophages, IRP2 activity was diminished. In further reports, the exposure of cells to an NO-releasing drug did not affect IRP2 expression [165,186,187]. To some extent, the apparent discrepancy can be attributed to differences in the experimental approaches and the utilized sources of NO [129]. IRE/IRP regulatory system % A decline in IRP2 activity has also been observed following exposure of cells to sodium nitroprusside (SNP) [188]. This iron-containing compound is known to release the nitrosonium cation (NO+) that efficiently nitrosylates protein thiols [189]. It has been proposed that SNP nitrosylates IRP2 at C178 within the 73-aa-region and that this serves as a signal for ubiquitination and degradation by the proteasome [172]. However, in other settings, IRP2 variants bearing point mutations at C178 [165,173] or lacking the entire 73-aa-region [173] remained sensitive to SNP-mediated degradation. A treatment of cells with active or photodegraded SNP profoundly increased the calcein-accessible labile iron pool (LIP), which reflects the intracellular iron status [190], suggesting that the effects of SNP on IRP2 depend on the iron moiety of the drug and not on nitrogen derivatives [173]. The iron-donating properties of SNP are also underscored by experiments showing that the activation of heme oxygenase 1 by SNP involves the release of iron by the drug (rather than nitrogen derivatives) and the subsequent activation of a cAMP-dependent signaling pathway [191,192]. A co-culture system consisting of B6.NOS cells and H1299 cells expressing a hemagglutinin (HA) epitope-tagged IRP2 was utilized to better understand the nature of IRP2 regulation by NO in the absence of potential confounding side effects of pharmacological treatments. In this model, generation of NO by B6.NOS cells was expected to diffuse to neighboring H1299 cells and modulate HA-IRP2. Indeed, physiologically generated NO by B6.NOS cells stabilized HA-IRP2 against proteasomal degradation [193]. The kinetics of IRP2 stabilization by NO were similar to those elicited by iron chelation, supporting a model where NO may promote an iron deficient phenotype [193]. IRP2 may also be regulated by further stimuli (for example its activity is decreased in menadione-treated cells [136]), but this area of research is largely unexplored. Finally, the activity of IRP2 is enhanced by phosphorylation [194]. The phosphorylation site has not been mapped as yet. 9. Expression and physiological functions of IRP1 and IRP2 IRP1 and IRP2 are expressed in the cytosol of most mammalian cell types. IRP1 is predominantly found in the kidney and liver, whereas IRP2 levels are particularly high in the brain [195,196]. Due to technical constrains, IRP1 was earlier believed to be more abundant than IRP2. Even though IRP1 and IRP2 show some diversity in binding specificity and a preference towards distinct sets of RNA targets [27,95,197], they do not differ in their ability to regulate ferritin mRNA translation in vitro [198]. However, cell culture experiments have provided some evidence for differential responses of IRP1 and IRP2 to downstream targets. Thus, in mouse macrophage cell lines, a selective induction of IRP2 sufficed to regulate TfR1 and ferritin expression [183,186,188]. On the other hand, the constitutive IRP1 C437S mutant was capable of regulating TfR1 and ferritin expression, independently of IRP2 [199,200]. In animal tissues, IRP1 is primarily expressed in the aconitase form and there is increasing evidence that it may not readily respond to iron starvation as efficiently as IRP2 [195,201]. Furthermore, in cell culture experiments, IRP1 only compensated for IRP2 deficiency when IRP2(–/–) cells were grown at 21% oxygen, and not at 3%–6%, which likely mimics physiological oxygenation of most tissues [128]. The implication of this notion is that in vivo, IRP2 is a dominant regulator, providing the vast fraction of IRE- %) Carine Fillebeen et al. binding activity, while IRP1 primarily functions as a c-aconitase. This enzyme activity may serve an antioxidant role as part of a network to provide NADPH in the cytosol, in conjunction with cytosolic isocitrate dehydrogenase [202]. A more complete picture is expected to emerge with the characterization of IRP1(–/–) and IRP2(–/–) animals. The targeted disruption of IRP1 and IRP2 in mice has already provided important clues on the function of IRPs in tissues. The ablation of both IRP1 and IRP2 is associated with embryonic lethality [203], underlying an important role of the IRE/IRP system in early development. Preliminary data associated the tissue specific disruption of both IRP1 and IRP2 in the small intestine by Cre/Lox homologous recombination with growth retardation, weight loss and early lethality a few weeks after birth [204]. IRP1(–/–) mice generated