Conditional derepression of ferritin synthesis in cells expressing a constitutive IRP1 mutant

Expression of genes related to iron homeostasis in breast cancer

BACKGROUND

The dysfunctions in the metabolism of iron have an important role in many pathological conditions, ranging from disease with iron deposition to cancer. Studies on malignant diseases of the breast reported irregular expression in genes associated with iron metabolism. The variations are related to findings that have prognostic significance. This study evaluated the relationship of the expression levels of transferrin receptor 1 (TFRC), iron regulatory protein 1 (IRP1), hepcidin (HAMP), ferroportin 1 (FPN1), hemojuvelin (HFE2), matriptase 2 (TMPRSS6), and miR-122 genes in the normal and malignant tissues of breast cancer patients.

METHODS & RESULTS

The normal and malignant tissues from 75 women with breast malignancies were used in this study. The patients did not receive any treatment previously. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was used in figuring the levels of gene expression associated with iron metabolism. When the malignant and normal tissues gene expression levels were analyzed, expression of TFRC increased (1.586-fold); IRP1 (0.594 fold) and miR-122 (0.320 fold) expression decreased; HAMP, FPN1, HFE2, and TMPRSS6 expressions did not change. FPN1 and IRP1 had a positive association, and this association was statistically significant (r = 0.266; p = 0.022). IRP1 and miR-122 had a positive association, and this association had statistical significance (r = 0.231; p = 0.048).

CONCLUSIONS

Our study portrayed the important association between genes involved in iron hemostasis and breast malignancy. The results could be used to establish new diagnostic techniques in the management of breast malignancies. The alterations in the metabolism of malignant breast cells with normal breast cells could be utilized to achieve advantages in treatment.

Emerging role of ferroptosis in breast cancer: New dawn for overcoming tumor progression

Breast cancer has become a serious threat to women's health. Cancer progression is mainly derived from resistance to apoptosis induced by procedures or therapies. Therefore, new drugs or models that can overcome apoptosis resistance should be identified. Ferroptosis is a recently identified mode of cell death characterized by excess reactive oxygen species-induced lipid peroxidation. Since ferroptosis is distinct from apoptosis, necrosis and autophagy, its induction successfully eliminates cancer cells that are resistant to other modes of cell death. Therefore, ferroptosis may become a new direction around which to design breast cancer treatment. Unfortunately, the complete appearance of ferroptosis in breast cancer has not yet been fully elucidated. Furthermore, whether ferroptosis inducers can be used in combination with traditional anti- breast cancer drugs is still unknown. Moreover, a summary of ferroptosis in breast cancer progression and therapy is currently not available. In this review, we discuss the roles of ferroptosis-associated modulators glutathione, glutathione peroxidase 4, iron, nuclear factor erythroid-2 related factor-2, superoxide dismutases, lipoxygenase and coenzyme Q in breast cancer. Furthermore, we provide evidence that traditional drugs against breast cancer induce ferroptosis, and that ferroptosis inducers eliminate breast cancer cells. Finally, we put forward prospect of using ferroptosis inducers in breast cancer therapy, and predict possible obstacles and corresponding solutions. This review will deepen our understanding of the relationship between ferroptosis and breast cancer, and provide new insights into breast cancer-related therapeutic strategies.

Mdm2 is a target and mediator of IRP2 in cell growth control

Iron is an essential element to all living organisms and plays a vital role in many cellular processes, such as DNA synthesis and energy production. The Mdm2 oncogene is an E3 ligase and known to promote tumor growth. However, the role of Mdm2 in iron homeostasis is not certain. Here, we showed that Mdm2 expression was increased by iron depletion but decreased by iron repletion. We also showed that Iron Regulatory Protein 2 (IRP2) mediated iron-regulated Mdm2 expression. Specifically, Mdm2 expression was increased by ectopic IRP2 but decreased by knockdown or knockout of IRP2 in human cancer cells as well as in mouse embryonic fibroblasts. In addition, we showed that IRP2-regulated Mdm2 expression was independent of tumor suppressor p53. Mechanistically, we found that IRP2 stabilized Mdm2 transcript via binding to an iron response element (IRE) in the 3'UTR of Mdm2 mRNA. Finally, we showed that Mdm2 is required for IRP2-mediated cell proliferation and Mdm2 expression is highly associated with IRP2 in both the normal and cancerous liver tissues. Together, we uncover a novel regulation of Mdm2 by IRP2 via mRNA stability and that the IRP2-Mdm2 axis may play a critical role in cell growth.

Identification and Characterization of Glycine Decarboxylase as a Direct Target of Snail in the Epithelial-Mesenchymal Transition of Cancer Cells

Context/Aims:

Metabolic reprogramming and cellular plasticity drive tumorigenesis. However, how these cellular events collectively contribute to the oncogenic process is poorly understood. Epithelial-mesenchymal transition (EMT), a fundamental mechanism of cellular plasticity, is governed by the EMT transcription repressors such as Snail. In the present study, through establishment and characterization of inducible overexpression of Snail in A549 lung cancer cells, we aim to define the metabolic reprogramming in response to Snail in the EMT of lung cancer cells.

Methods/Results:

Our metabolomic analysis suggests that forced expression of Snail accompanied reduced diversion of glycolytic metabolites to the serine/glycine metabolic shunt, a critical metabolic branch that distributes glucose catabolic intermediates to the major anabolic pathways. Our gene expression profiling and molecular characterization revealed that Snail actively suppressed the expression of glycine decarboxylase (GLDC), a key enzyme on the serine/glycine metabolic shunt, through binding to an evolutionarily conserved E-box motif and thereby inhibiting the promoter of the GLDC gene. Besides, knockdown of GLDC led to a cellular function shift from proliferation to migration.

Conclusion:

This study has revealed a novel molecular link that integrates the serine/glycine metabolism with the Snail-mediated EMT program in cancer cells.

