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

Inhibition of IRP2-dependent reprogramming of iron metabolism suppresses tumor growth in colorectal cancer

BACKGROUND

Dysregulation of iron metabolism is implicated in malignant transformation, cancer progression, and therapeutic resistance. Here, we demonstrate that iron regulatory protein 2 (IRP2) preferentially regulates iron metabolism and promotes tumor growth in colorectal cancer (CRC).

METHODS

IRP2 knockdown and knockout cells were generated using RNA interference and clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 methodologies, respectively. Cell viability was evaluated using both CCK-8 assay and cell counting techniques. Furthermore, IRP2 inhibition was determined by surface plasmon resonance (SPR) and RNA immunoprecipitation (IP). The suppressive effects of IRP2 were also corroborated in both organoid and mouse xenograft models, providing a comprehensive validation of IRP2's role.

RESULTS

We have elucidated the role of IRP2 as a preferential regulator of iron metabolism, actively promoting tumorigenesis within CRC. Elevated levels of IRP2 expression in patient samples are correlated with diminished overall survival, thereby reinforcing its potential role as a prognostic biomarker. The functional suppression of IRP2 resulted in a pronounced delay in tumor growth. Building on this proof of concept, we have developed IRP2 inhibitors that significantly reduce IRP2 expression and hinder its interaction with iron-responsive elements in key iron-regulating proteins, such as ferritin heavy chain 1 (FTH1) and transferrin receptor (TFRC), culminating in iron depletion and a marked reduction in CRC cell proliferation. Furthermore, these inhibitors are shown to activate the AMPK-ULK1-Beclin1 signaling cascade, leading to cell death in CRC models.

CONCLUSIONS

Collectively, these findings highlight the therapeutic potential of targeting IRP2 to exploit the disruption of iron metabolism in CRC, presenting a strategic advancement in addressing a critical area of unmet clinical need.

FGFR1 governs iron homeostasis via regulating intracellular protein degradation pathways of IRP2 in prostate cancer cells

The acquisition of ectopic fibroblast growth factor receptor 1 (FGFR1) expression is well documented in prostate cancer (PCa) progression, notably in conferring tumor growth advantage and facilitating metastasis. However, how FGFR1 contributes to PCa progression is not fully revealed. Here we report that ectopic FGFR1 in PCa cells promotes transferrin receptor 1 (TFR1) expression and expands the labile iron pool (LIP), and vice versa. We further demonstrate that FGFR1 stabilizes iron regulatory proteins 2 (IRP2) and therefore, upregulates TFR1 via promoting IRP2 binding to the IRE of TFR1. Deletion of FGFR1 in DU145 cells decreases the LIP, which potentiates the anticancer efficacy of iron chelator. Intriguingly, forced expression of IRP2 in FGFR1 depleted cells reinstates TFR1 expression and LIP, subsequently restoring the tumorigenicity of the cells. Together, our results here unravel a new mechanism by which FGFR1 drives PCa progression and suggest a potential novel target for PCa therapy.

A sporadic Parkinson's disease model via silencing of the ubiquitin-proteasome/E3 ligase component, SKP1A