by different strategies grow normally and do not display any discernible phenotype [195,205-207]. These animals only mildly misregulate TfR1 and ferritin expression in the kidney and brown fat [195]. Considering that under cell culture conditions IRP1 is modulated by NO and oxidative stress, it is possible that the IRP1(–/–) mice may reveal more significant defects when challenged with inflammatory stimuli. A strain of IRP2(–/–) mice shows clear pathological signs and the animals accumulate iron in the intestinal mucosa and the CNS [196,208]. The iron overload in specific areas of the brain leads to the development of a progressive neurodegenerative disorder. The phenotype is more severe in IRP1(+/–) IRP2(–/–) animals [209]. These data establish IRP2 as an important regulator of systemic iron metabolism [210]. How the loss of IRP2 function leads to iron overload in the brain is currently unclear. An appealing model postulates that the disruption of iron homeostasis in neurons may lead to functional iron deficiency due to overexpression of ferritin and iron sequestration within it in distal axons [211]. This situation may be encountered in neuroferritinopathy, a rare, dominant, adult-onset neurodegenerative disorder, characterized by iron deposition in the basal ganglia, due to a frameshift mutation in L-ferritin gene [212]. The overexpression of mutant ferritin may promote the non-reversible storage of metabolically active iron. More recently, defects in erythropoiesis and microcytosis have been reported in the above described IRP2(–/–) strain [213]. The lack of IRP2 leads to reduced TfR1 expression in erythroid precursors, culminating in the absence of iron stores in the bone marrow. In addition, loss of translational repression promotes over-expression of ALAS2, resulting in the accumulation of protoporphyrin IX in erythroid cells. These data emphasize the important function of IRP2 in erythroid cell development. The microcytic anemia phenotype has been recapitulated in IRP2(–/–) mice generated by a different targeting strategy, ensuring that no selection cassette was retained [205,214]. However, these animals do not develop neurological defects [205,215]. Further investigations and thorough analysis of additional biochemical, genetic and functional parameters is expected to uncover the nature of this discrepancy in the phenotypes of the two available IRP2(–/–) mouse models. 10. The IRE/IRP regulatory system and human disease On the basis of the animal data, it can be predicted that polymorphisms associated with IRP2 defects may also be relevant to human disease. Genetic screens will test the validity of this hypothesis. Nevertheless, the first characterized genetic disease of the IRE/IRP regulatory system is not associated with mutations in IRPs, but in their targets, the IREs. Mutations in L-ferritin IRE are etiologically linked to the hereditary IRE/IRP regulatory system %$ hyperferritinemia-cataract syndrome (HHCS), an autosomal dominant disorder, which, ironically, does not present with any apparent (in terms of general clinical parameters) abnormalities on body iron metabolism. HHCS is characterized by a substantial (up to 20-fold) increase of serum L-ferritin levels in the absence of iron overload, correlating with the development of early-onset cataract [216]. The mutations in the IRE of L-ferritin mRNA prevent binding of IRPs and thus, abolish its translational regulation. Experimental evidence that the hyperferritinemic phenotype is due to failure of IRPs to control L-ferritin synthesis was first provided by gelshift assays with an L-ferritin IRE transcript bearing the disease-associated A32G mutation [217]. This mutant fails to compete wild type L-ferritin IRE for IRP binding, even at 500-fold molar excess. A series of various disease-associated mutations in L-ferritin IRE have been identified in HHCS patients of different ethnic backgrounds [216]. These can be classified into four groups (Figure 6): o o o o deletions in nucleotides forming part of the stem or the loop, point mutations in the loop or the unpaired C in the bulge, point mutations in the upper stem or other nucleotides of the bulge, or point mutations in the lower stem. Type (i) and (ii) deletions / mutations yield the most acute phenotypes; serum Lferritin 1200–2700 µg/l and severe cataract [218]. Type (iii) mutations are associated with slightly lower levels of L-ferritin (950–1900 µg/l) and milder cataract. Type (iv) mutations lead to only a moderate increase in L-ferritin levels (350–650 µg/l) and asymptomatic cataract [218]. The degree of inhibition of IRP1 and IRP2 binding in different mutants correlates well with the gravity of HHCS [219]. Clinical variability among individuals sharing the same mutation suggests the involvement of additional factors [216]. How hyperferritinemia contributes to the pathogenesis of cataract remains a matter of speculation. In lymphoblastoid cell lines, as well as in lens from HHCS patients (recovered from surgery), overproduction of L-ferritin shifts the H-/L- equilibrium in holo-ferritin, leading to the accumulation of L-homopolymers [220]. The development of animal models may be an essential step towards the unraveling of the mechanisms of disease pathogenesis. Interestingly, a point mutation (A49U) in H-ferritin IRE has been linked to an autosomal dominant iron overload disease in a Japanese pedigree, with iron deposition primarily in hepatocytes [221]. This mutation may increase the affinity of IRPs, resulting in inhibition of H-ferritin synthesis, but it is unclear whether this is the causative defect leading to iron overload. 11. Pharmacological modulation of IRP1 and IRP2 Doxorubicin (DOX), a quinone-containing anthracycline is widely used as an antitumor drug to treat a broad spectrum of human cancers. One of the major long-term side effects of DOX is the development of cardiomyopathy, leading to heart failure. It has been proposed that redox activation of this drug results in the formation of reactive oxygen species (ROS) accounting for acute toxicity [222]. Iron is a significant player in % Carine Fillebeen et al. Figure 6. Mutations in the IRE within the human L-ferritin mRNA associated with the hereditary hyperferritinemia-cataract syndrome (HHCS). Nucleotide deletions corresponding to group (i) are shown in bold italics; the deleted region is indicated by arrows. Nucleotides amenable to point mutations that correspond to groups (ii)–(iv) are encircled. DOX cardiotoxicity, which is reduced when patients receive iron chelation therapy. The precise mechanisms responsible for DOX iron-dependent toxicity are not well known. In vitro studies suggest that oxidation of DOX promotes the release of iron from ferritin [223]. However it appears that this is not the only mechanism leading to cardiotoxicity; thus, DOX also disturbs IRP1 and IRP2 activity in cultured cells [224], showing some similarities with the effects of menadione [136]. While IRP2 undergoes proteasomal degradation in cardiomyocytes exposed to DOX, IRP1 switches to its IRE-binding form even in iron-replete cells. The DOX-mediated stimulation of IRP1 enhances TfR1 expression, promoting increased uptake of iron [225], which in turn may catalyze the production of ROS and contribute to toxicity. However, experiments with wild type and IRP1(–/–) mice showed that the DOX-induced alterations in cardiac iron homeostasis are IRP1-independent [226]. Chronic administration of DOX results in the formation of secondary alcohol metabolites, such as doxorubicinol (DOXol) [222]. These metabolites are thought to IRE/IRP regulatory system % contribute to the course of chronic cardiomyopathy when patients have been exposed to durable therapy. The involvement of iron provides a common denominator for DOX and DOXol cardiotoxicity, however the underlying mechanisms are different. In contrast to DOX and independently of the presence of ROS, DOXol converts IRP1 to a null protein, without any enzymatic or RNA-binding activities [224]. The concomitant loss of the two IRP1 activities involves oxidation of critical cysteine residues, which participate in both functions of IRP1. The specific mechanism responsible for damaging the cysteine residues is unclear. Another major dissimilarity with DOX is the absence of sensitivity of IRP2 to DOXol [224]. Interestingly, a ROS-dependent degradation of IRP2 in the early phase of DOX exposure may correct the transitory activation of IRP1. However, in the chronic phase of the therapy the opposite occurs: conversion of DOX to DOXol inactivates permanently IRP1 whereas newly synthesized IRP2 is not affected. The combined effect of the different agents, DOX, DOXol and the other metabolites is more obscure, leading to disruption of iron homeostasis. It is believed that the particular sensitivity of cardiomyocytes to DOX arises from their relatively poor content in antioxidants. However, antioxidant therapy generally fails to improve the condition of the long-term DOX-treated patients, which is in contrast to iron chelation therapy. This is supporting the concept to restore cellular iron balance during DOX administration. Finally, the intricate relationship between the pharmacological effects of anthracyclines and iron metabolism may not merely depend on the direct targeting of IRPs, but also on signal transduction pathways, possibly involving ROS [227]. 12. Conclusions By virtue of their roles in cellular iron uptake and storage, respectively, TfR1 and ferritin can be considered as key proteins for cellular iron homeostasis. 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