Perturbation of Iron Metabolism by Cisplatin through Inhibition of Iron Regulatory Protein 2

Cisplatin is classically known to exhibit anticancer activity through DNA damage in the nucleus. Here we found a mechanism by which cisplatin affects iron metabolism, leading to toxicity and cell death. Cisplatin causes intracellular iron deficiency through direct inhibition of the master regulator of iron metabolism, iron regulatory protein 2 (IRP2) with marginal effects on IRP1. Cisplatin, but not carboplatin or transplatin, binds human IRP2 at Cys512 and Cys516 and impairs IRP2 binding to iron-responsive elements of ferritin and transferrin receptor-1 (TfR1) mRNAs. IRP2 inhibition by cisplatin caused ferritin upregulation and TfR1 downregulation leading to sustained intracellular iron deficiency. Cys512/516Ala mutant IRP2 made cells more resistant to cisplatin. Furthermore, combination of cisplatin and the iron chelator desferrioxamine enhanced cytotoxicity through augmented iron depletion in culture and xenograft mouse model. Collectively, cisplatin is an inhibitor of IRP2 that induces intracellular iron deficiency.

Loss of ABHD5 promotes the aggressiveness of prostate cancer cells

The accumulation of neutral lipids in intracellular lipid droplets has been associated with the formation and progression of many cancers, including prostate cancer (PCa). Alpha-beta Hydrolase Domain Containing 5 (ABHD5) is a key regulator of intracellular neutral lipids that has been recently identified as a tumor suppressor in colorectal cancer, yet its potential role in PCa has not been investigated. Through mining publicly accessible PCa gene expression datasets, we found that ABHD5 gene expression is markedly decreased in metastatic castration-resistant PCa (mCRPC) samples. We further demonstrated that RNAi-mediated ABHD5 silencing promotes, whereas ectopic ABHD5 overexpression inhibits, the invasion and proliferation of PCa cells. Mechanistically, we found that ABHD5 knockdown induces epithelial to mesenchymal transition, increasing aerobic glycolysis by upregulating the glycolytic enzymes hexokinase 2 and phosphofrucokinase, while decreasing mitochondrial respiration by downregulating respiratory chain complexes I and III. Interestingly, knockdown of ATGL, the best-known molecular target of ABHD5, impeded the proliferation and invasion, suggesting an ATGL-independent role of ABHD5 in modulating PCa aggressiveness. Collectively, these results provide evidence that ABHD5 acts as a metabolic tumor suppressor in PCa that prevents EMT and the Warburg effect, and indicates that ABHD5 is a potential therapeutic target against mCRPC, the deadly aggressive PCa.

Iron homeostasis and tumorigenesis: molecular mechanisms and therapeutic opportunities

Excess iron is tightly associated with tumorigenesis in multiple human cancer types through a variety of mechanisms including catalyzing the formation of mutagenic hydroxyl radicals, regulating DNA replication, repair and cell cycle progression, affecting signal transduction in cancer cells, and acting as an essential nutrient for proliferating tumor cells. Thus, multiple therapeutic strategies based on iron deprivation have been developed in cancer therapy. During the past few years, our understanding of genetic association and molecular mechanisms between iron and tumorigenesis has expanded enormously. In this review, we briefly summarize iron homeostasis in mammals, and discuss recent progresses in understanding the aberrant iron metabolism in numerous cancer types, with a focus on studies revealing altered signal transduction in cancer cells.

The IRP/IRE system in vivo: insights from mouse models

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) post-transcriptionally control the expression of several mRNAs encoding proteins of iron, oxygen and energy metabolism. The mechanism involves their binding to iron responsive elements (IREs) in the untranslated regions of target mRNAs, thereby controlling mRNA translation or stability. Whereas IRP2 functions solely as an RNA-binding protein, IRP1 operates as either an RNA-binding protein or a cytosolic aconitase. Early experiments in cultured cells established a crucial role of IRPs in regulation of cellular iron metabolism. More recently, studies in mouse models with global or localized Irp1 and/or Irp2 deficiencies uncovered new physiological functions of IRPs in the context of systemic iron homeostasis. Thus, IRP1 emerged as a key regulator of erythropoiesis and iron absorption by controlling hypoxia inducible factor 2α (HIF2α) mRNA translation, while IRP2 appears to dominate the control of iron uptake and heme biosynthesis in erythroid progenitor cells by regulating the expression of transferrin receptor 1 (TfR1) and 5-aminolevulinic acid synthase 2 (ALAS2) mRNAs, respectively. Targeted disruption of either Irp1 or Irp2 in mice is associated with distinct phenotypic abnormalities. Thus, Irp1(-/-) mice develop polycythemia and pulmonary hypertension, while Irp2(-/-) mice present with microcytic anemia, iron overload in the intestine and the liver, and neurologic defects. Combined disruption of both Irp1 and Irp2 is incombatible with life and leads to early embryonic lethality. Mice with intestinal- or liver-specific disruption of both Irps are viable at birth but die later on due to malabsorption or liver failure, respectively. Adult mice lacking both Irps in the intestine exhibit a profound defect in dietary iron absorption due to a "mucosal block" that is caused by the de-repression of ferritin mRNA translation. Herein, we discuss the physiological function of the IRE/IRP regulatory system.

Abnormal body iron distribution and erythropoiesis in a novel mouse model with inducible gain of iron regulatory protein (IRP)-1 function

Disorders of iron metabolism account for some of the most common human diseases. Cellular iron homeostasis is maintained by iron regulatory proteins (IRP)-1 and 2 through their binding to cis-regulatory iron-responsive elements (IREs) in target mRNAs. Mouse models with IRP deficiency have yielded valuable insights into iron biology, but the physiological consequences of gain of IRP function in mammalian organisms have remained unexplored. Here, we report the generation of a mouse line allowing conditional expression of a constitutively active IRP1 mutant (IRP1*) using Cre/Lox technology. Systemic activation of the IRP1* transgene from the Rosa26 locus yields viable animals with gain of IRE-binding activity in all the organs analyzed. IRP1* activation alters the expression of IRP target genes and is accompanied by iron loading in the same organs. Furthermore, mice display macrocytic erythropenia with decreased hematocrit and hemoglobin levels as well as impaired erythroid differentiation. Thus, inappropriately high IRP1 activity causes disturbed body iron distribution and erythropoiesis. This new mouse model further highlights the importance of appropriate IRP regulation in central organs of iron metabolism. Moreover, it opens novel avenues to study diseases associated with abnormally high IRP1 activity, such as Parkinson’s disease or Friedreich’s ataxia.