Our and other's laboratory microarray-derived transcriptomic studies in human PD substantia nigra pars compacta (SNpc) samples have opened an avenue to concentrate on potential gene intersections or cross-talks along the dopaminergic (DAergic) neurodegenerative cascade in sporadic PD (SPD). One emerging gene candidate identified was SKP1A (p19, S-phase kinase-associated protein 1A), found significantly decreased in the SNpc as confirmed later at the protein level. SKP1 is part of the Skp1, Cullin 1, F-box protein (SCF) complex, the largest known class of sophisticated ubiquitin-proteasome/E3-ligases and was found to directly interact with FBXO7, a gene defective in PARK15-linked PD. This finding has led us to the hypothesis that a targeted site-specific reduction of Skp1 levels in DAergic neuronal cell culture and animal systems may result in a progressive loss of DAergic neurons and hopefully recreate motor disabilities in animals. The second premise considers the possibility that both intrinsic and extrinsic factors (e.g., manipulation of selected genes and mitochondria impairing toxins), alleged to play central roles in DAergic neurodegeneration in PD, may act in concert as modifiers of Skp1 deficiency-induced phenotype alterations ('dual-hit' hypothesis of neurodegeneration). To examine a possible role of Skp1 in DAergic phenotype, we have initially knocked down the expression of SKP1A gene in an embryonic mouse SN-derived cell line (SN4741) with short hairpin RNA (shRNA) lentiviruses (LVs). The deficiency of SKP1A closely recapitulated cardinal features of the DAergic pathology of human PD, such as decreased expression of DAergic phenotypic markers and cell cycle aberrations. Furthermore, the knocked down cells displayed a lethal phenotype when induced to differentiate exhibiting proteinaceous round inclusion structures, which were almost identical in composition to human Lewy bodies, a hallmark of PD. These findings support a role for Skp1 in neuronal phenotype, survival, and differentiation. The identification of Skp1 as a key player in DAergic neuron function suggested that a targeted site-specific reduction of Skp1 levels in mice SNpc may result in a progressive loss of DAergic neurons and terminal projections in the striatum. The injected LV SKP1shRNA to mouse SN resulted in decreased expression of Skp1 protein levels within DAergic neurons and loss of tyrosine hydroxylase immunoreactivity (TH-IR) in both SNpc and striatum that was accompanied by time-dependent motor disabilities. The reduction of the vertical movements, that is rearing, may be reminiscent of the early occurrence of hypokinesia and axial, postural instability in PD. According to the 'dual-hit' hypothesis of neurodegenerative diseases, it is predicted that gene-gene and/or gene-environmental factors would act in concert or sequentially to propagate the pathological process of PD. Our findings are compatible with this conjecture showing that the genetic vulnerability caused by knock down of SKP1A renders DAergic SN4741 cells especially sensitive to genetic reduction of Aldh1 and exposure to the external stressors MPP+ and DA, which have been implicated in PD pathology. Future consideration should be given in manipulation SKP1A expression as therapeutic window, via its induction genetically or pharmacological, to prevent degeneration of the nigra striatal dopamine neurons, since UPS is defective.

Clozapine-N-oxide protects dopaminergic neurons against rotenone-induced neurotoxicity by preventing ferritinophagy-mediated ferroptosis

Parkinson's disease (PD) is the second most common neurodegenerative disorder, yet treatment options are limited. Clozapine (CLZ), an antipsychotic used for schizophrenia, has potential as a PD treatment. CLZ and its metabolite, Clozapine-N-Oxide (CNO), show neuroprotective effects on dopaminergic neurons, with mechanisms needing further investigation. This study aimed to confirm the neuroprotective effects of CLZ and CNO in a rotenone-induced mouse model and further explore the underlying mechanisms of CNO-afforded protection. Gait pattern and rotarod activity evaluations showed motor impairments in rotenone-exposed mice, with CLZ or CNO administration ameliorating behavioral deficits. Cell counts and biochemical analysis demonstrated CLZ and CNO's effectiveness in reducing rotenone-induced neurodegeneration of dopaminergic neurons in the nigrostriatal system in mice. Mechanistic investigations revealed that CNO suppressed rotenone-induced ferroptosis of dopaminergic neurons by rectifying iron imbalances, curtailing lipid peroxidation, and mitigating mitochondrial morphological changes. CNO also reversed autolysosome and ferritinophagic activation in rotenone-exposed mice. SH-SY5Y cell cultures validated these findings, indicating ferritinophage involvement, where CNO-afforded protection was diminished by ferritinophagy enhancers. Furthermore, knockdown of NCOA4, a crucial cargo receptor for ferritin degradation in ferritinophagy, hampered rotenone-induced ferroptosis and NCOA4 overexpression countered the anti-ferroptotic effects of CNO. Whereas, iron-chelating agents and ferroptosis enhancers had no effect on the anti-ferritinophagic effects of CNO in rotenone-treated cells. In summary, CNO shielded dopaminergic neurons in the rotenone-induced PD model by modulating NCOA4-mediated ferritinophagy, highlighting a potential therapeutic pathway for PD treatment. This research provided insights into the role of NCOA4 in ferroptosis and suggested new approaches for PD therapy.

Induction of Hepcidin Expression in the Renal Cortex of Sickle Cell Disease Mice

In patients with sickle cell disease (SCD), chronic hemolysis and frequent blood transfusions cause iron overload and accumulation in the kidneys. The iron deposition is found in the renal cortex and correlates with the severity of hemolysis. In this study, we observed a significant accumulation of iron in the renal cortex of a mouse model of SCD, and assessed the expression of the proteins involved in maintaining renal iron homeostasis. Despite the intracellular iron accumulation, the levels of the transferrin receptor in the kidneys were increased, but the levels of the iron exporter ferroportin were not altered in SCD mice. Ferroportin is regulated by hepcidin, which binds to it and promotes its degradation. We found reduced serum hepcidin levels but increased renal hepcidin production in SCD mice. Furthermore, we observed significant macrophage infiltration and increased expression of intercellular adhesion molecule 1 in the endothelial cells of the kidneys in SCD mice. These observations correlated with elevated levels of proinflammatory cytokines IL-1β and IL-6, which can potentially stimulate hepcidin expression. Taken together, our results demonstrate that in individuals with SCD, a renal inflammation state induces renal hepcidin production that blocks the upregulation of ferroportin levels, resulting in dysregulation of iron homeostasis in the kidney and iron deposition in the renal cortex.

Detection of Ferritin Expression in Soft Tissue Sarcomas With MRI: Potential Implications for Iron Metabolic Therapy

BACKGROUND

Cancer cells often have altered iron metabolism relative to non-malignant cells with increased transferrin receptor and ferritin expression. Targeting iron regulatory proteins as part of a cancer therapy regimen is currently being investigated in various malignancies. Anti-cancer therapies that exploit the differences in iron metabolism between malignant and non-malignant cells (e.g. pharmacological ascorbate and iron chelation therapy) have shown promise in various cancers, including glioblastoma, lung, and pancreas cancers. Non-invasive techniques that probe tissue iron metabolism may provide valuable information for the personalization of iron-based cancer therapies. T₂* mapping is a clinically available MRI technique that assesses tissue iron content in the heart and liver. We aimed to investigate the capacity of T₂* mapping to detect iron stores in soft tissue sarcomas (STS).

METHODS

In this study, we evaluated T₂* relaxation times ex vivo in five STS samples from subjects enrolled on a phase Ib/IIa clinical trial combining pharmacological ascorbate with neoadjuvant radiation therapy. Iron protein expression levels (ferritin, transferrin receptor, iron response protein 2) were evaluated by Western blot analysis. Bioinformatic data relating clinical outcomes in STS patients and iron protein expression levels were evaluated using the KMplotter database.

RESULTS

There was a high level of inter-subject variability in the expression of iron protein and T₂* relaxation times. We identified that T₂* relaxation time is capable of accurately detecting ferritin-heavy chain expression (r = -0.96) in these samples. Bioinformatic data acquired from the KMplot database revealed that transferrin receptor and iron-responsive protein 2 may be negative prognostic markers while ferritin expression may be a positive prognostic marker in the management of STS.

CONCLUSION

These data suggest that targeting iron regulatory proteins may provide a therapeutic approach to enhance STS management. Additionally, T₂* mapping has the potential to be used a clinically accessible, non-invasive marker of STS iron regulatory protein expression and influence cancer therapy decisions that warrants further investigation. Level of Evidence: IV.

Aging is associated with increased brain iron through cortex-derived hepcidin expression

Iron is an essential molecule for biological processes, but its accumulation can lead to oxidative stress and cellular death. Due to its oxidative effects, iron accumulation is implicated in the process of aging and neurodegenerative diseases. However, the mechanism for this increase in iron with aging, and whether this increase is localized to specific cellular compartment(s), are not known. Here, we measured the levels of iron in different tissues of aged mice, and demonstrated that while cytosolic non-heme iron is increased in the liver and muscle tissue, only the aged brain cortex exhibits an increase in both the cytosolic and mitochondrial non-heme iron. This increase in brain iron is associated with elevated levels of local hepcidin mRNA and protein in the brain. We also demonstrate that the increase in hepcidin is associated with increased ubiquitination and reduced levels of the only iron exporter, ferroportin-1 (FPN1). Overall, our studies provide a potential mechanism for iron accumulation in the brain through increased local expression of hepcidin, and subsequent iron accumulation due to decreased iron export. Additionally, our data support that aging is associated with mitochondrial and cytosolic iron accumulation only in the brain and not in other tissues.