Alternative ferritin mRNA translation via internal initiation

Ferritin stores and detoxifies an excess of intracellular iron, and thereby plays an important role in the metabolism of this metal. As unshielded iron promotes oxidative stress, ferritin is crucial in maintaining cellular redox balance and may also modulate cell growth, survival, and apoptosis. The expression of ferritin is controlled by transcriptional and post-transcriptional mechanisms. In light of the well-established transcriptional induction of ferritin by inflammatory signals, we examined how ferritin mRNA translation responds to stress conditions. We first used HT1080 fibrosarcoma cells engineered for coumermycin-inducible expression of PKR, a stress kinase that inhibits protein synthesis in virus-infected cells by phosphorylating eIF2α. In this setting, iron triggered partial ferritin mRNA translation despite a PKR-induced global shutdown in protein synthesis. Moreover, iron-mediated ferritin synthesis was evident in cells infected with an attenuated strain of poliovirus; when eIF4GI was cleaved, eIF2α was phosphorylated, and host protein synthesis was inhibited. Under global inhibition of protein synthesis or specific inhibition of ferritin mRNA translation in cells overexpressing PKR or IRP1, respectively, we demonstrate association of ferritin mRNA with heavy polysomes. Further experiments revealed that the 5' untranslated region (5' UTR) of ferritin mRNA contains a bona fide internal ribosomal entry site (IRES). Our data are consistent with the existence of an alternative, noncanonical mechanism for ferritin mRNA translation, which may primarily operate under stress conditions to protect cells from oxidative stress.

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.

Tumorigenic properties of iron regulatory protein 2 (IRP2) mediated by its specific 73-amino acids insert

Iron regulatory proteins, IRP1 and IRP2, bind to mRNAs harboring iron responsive elements and control their expression. IRPs may also perform additional functions. Thus, IRP1 exhibited apparent tumor suppressor properties in a tumor xenograft model. Here we examined the effects of IRP2 in a similar setting. Human H1299 lung cancer cells or clones engineered for tetracycline-inducible expression of wild type IRP2, or the deletion mutant IRP2_Δ73 (lacking a specific insert of 73 amino acids), were injected subcutaneously into nude mice. The induction of IRP2 profoundly stimulated the growth of tumor xenografts, and this response was blunted by addition of tetracycline in the drinking water of the animals, to turnoff the IRP2 transgene. Interestingly, IRP2_Δ73 failed to promote tumor growth above control levels. As expected, xenografts expressing the IRP2 transgene exhibited high levels of transferrin receptor 1 (TfR1); however, the expression of other known IRP targets was not affected. Moreover, these xenografts manifested increased c-MYC levels and ERK1/2 phosphorylation. A microarray analysis identified distinct gene expression patterns between control and tumors containing IRP2 or IRP1 transgenes. By contrast, gene expression profiles of control and IRP2_Δ73-related tumors were more similar, consistently with their growth phenotype. Collectively, these data demonstrate an apparent pro-oncogenic activity of IRP2 that depends on its specific 73 amino acids insert, and provide further evidence for a link between IRPs and cancer biology.

Phosphinothricin-tripeptide biosynthesis: an original version of bacterial secondary metabolism?

Streptomyces viridochromogenes Tü494 produces the herbicide phosphinothricyl-alanyl-alanine (phosphinothricin-tripeptide=PTT; bialaphos). Its bioactive moiety phosphinothricin competitively inhibits bacterial and plant glutamine synthetases. The biosynthesis of PTT includes the synthesis of the unusual amino acid N-acetyl-demethyl-phosphinothricin and a three step condensation via non-ribosomal peptide synthetases. Two characteristics within the PTT biosynthesis make it suitable to study the evolution of secondary metabolism biosynthesis. First, PTT biosynthesis represents the only known system where all peptide synthetase modules are located on separate proteins. This 'single enzyme system' might be an archetype of the multimodular and multienzymatic non-ribosomal peptide synthetases in evolutionary terms. The second interesting feature of PTT biosynthesis is the pathway-specific aconitase Pmi that is involved in the supply of N-acetyl-demethyl-phosphinothricin. Pmi is highly similar to the tricarboxylic acid aconitase AcnA. They share 64% identity at the DNA level and both belong to the Iron-Regulatory-Protein/AcnA family. Despite their high sequence similarity, AcnA and Pmi catalyze different reactions and are not able to substitute for each other. Thus, the enzyme pair AcnA/Pmi presents an example of the evolution of a secondary metabolite-specific enzyme from a primary metabolism enzyme.

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.

Oxidative stress and cell death in cells expressing L-ferritin variants causing neuroferritinopathy

Neuroferritinopathies are dominantly inherited movement disorders associated with nucleotide insertions in the L-ferritin gene that modify the protein's C-terminus. The insertions alter physical and functional properties of the ferritins, causing an imbalance in brain iron homeostasis. We describe the effects produced by the over-expression in HeLa and neuroblastoma SH-SY5Y cells of two pathogenic L-ferritin variants, 460InsA and 498InsTC. Both peptides co-assembled with endogenous ferritins, producing molecules with reduced iron incorporation capacity, acting in a dominant negative manner. The cells showed an increase in cell death and a decrease in proteasomal activity. The formation of iron-ferritin aggregates became evident after 10 days of variant expression and was not associated with increased cell death. The addition of iron chelators or antioxidants restored proteasomal activity and reduced aggregate formation. The data indicate that cellular iron imbalance and oxidative damage are primary causes of cell death, while aggregate formation is a secondary effect.