Dietary Iron Overload and Hfe-/- Related Hemochromatosis Alter Hepatic Mitochondrial Function

Iron is an essential co-factor for many cellular metabolic processes, and mitochondria are main sites of utilization. Iron accumulation promotes production of reactive oxygen species (ROS) via the catalytic activity of iron species. Herein, we investigated the consequences of dietary and genetic iron overload on mitochondrial function. C57BL/6N wildtype and Hfe^-/- mice, the latter a genetic hemochromatosis model, received either normal diet (ND) or high iron diet (HI) for two weeks. Liver mitochondrial respiration was measured using high-resolution respirometry along with analysis of expression of specific proteins and ROS production. HI promoted tissue iron accumulation and slightly affected mitochondrial function in wildtype mice. Hepatic mitochondrial function was impaired in Hfe^-/- mice on ND and HI. Compared to wildtype mice, Hfe^-/- mice on ND showed increased mitochondrial respiratory capacity. Hfe^-/- mice on HI showed very high liver iron levels, decreased mitochondrial respiratory capacity and increased ROS production associated with reduced mitochondrial aconitase activity. Although Hfe^-/- resulted in increased mitochondrial iron loading, the concentration of metabolically reactive cytoplasmic iron and mitochondrial density remained unchanged. Our data show multiple effects of dietary and genetic iron loading on mitochondrial function and linked metabolic pathways, providing an explanation for fatigue in iron-overloaded hemochromatosis patients, and suggests iron reduction therapy for improvement of mitochondrial function.

Iron overload inhibits BMP/SMAD and IL-6/STAT3 signaling to hepcidin in cultured hepatocytes

Hepcidin is a peptide hormone that targets the iron exporter ferroportin, thereby limiting iron entry into the bloodstream. It is generated in hepatocytes mainly in response to increased body iron stores or inflammatory cues. Iron stimulates expression of bone morphogenetic protein 6 (BMP6) from liver sinusoidal endothelial cells, which in turn binds to BMP receptors on hepatocytes and induces the SMAD signaling cascade for transcriptional activation of the hepcidin-encoding HAMP mRNA. SMAD signaling is also essential for inflammatory HAMP mRNA induction by the IL-6/STAT3 pathway. Herein, we utilized human Huh7 hepatoma cells and primary murine hepatocytes to assess the effects of iron perturbations on signaling to hepcidin. Iron chelation appeared to slightly impair signaling to hepcidin. Subsequent iron supplementation not only failed to reverse these effects, but drastically reduced basal HAMP mRNA and inhibited HAMP mRNA induction by BMP6 and/or IL-6. Thus, treatment of cells with excess iron inhibited basal and BMP6-mediated SMAD5 phosphorylation and induction of HAMP, ID1 and SMAD7 mRNAs in a dose-dependent manner. Iron also inhibited IL-6-mediated STAT3 phosphorylation and induction of HAMP and SOCS3 mRNAs. These responses were accompanied by induction of GCLC and HMOX1 mRNAs, known markers of oxidative stress. We conclude that hepatocellular iron overload suppresses hepcidin by inhibiting the SMAD and STAT3 signaling pathways downstream of their respective ligands.

Inflammation drives alternative first exon usage to regulate immune genes including a novel iron-regulated isoform of Aim2

Determining the layers of gene regulation within the innate immune response is critical to our understanding of the cellular responses to infection and dysregulation in disease. We identified a conserved mechanism of gene regulation in human and mouse via changes in alternative first exon (AFE) usage following inflammation, resulting in changes to the isoforms produced. Of these AFE events, we identified 95 unannotated transcription start sites in mice using a de novo transcriptome generated by long-read native RNA-sequencing, one of which is in the cytosolic receptor for dsDNA and known inflammatory inducible gene, Aim2. We show that this unannotated AFE isoform of Aim2 is the predominant isoform expressed during inflammation and contains an iron-responsive element in its 5′UTR enabling mRNA translation to be regulated by iron levels. This work highlights the importance of examining alternative isoform changes and translational regulation in the innate immune response and uncovers novel regulatory mechanisms of Aim2.