Regulating amyloid precursor protein synthesis through an internal ribosomal entry site

Expression of amyloid precursor protein (APP) is critical to the etiology of Alzheimer's disease (AD). Consequently, regulating APP expression is one approach to block disease progression. To this end, APP can be targeted at the levels of transcription, translation, and protein stability. Little is currently known about the translation of APP mRNA. Here, we report that endogenous APP mRNA is translated in neural cell lines via an internal ribosome entry site (IRES) located in the 5'-untranslated leader. The functional unit of the APP IRES is located within the 5' 50 nucleotides of the 5'-leader. In addition, we found that the APP IRES is positively regulated by two conditions correlated with AD, increased intracellular iron concentration and ischemia. Interestingly, the enhancement of APP IRES activity is dependent upon de novo transcription. Taken together, our data suggest that internal initiation of translation of the APP mRNA is an important mode for synthesis of APP, a mechanism which is regulated by conditions that also contribute to AD.

Iron-dependent degradation of IRP2 requires its C-terminal region and IRP structural integrity

Background

Iron regulatory protein 2 (IRP2), a post-transcriptional regulator of cellular iron metabolism, undergoes iron-dependent degradation via the ubiquitin-proteasome pathway. A stretch of 73 amino acids within the N-terminal domain 1 of the protein was reported to function as an iron sensor. However, mutants lacking this fragment remain sensitive to degradation in iron-replete cells.

Results

To identify elements within IRP2 involved in the control of its stability, we undertook a systematic mutagenesis approach. Truncated versions of IRP2 were expressed in H1299 cells and analyzed for their response to iron. Deletion mutants lacking the entire C-terminal domain 4 (amino acids 719–963) of IRP2 remained stable following iron treatments. Moreover, the replacement of domain 4 of IRP1 with the corresponding region of IRP2 sensitized the chimerical IRP1_1–3/IRP2₄ protein to iron-dependent degradation, while the reverse manipulation gave rise to a stable chimerical IRP2_1–3/IRP1₄ protein. The deletion of just 26 or 34 C-terminal amino acids stabilized IRP2 against iron. However, the fusion of C-terminal IRP2 fragments to luciferase failed to sensitize the indicator protein for degradation in iron-loaded cells.

Conclusion

Our data suggest that the C-terminus of IRP2 contains elements that are necessary but not sufficient for iron-dependent degradation. The functionality of these elements depends upon the overall IRP structure.

Human iron regulatory protein 2 is easily cleaved in its specific domain: consequences for the haem binding properties of the protein

Mammalian IRPs (iron regulatory proteins), IRP1 and IRP2, are cytosolic RNA-binding proteins that post-transcriptionally control the mRNA of proteins involved in storage, transport, and utilization of iron. In iron-replete cells, IRP2 undergoes degradation by the ubiquitin/proteasome pathway. Binding of haem to a 73aa-Domain (73-amino-acid domain) that is unique in IRP2 has been previously proposed as the initial iron-sensing mechanism. It is shown here that recombinant IRP2 and the 73aa-Domain are sensitive to proteolysis at the same site. NMR results suggest that the isolated 73aa-Domain is not structured. Iron-independent cleavage of IRP2 within the 73aa-Domain also occurs in lung cancer (H1299) cells. Haem interacts with a cysteine residue only in truncated forms of the 73aa-Domain, as shown by a series of complementary physicochemical approaches, including NMR, EPR and UV-visible absorption spectroscopy. In contrast, the cofactor is not ligated by the same residue in the full-length peptide or intact IRP2, although non-specific interaction occurs between these molecular forms and haem. Therefore it is unlikely that the iron-dependent degradation of IRP2 is mediated by haem binding to the intact 73aa-Domain, since the sequence resembling an HRM (haem-regulatory motif) in the 73aa-Domain does not provide an axial ligand of the cofactor unless this domain is cleaved.

Expression of the subgenomic hepatitis C virus replicon alters iron homeostasis in Huh7 cells

BACKGROUND/AIMS

Infection with hepatitis C virus (HCV) is associated with alterations in body iron homeostasis by poorly defined mechanisms. To seek for molecular links, we employed an established cell culture model for viral replication, and assessed how the expression of an HCV subgenomic replicon affects iron metabolism in host Huh7 hepatoma cells.

METHODS

The expression of iron metabolism genes and parameters defining the cellular iron status were analyzed and compared between parent and replicon Huh7 cells.

RESULTS

By using the IronChip microarray platform, we observed replicon-induced changes in expression profiles of iron metabolism genes. Notably, ceruloplasmin mRNA and protein expression were decreased in replicon cells. In addition, transferrin receptor 1 (TfR1) was also downregulated, while ferroportin levels were elevated, resulting in reduced iron uptake and increased iron release capacity of replicon cells. These responses were associated with an iron-deficient phenotype, manifested in decreased levels of the "labile iron pool" and concomitant induction of IRE-binding activity and IRP2 expression. Furthermore, hemin-treated replicon cells exhibited a defect in retaining iron. The clearance of the replicon by prolonged treatment with interferon-alpha only partially reversed the iron-deficient phenotype but almost completely restored the capacity of cured cells to retain iron.

CONCLUSIONS

We propose that Huh7 cells undergo genetic reprogramming to permit subgenomic viral replication that results in reduction of intracellular iron levels. This response may provide a mechanism to bypass iron-mediated inactivation of the viral RNA polymerase NS5B.

Strong iron demand during hypoxia-induced erythropoiesis is associated with down-regulation of iron-related proteins and myoglobin in human skeletal muscle

Iron is essential for oxygen transport because it is incorporated in the heme of the oxygen-binding proteins hemoglobin and myoglobin. An interaction between iron homeostasis and oxygen regulation is further suggested during hypoxia, in which hemoglobin and myoglobin syntheses have been reported to increase. This study gives new insights into the changes in iron content and iron-oxygen interactions during enhanced erythropoiesis by simultaneously analyzing blood and muscle samples in humans exposed to 7 to 9 days of high altitude hypoxia (HA). HA up-regulates iron acquisition by erythroid cells, mobilizes body iron, and increases hemoglobin concentration. However, contrary to our hypothesis that muscle iron proteins and myoglobin would also be up-regulated during HA, this study shows that HA lowers myoglobin expression by 35% and down-regulates iron-related proteins in skeletal muscle, as evidenced by decreases in L-ferritin (43%), transferrin receptor (TfR; 50%), and total iron content (37%). This parallel decrease in L-ferritin and TfR in HA occurs independently of increased hypoxia-inducible factor 1 (HIF-1) mRNA levels and unchanged binding activity of iron regulatory proteins, but concurrently with increased ferroportin mRNA levels, suggesting enhanced iron export. Thus, in HA, the elevated iron requirement associated with enhanced erythropoiesis presumably elicits iron mobilization and myoglobin down-modulation, suggesting an altered muscle oxygen homeostasis.

Iron-dependent degradation of apo-IRP1 by the ubiquitin-proteasome pathway

Iron regulatory protein 1 (IRP1) controls the translation or stability of several mRNAs by binding to "iron-responsive elements" within their untranslated regions. In iron-replete cells, IRP1 assembles a cubane iron-sulfur cluster (ISC) that inhibits RNA-binding activity and converts the protein to cytosolic aconitase. We show that the constitutive IRP1(C437S) mutant, which fails to form an ISC, is destabilized by iron. Thus, exposure of H1299 cells to ferric ammonium citrate reduced the half-life of transfected IRP1(C437S) from approximately 24 h to approximately 10 h. The iron-dependent degradation of IRP1(C437S) involved ubiquitination, required ongoing transcription and translation, and could be efficiently blocked by the proteasomal inhibitors MG132 and lactacystin. Similar results were obtained with overexpressed wild-type IRP1, which predominated in the apo-form even in iron-loaded H1299 cells, possibly due to saturation of the ISC assembly machinery. Importantly, inhibition of ISC biogenesis in HeLa cells by small interfering RNA knockdown of the cysteine desulfurase Nfs1 sensitized endogenous IRP1 for iron-dependent degradation. Collectively, these data uncover a mechanism for the regulation of IRP1 abundance as a means to control its RNA-binding activity, when the ISC assembly pathway is impaired.

Overexpression of iron regulatory protein 1 suppresses growth of tumor xenografts

Iron is essential for proliferation of normal and neoplastic cells. Cellular iron uptake, utilization and storage are regulated by transcriptional and post-transcriptional mechanisms. We hypothesized that the disruption of iron homeostasis may modulate the growth properties of cancer cells. To address this, we employed H1299 lung cancer cells engineered for tetracycline-inducible overexpression of the post-transcriptional regulator iron regulatory protein 1 (IRP1). The induction of IRP1 (wild-type or the constitutive IRP1(C437S) mutant) did not affect the proliferation of the cells in culture, and only modestly reduced their efficiency to form colonies in soft agar. However, IRP1 dramatically impaired the capacity of the cells to form solid tumor xenografts in nude mice. Tumors derived from IRP1-transfectants were <20% in size compared to those from parent cells. IRP1 coordinately controls the expression of transferrin receptor 1 (TfR1) and ferritin by binding to iron-responsive elements (IREs) within their mRNAs. Biochemical analysis revealed high expression of epitope-tagged IRP1 in tumor tissue, which was associated with a profound increase in IRE-binding activity. As expected, this response misregulated iron metabolism by increasing TfR1 levels. Surprisingly, IRP1 failed to suppress ferritin expression and did not affect the levels of the iron transporter ferroportin. Our results show that the overexpression of IRP1 is associated with an apparent tumor suppressor phenotype and provide a direct regulatory link between the IRE/IRP system and cancer.

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.

In vivo tumor growth is inhibited by cytosolic iron deprivation caused by the expression of mitochondrial ferritin

Mitochondrial ferritin (MtFt) is a mitochondrial iron-storage protein whose function and regulation is largely unknown. Our previous results have shown that MtFt overexpression markedly affects intracellular iron homeostasis in mammalian cells. Using tumor xenografts, we examined the effects of MtFt overexpression on tumor iron metabolism and growth. The expression of MtFt dramatically reduced implanted tumor growth in nude mice. Mitochondrial iron deposition in MtFt-expressing tumors was directly observed by transmission electron microscopy. A cytosolic iron starvation phenotype in MtFt-expressing tumors was revealed by increased RNA-binding activity of iron regulatory proteins, and concomitantly both an increase in transferrin receptor levels and a decrease in cytosolic ferritin. MtFt overexpression also led to decreases in total cellular heme content and heme oxygenase-1 levels. In addition, elevated MtFt in tumors was also associated with a decrease in total aconitase activity and lower frataxin protein level. In conclusion, our study shows that high MtFt levels can significantly affect tumor iron homeostasis by shunting iron into mitochondria; iron scarcity resulted in partially deficient heme and iron-sulfur cluster synthesis. It is likely that deprivation of iron in the cytosol is the cause for the significant inhibition of xenograft tumor growth.

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.

Secreted ferritin: mosquito defense against iron overload?