Ferritinophagy is involved in the zinc oxide nanoparticles-induced ferroptosis of vascular endothelial cells

Zinc oxide nanoparticles (ZnONPs) hold great promise for biomedical applications. Previous studies have revealed that ZnONPs exposure can induce toxicity in endothelial cells, but the underlying mechanisms have not been fully elucidated. In this study, we report that ZnONPs can induce ferroptosis of both HUVECs and EA.hy926 cells, as evidenced by the elevation of intracellular iron levels, lipid peroxidation and cell death in a dose- and time-dependent manner. In addition, both the lipid reactive oxygen species (ROS) scavenger ferrostatin-1 and the iron chelator deferiprone attenuated ZnONPs-induced cell death. Intriguingly, we found that ZnONPs-induced ferroptosis is macroautophagy/autophagy-dependent, because the inhibition of autophagy with a pharmacological inhibitor or by ATG5 gene knockout profoundly mitigated ZnONPs-induced ferroptosis. We further demonstrated that NCOA4 (nuclear receptor coactivator 4)-mediated ferritinophagy (autophagic degradation of the major intracellular iron storage protein ferritin) was required for the ferroptosis induced by ZnONPs, by showing that NCOA4 knockdown can reduce the intracellular iron level and lipid peroxidation, and subsequently alleviate ZnONPs-induced cell death. Furthermore, we showed that ROS originating from mitochondria (mtROS) probably activated the AMPK-ULK1 axis to trigger ferritinophagy. Most importantly, pulmonary ZnONPs exposure caused vascular inflammation and ferritinophagy in mice, and ferrostatin-1 supplementation significantly reversed the vascular injury induced by pulmonary ZnONPs exposure. Overall, our study indicates that ferroptosis is a novel mechanism for ZnONPs-induced endothelial cytotoxicity, and that NCOA4-mediated ferritinophagy is required for ZnONPs-induced ferroptotic cell death.

Ascorbate and Tumor Cell Iron Metabolism: The Evolving Story and Its Link to Pathology

Significance: Vitamin C or ascorbate (Asc) is a water-soluble vitamin and an antioxidant that is involved in many crucial biological functions. Asc's ability to reduce metals makes it an essential enzyme cofactor. Recent Advances: The ability of Asc to act as a reductant also plays an important part in its overall role in iron metabolism, where Asc induces both nontransferrin-bound iron and transferrin-bound iron uptake at physiological concentrations (∼50 μM). Moreover, Asc has emerged to play an important role in multiple diseases and its effects at pharmacological doses could be important for their treatment. Critical Issues: Asc's role as a regulator of cellular iron metabolism, along with its cytotoxic effects and different roles at pharmacological concentrations, makes it a candidate as an anticancer agent. Ever since the controversy regarding the studies from the Mayo Clinic was finally explained, there has been a renewed interest in using Asc as a therapeutic approach toward cancer due to its minimal side effects. Numerous studies have been able to demonstrate the anticancer activity of Asc through selective oxidative stress toward cancer cells via H₂O₂ generation at pharmacological concentrations. Studies have demonstrated that Asc's cytotoxic mechanism at concentrations (>1 mM) has been associated with decreased cellular iron uptake. Future Directions: Recent studies have also suggested other mechanisms, such as Asc's effects on autophagy, polyamine metabolism, and the cell cycle. Clearly, more has yet to be discovered about Asc's mechanism of action to facilitate safe and effective treatment options for cancer and other diseases.