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

Inhibition of transferrin receptor 1 transcription by a cell density response element

TfR1 (transferrin receptor 1) mediates the uptake of transferrin-bound iron and thereby plays a critical role in cellular iron metabolism. Its expression is coupled to cell proliferation/differentiation and controlled in response to iron levels and other signals by transcriptional and post-transcriptional mechanisms. It is well established that TfR1 levels decline when cultured cells reach a high density and in the present study we have investigated the underlying mechanisms. Consistent with previous findings, we demonstrate that TfR1 expression is attenuated in a cell-density-dependent manner in human lung cancer H1299 cells and in murine B6 fibroblasts as the result of a marked decrease in mRNA content. This response is not associated with alterations in the RNA-binding activity of iron regulatory proteins that are indicative of a transcriptional mechanism. Reporter assays reveal that the human TfR1 promoters contains sequences mediating cell-density-dependent transcriptional inhibition. Mapping of the human and mouse TfR1 promoters identified a conserved hexa-nucleotide 5'-GAGGGC-3' motif with notable sequence similarity to a previously described element within the IGF-2 (insulin-like growth factor-2) promoter. We show that this motif is necessary for the formation of specific complexes with nuclear extracts and for cell-density-dependent regulation in reporter gene assays. Thus the TfR1 promoter contains a functional 'cell density response element' (CDRE).

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.

Mammalian tissue oxygen levels modulate iron-regulatory protein activities in vivo

The iron-regulatory proteins (IRPs) posttranscriptionally regulate expression of transferrin receptor, ferritin, and other iron metabolism proteins. Although both IRPs can regulate expression of the same target genes, IRP2-/- mice significantly misregulate iron metabolism and develop neurodegeneration, whereas IRP1-/- mice are spared. We found that IRP2-/- cells misregulated iron metabolism when cultured in 3 to 6% oxygen, which is comparable to physiological tissue concentrations, but not in 21% oxygen, a concentration that activated IRP1 and allowed it to substitute for IRP2. Thus, IRP2 dominates regulation of mammalian iron homeostasis because it alone registers iron concentrations and modulates its RNA-binding activity at physiological oxygen tensions.

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.

Overexpression of mitochondrial ferritin causes cytosolic iron depletion and changes cellular iron homeostasis

Cytosolic ferritin sequesters and stores iron and, consequently, protects cells against iron-mediated free radical damage. However, the function of the newly discovered mitochondrial ferritin (MtFt) is unknown. To examine the role of MtFt in cellular iron metabolism, we established a cell line that stably overexpresses mouse MtFt under the control of a tetracycline-responsive promoter. The overexpression of MtFt caused a dose-dependent iron deficiency in the cytosol that was revealed by increased RNA-binding activity of iron regulatory proteins (IRPs) along with an increase in transferrin receptor levels and decrease in cytosolic ferritin. Consequently, the induction of MtFt resulted in a dramatic increase in cellular iron uptake from transferrin, most of which was incorporated into MtFt. The induction of MtFt caused a shift of iron from cytosolic ferritin to MtFt. In addition, iron inserted into MtFt was less available for chelation than that in cytosolic ferritin and the expression of MtFt was associated with decreased mitochondrial and cytosolic aconitase activities, the latter being consistent with the increase in IRP-binding activity. In conclusion, our results indicate that overexpression of MtFt causes a dramatic change in intracellular iron homeostasis and that shunting iron to MtFt likely limits its availability for active iron proteins.

ANP-induced decrease of iron regulatory protein activity is independent of HO-1 induction

Atrial natriuretic peptide (ANP)-preconditioned livers are protected from ischemia-reperfusion injury. ANP-treated organs show increased expression of heme oxygenase (HO)-1. Because HO-1 liberates bound iron, the aim of our study was to determine whether ANP affects iron regulatory protein (IRP) activity and, thus, the levels of ferritin. Rat livers were perfused with Krebs-Henseleit buffer [+/-ANP, 8-bromo-cGMP (8-Br-cGMP), and tin protoporphyrin, 20 min], stored in University of Wisconsin solution (4 degrees C, 24 h), and reperfused (120 min). IRP activity was assessed by gel-shift assays, and ferritin, IRP phosphorylation, and PKC localization were assessed by Western blot. Control livers displayed decreased IRP activity at the end of ischemia but no change in ferritin content during ischemia and reperfusion. ANP-pretreated livers showed reduced IRP activity, an effect mimicked by 8-Br-cGMP. Ferritin levels were increased in ANP-pretreated organs. Simultaneous perfusion of livers with ANP and tin protoporphyrin did not reduce ANP-induced action, arguing against a role for HO-1 in changes in IRP activity. ANP and 8-Br-cGMP decreased membrane localization of PKC-alpha and PKC-epsilon, but this modulation of PKC seems unrelated to inhibition of IRP binding. This work shows the cGMP-mediated attenuation of IRP binding activity by ANP, which results in increased hepatic ferritin levels. This change in IRPs is independent of ANP-induced HO-1 and reduced PKC activation.

Iron metabolism and the IRE/IRP regulatory system: an update

Cellular iron homeostasis is accomplished by the coordinated regulated expression of the transferrin receptor and ferritin, which mediate iron uptake and storage, respectively. The mechanism is posttranscriptional and involves two cytoplasmic iron regulatory proteins, IRP1 and IRP2. Under conditions of iron starvation, IRPs stabilize the transferrin receptor and inhibit the translation of ferritin mRNAs by binding to "iron responsive elements" (IREs) within their untranslated regions. The IRE/IRP system also controls the expression of additional IRE-containing mRNAs, encoding proteins of iron and energy metabolism. The activities of IRP1 and IRP2 are regulated by distinct posttranslational mechanisms in response to cellular iron levels. Thus, in iron-replete cells, IRP1 assembles a cubane iron-sulfur cluster, which prevents IRE binding, while IRP2 undergoes proteasomal degradation. IRP1 and IRP2 also respond, albeit differentially, to iron-independent signals, such as hydrogen peroxide, hypoxia, or nitric oxide. Basic principles of the IRE/IRP system and recent advances in understanding the regulation and the function of IRP1 and IRP2 are discussed.