Genetic Analysis of EGLN1 C127S Variant in Taiwanese Parkinson’s Disease

Parkinson's disease (PD) is a neurodegenerative disorder related to nigrostriatal dopaminergic neuron degeneration and iron accumulation. As a cellular oxygen sensor, prolyl hydroxylase domain containing protein 2 (PHD2, encoded by egl-9 family hypoxia inducible factor 1, EGLN1) modifies hypoxia-inducible factor alpha (HIF-α) protein for proteasomal destruction under normoxic condition. In addition, 2-oxoglutarate- (OG-) dependent dioxygenase activity of PHD2 is involved in the oxygen and iron regulation of iron-responsive element binding protein 2 (IRP2) stability. Previously increased expression of EGLN1 was found in the substantia nigra of the parkinsonian brain. We investigated the possible role of c.380 G > C (p.C127S) of EGLN1 gene in Taiwanese patients with PD. 479 patients and 435 healthy controls were recruited. Polymerase chain reaction and BsmAI restriction enzyme analysis were applied for analysis. An association between CC genotype and reduced PD risk in the recessive model (CC vs. GG + GC) was found. Our study provides a link between EGLN1 c.380 G > C SNP and the development of PD.

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.

Chemistry and Biology of Siderophores from Marine Microbes

Microbial siderophores are multidentate Fe(III) chelators used by microbes during siderophore-mediated assimilation. They possess high affinity and selectivity for Fe(III). Among them, marine siderophore-mediated microbial iron uptake allows marine microbes to proliferate and survive in the iron-deficient marine environments. Due to their unique iron(III)-chelating properties, delivery system, structural diversity, and therapeutic potential, marine microbial siderophores have great potential for further development of various drug conjugates for antibiotic-resistant bacteria therapy or as a target for inhibiting siderophore virulence factors to develop novel broad-spectrum antibiotics. This review covers siderophores derived from marine microbes.

Dietary and Sentinel Factors Leading to Hemochromatosis

Although hereditary hemochromatosis is associated with the mutation of genes involved in iron transport and metabolism, secondary hemochromatosis is due to external factors, such as intended or unintended iron overload, hemolysis-linked iron exposure or other stress-impaired iron metabolism. The present review addresses diet-linked etiologies of hemochromatosis and their pathogenesis in the network of genes and nutrients. Although the mechanistic association to diet-linked etiologies can be complicated, the stress sentinels are pivotally involved in the pathological processes of secondary hemochromatosis in response to iron excess and other external stresses. Moreover, the mutations in these sentineling pathway-linked genes increase susceptibility to secondary hemochromatosis. Thus, the crosstalk between nutrients and genes would verify the complex procedures in the clinical outcomes of secondary hemochromatosis and chronic complications, such as malignancy. All of this evidence provides crucial insights into comprehensive clinical or nutritional interventions for hemochromatosis.

Thermodynamic and Kinetic Analyses of Iron Response Element (IRE)-mRNA Binding to Iron Regulatory Protein, IRP1

Comparison of kinetic and thermodynamic properties of IRP1 (iron regulatory protein1) binding to FRT (ferritin) and ACO2 (aconitase2) IRE-RNAs, with or without Mn²⁺, revealed differences specific to each IRE-RNA. Conserved among animal mRNAs, IRE-RNA structures are noncoding and bind Fe²⁺ to regulate biosynthesis rates of the encoded, iron homeostatic proteins. IRP1 protein binds IRE-RNA, inhibiting mRNA activity; Fe²⁺ decreases IRE-mRNA/IRP1 binding, increasing encoded protein synthesis. Here, we observed heat, 5 °C to 30 °C, increased IRP1 binding to IRE-RNA 4-fold (FRT IRE-RNA) or 3-fold (ACO2 IRE-RNA), which was enthalpy driven and entropy favorable. Mn²⁺ (50 µM, 25 °C) increased IRE-RNA/IRP1 binding (K_d) 12-fold (FRT IRE-RNA) or 6-fold (ACO2 IRE-RNA); enthalpic contributions decreased ~61% (FRT) or ~32% (ACO2), and entropic contributions increased ~39% (FRT) or ~68% (ACO2). IRE-RNA/IRP1 binding changed activation energies: FRT IRE-RNA 47.0 ± 2.5 kJ/mol, ACO2 IRE-RNA 35.0 ± 2.0 kJ/mol. Mn²⁺ (50 µM) decreased the activation energy of RNA-IRP1 binding for both IRE-RNAs. The observations suggest decreased RNA hydrogen bonding and changed RNA conformation upon IRP1 binding and illustrate how small, conserved, sequence differences among IRE-mRNAs selectively influence thermodynamic and kinetic selectivity of the protein/RNA interactions.