Iron-mediated degradation of IRP2, an unexpected pathway involving a 2-oxoglutarate-dependent oxygenase activity

Iron regulatory protein 2 (IRP2), a central posttranscriptional regulator of cellular and systemic iron metabolism, undergoes proteasomal degradation in iron-replete cells. The prevailing model postulates that the mechanism involves site-specific oxidation of 3 cysteine residues (C168, C174, and C178) within a 73-amino-acid (73-aa) degradation domain. By expressing wild-type and mutated versions of IRP2 in H1299 cells, we find that a C168S C174S C178S triple mutant, or a deletion mutant lacking the entire "73-aa domain," is sensitive to iron-mediated degradation, like wild-type IRP2. The antioxidants N-acetylcysteine, ascorbate, and alpha-tocopherol not only fail to stabilize IRP2 but, furthermore, promote its proteasomal degradation. The pathway for IRP2 degradation is saturable, which may explain earlier data supporting the "cysteine oxidation model," and shows remarkable similarities with the degradation of the hypoxia-inducible factor 1 alpha (HIF-1 alpha): dimethyl-oxalylglycine, a specific inhibitor of 2-oxoglutarate-dependent oxygenases, stabilizes IRP2 following the administration of iron to iron-deficient cells. Our results challenge the current model for IRP2 regulation and provide direct pharmacological evidence for the involvement of 2-oxoglutarate-dependent oxygenases in a pathway for IRP2 degradation.

Genetic ablations of iron regulatory proteins 1 and 2 reveal why iron regulatory protein 2 dominates iron homeostasis

The two iron regulatory proteins IRP1 and IRP2 bind to transcripts of ferritin, transferrin receptor and other target genes to control the expression of iron metabolism proteins at the post-transcriptional level. Here we compare the effects of genetic ablation of IRP1 to IRP2 in mice. IRP1-/- mice misregulate iron metabolism only in the kidney and brown fat, two tissues in which the endogenous expression level of IRP1 greatly exceeds that of IRP2, whereas IRP2-/- mice misregulate the expression of target proteins in all tissues. Surprisingly, the RNA-binding activity of IRP1 does not increase in animals on a low-iron diet that is sufficient to activate IRP2. In animal tissues, most of the bifunctional IRP1 is in the form of cytosolic aconitase rather than an RNA-binding protein. Our findings indicate that the small RNA-binding fraction of IRP1, which is insensitive to cellular iron status, contributes to basal mammalian iron homeostasis, whereas IRP2 is sensitive to iron status and can compensate for the loss of IRP1 by increasing its binding activity. Thus, IRP2 dominates post-transcriptional regulation of iron metabolism in mammals.

Mechanisms of TfR-mediated transcytosis and sorting in epithelial cells and applications toward drug delivery

Transferrin receptor has been an important protein for many of the advances made in understanding the intricacies of the intramolecular sorting pathways of endocytosed molecules. The unique internalization and recycling functions of transferrin receptor have also made it an attractive choice for drug targeting and delivery of large protein-based therapeutics and toxins. Recent advances in elucidating the role of the intracellular controllers of transferrin recycling and sorting, such as Rab proteins and their effectors, have led to enhancement of transferrin receptor as a drug delivery vehicle. This review focuses on the use of transferrin receptor as an agent for facilitating drug delivery and targeting, and the role that mechanisms of transferrin receptor sorting and transcytosis play in these events.

Analysis of the biologic functions of H- and L-ferritins in HeLa cells by transfection with siRNAs and cDNAs: evidence for a proliferative role of L-ferritin

We describe the use of small interfering RNAs (siRNAs) to down-regulate H- and L-ferritin levels in HeLa cells. siRNAs repressed H- and L-ferritin expression to about 20% to 25% of the background level in both stable and transient transfections. HeLa cells transfected with H- and L-ferritin cDNAs were analyzed in parallel to compare the effects of ferritin up- and down-regulation. We found that large modifications of L-ferritin levels did not affect iron availability in HeLa cells but positively affected cell proliferation rate in an iron-independent manner. The transient down-regulation of H-ferritin modified cellular iron availability and resistance to oxidative damage, as expected. In contrast, the stable suppression of H-ferritin in HeLa cell clones transfected with siRNAs did not increase cellular iron availability but made cells less resistant to iron supplementation and chelation. The results indicate that L-ferritin has no direct effects on cellular iron homeostasis in HeLa cells, while it has new, iron-unrelated functions. In addition, they suggest that H-ferritin function is to act as an iron buffer.

A phosphomimetic mutation at Ser-138 renders iron regulatory protein 1 sensitive to iron-dependent degradation

Iron regulatory protein 1 (IRP1) binds to mRNA iron-responsive elements (IREs) and thereby controls the expression of IRE-containing mRNAs. In iron-replete cells, assembly of a cubane [4Fe-4S] cluster inhibits IRE-binding activity and converts IRP1 to a cytosolic aconitase. Earlier experiments with Saccharomyces cerevisiae suggested that phosphomimetic mutations of Ser-138 negatively affect the stability of the cluster (N. M. Brown, S. A. Anderson, D. W. Steffen, T. B. Carpenter, M. C. Kennedy, W. E. Walden, and R. S. Eisenstein, Proc. Natl. Acad. Sci. USA 95:15235-15240, 1998). Along these lines, we show here that a highly purified preparation of recombinant human IRP1 bearing a phosphomimetic S138E substitution (IRP1(S138E)) lacks aconitase activity, which is a hallmark of [4Fe-4S] cluster integrity. Similarly, IRP1(S138E) expressed in mammalian cells fails to function as aconitase. Furthermore, we demonstrate that the impairment of [4Fe-4S] cluster assembly in mammalian cells sensitizes IRP1(S138E) to iron-dependent degradation. This effect can be completely blocked by the iron chelator desferrioxamine or by the proteasome inhibitors MG132 and lactacystin. As expected, the stability of wild-type or phosphorylation-deficient IRP1(S138A) is not affected by iron manipulations. Ser-138 and flanking sequences appear to be highly conserved in the IRP1s of vertebrates, whereas insect IRP1 orthologues and nonvertebrate IRP1-like molecules contain an S138A substitution. Our data suggest that phosphorylation of Ser-138 may provide a basis for an additional mechanism for the control of vertebrate IRP1 activity at the level of protein stability.

Iron regulatory proteins as NO signal transducers

The iron regulatory proteins (IRPs) are an example of different proteins regulating the same metabolic process, iron uptake and metabolism. IRP1 is an iron-sulfur cluster-containing protein that can be converted from a cytosolic aconitase to an RNA binding posttranscriptional regulator in response to nitric oxide (NO). IRP2 lacks aconitase activity and its expression is decreased by NO signaling. In macrophages, NO is produced in response to such inflammatory ligands as interferon-gamma, which is expressed in response to mitogenic and antigenic stimuli, and lipopolysaccharide, a marker of bacterial invasion. Until recently, research results predict that the cellular response to increased NO production should be a decrease in ferritin synthesis, due to IRP1 binding to ferritin mRNA, and an increase in transferrin receptor biosynthesis, due to IRP1 binding to the transferrin mRNA. Surprisingly, however, macrophages exhibit decreased transferrin receptor concentration in response to inflammatory ligands. Bouton and Drapier discuss the physiological role and the mechanisms that may underlie this contradictory response.

Anthracyclines induce accumulation of iron in ferritin in myocardial and neoplastic cells: inhibition of the ferritin iron mobilization pathway

Anthracyclines are potent antitumor agents that cause cardiotoxicity at high cumulative doses. Because anthracycline cardiotoxicity is attributed to their ability to avidly bind iron (Fe), we examined the effect of anthracyclines on intracellular Fe trafficking in neoplastic cells and differentiated cardiomyocytes. In both cell types, incubation with doxorubicin (DOX) resulted in a significant (p < 0.004) accumulation of Fe in the storage protein, ferritin. Pulse-chase experiments using control cells demonstrated that within 6 h, the majority of (59)Fe donated from transferrin was incorporated into ferritin. Over longer incubation periods up to 18 to 24 h, (59)Fe was subsequently mobilized from ferritin into other compartments in control cells. However, anthracyclines inhibited ferritin-(59)Fe redistribution during the 18- to 24-h period, resulting in a significant (p < 0.0003) 3- to 5-fold accumulation of ferritin-(59)Fe compared with control cells. The increase in ferritin-(59)Fe after a 24-h incubation with DOX could not be correlated with increased ferritin expression, suggesting that (59)Fe accumulation occurred in pre-existing ferritin. In addition to DOX, other redox-cycling agents (i.e., menadione and paraquat) also increased ferritin-(59)Fe levels. Moreover, the intracellular superoxide scavenger, Mn(III) tetrakis(4-benzoic acid)-porphyrin complex, partially prevented the ability of DOX and menadione at inducing this effect. Hence, superoxide generation by these compounds could play a role in causing ferritin-(59)Fe accumulation. This study is the first to demonstrate the effect of anthracyclines at inhibiting Fe mobilization from ferritin, resulting in marked Fe accumulation within the molecule. This response may have consequences in terms of the cytotoxic effects of anthracyclines.

The haemochromatosis protein HFE induces an apparent iron-deficient phenotype in H1299 cells that is not corrected by co-expression of beta 2-microglobulin

HFE, an atypical MHC class I type molecule, has a critical, yet still elusive function in the regulation of systemic iron metabolism. HFE mutations are linked to hereditary haemochromatosis type 1, a common autosomal recessive disorder of iron overload. Most patients are homozygous for a C282Y point mutation that abrogates the interaction of HFE with beta(2)-microglobulin (beta(2)M) and, thus, impairs its proper processing and expression on the cell surface. An H63D substitution is also associated with disease. To investigate the function of HFE we have generated clones of human H1299 lung cancer cells that express wild-type, C282Y or H63D HFE under the control of a tetracycline-inducible promoter. Consistent with earlier observations in other cell lines, the expression of wild-type or H63D, but not C282Y, HFE induces an apparent iron-deficient phenotype, manifested in the activation of iron-regulatory protein and concomitant increase in transferrin receptor levels and decrease in ferritin content. This phenotype persists in cells expressing wild-type HFE after transfection with a beta(2)M cDNA. Whereas endogenous beta(2)M is sufficient for the presentation of at least a fraction of chimeric HFE on the cell surface, this effect is stimulated by approx. 2.8-fold in beta(2)M transfectants. The co-expression of exogenous beta(2)M does not significantly affect the half-life of HFE. These results suggest that the apparent iron-deficient phenotype elicited by HFE is not linked to beta(2)M insufficiency.

Post-transcriptional regulation of human iron metabolism by iron regulatory proteins

In mammalian iron metabolism, ferritin, transferrin receptor and several other iron metabolism genes are post-transcriptionally regulated. Iron regulatory proteins 1 and 2 are cytosolic proteins that bind to RNA stem-loops known as iron-responsive elements in several transcripts. We have studied the role of these proteins in knockout mice and discovered that misregulation of iron metabolism can be a primary cause of neurodegeneration.

The prolyl 4-hydroxylase inhibitor ethyl-3,4-dihydroxybenzoate generates effective iron deficiency in cultured cells

Ethyl-3,4-dihydroxybenzoate (EDHB) is commonly utilized as a substrate analog and competitive inhibitor of prolyl 4-hydroxylases. These iron-dependent enzymes have received a lot of attention for their involvement in crucial biochemical pathways such as collagen maturation and oxygen sensing. Since EDHB is also capable of chelating the enzyme-bound iron, we study here its function as a chelator. We show that the affinity of EDHB for ferric iron is significantly lower than that of desferrioxamine. Nevertheless, EDHB is sufficient to promote effective iron deficiency in cells, reflected in the activation of the iron-responsive element/iron regulatory protein regulatory network. Thus, treatment of B6 fibroblasts with EDHB results in slow activation of iron regulatory protein 1 accompanied by an increase in transferrin receptor levels and reduction of the ferritin pool.