Tumor-initiating cells of breast and prostate origin show alterations in the expression of genes related to iron metabolism

The importance of iron in the growth and progression of tumors has been widely documented. In this report, we show that tumor-initiating cells (TICs), represented by spheres derived from the MCF7 cell line, exhibit higher intracellular labile iron pool, mitochondrial iron accumulation and are more susceptible to iron chelation. TICs also show activation of the IRP/IRE system, leading to higher iron uptake and decrease in iron storage, suggesting that level of properly assembled cytosolic iron-sulfur clusters (FeS) is reduced. This finding is confirmed by lower enzymatic activity of aconitase and FeS cluster biogenesis enzymes, as well as lower levels of reduced glutathione, implying reduced FeS clusters synthesis/utilization in TICs. Importantly, we have identified specific gene signature related to iron metabolism consisting of genes regulating iron uptake, mitochondrial FeS cluster biogenesis and hypoxic response (ABCB10, ACO1, CYBRD1, EPAS1, GLRX5, HEPH, HFE, IREB2, QSOX1 and TFRC). Principal component analysis based on this signature is able to distinguish TICs from cancer cells in vitro and also Leukemia-initiating cells (LICs) from non-LICs in the mouse model of acute promyelocytic leukemia (APL). Majority of the described changes were also recapitulated in an alternative model represented by MCF7 cells resistant to tamoxifen (TAMR) that exhibit features of TICs. Our findings point to the critical importance of redox balance and iron metabolism-related genes and proteins in the context of cancer and TICs that could be potentially used for cancer diagnostics or therapy.

Hypoxic Adaptation in the Nervous System: Promise for Novel Therapeutics for Acute and Chronic Neurodegeneration

Homeostasis is the process by which cells adapt to stress and prevent or repair injury. Unique programs have evolved to sense and activate these homeostatic mechanisms and as such, homeostatic sensors may be potent therapeutic targets. The hypoxic response mediated by hypoxia inducible factor (HIF) downstream of oxygen sensing by HIF prolyl 4-hydroxylases (PHDs) has been well-studied, revealing cell-type specific regulation of HIF stability, activity, and transcriptional targets. HIF's paradoxical roles in nervous system development, physiology, and pathology arise from its complex roles in hypoxic adaptation and normoxic biology. Understanding how to engage the hypoxic response so as to recapitulate the protective mechanism of ischemic preconditioning is a high priority. Indeed, small molecules that activate the hypoxic response provide broad neuroprotection in several clinically relevant injury models. Screens for PHD inhibitors have identified novel therapeutics for neuroprotection that are ready to proceed to clinical trials for ischemic stroke. Better understanding the mechanisms of how to engage hypoxic adaption without altering development or physiology may identify additional novel therapeutic targets for diverse acute and chronic neuropathologies.

Iron regulatory proteins and their role in controlling iron metabolism

Cellular iron homeostasis is regulated by post-transcriptional feedback mechanisms, which control the expression of proteins involved in iron uptake, release and storage. Two cytoplasmic proteins with mRNA-binding properties, iron regulatory proteins 1 and 2 (IRP1 and IRP2) play a central role in this regulation. Foremost, IRPs regulate ferritin H and ferritin L translation and thus iron storage, as well as transferrin receptor 1 (TfR1) mRNA stability, thereby adjusting receptor expression and iron uptake via receptor-mediated endocytosis of iron-loaded transferrin. In addition splice variants of iron transporters for import and export at the plasma-membrane, divalent metal transporter 1 (DMT1) and ferroportin are regulated by IRPs. These mechanisms have probably evolved to maintain the cytoplasmic labile iron pool (LIP) at an appropriate level. In certain tissues, the regulation exerted by IRPs influences iron homeostasis and utilization of the entire organism. In intestine, the control of ferritin expression limits intestinal iron absorption and, thus, whole body iron levels. In bone marrow, erythroid heme biosynthesis is coordinated with iron availability through IRP-mediated translational control of erythroid 5-aminolevulinate synthase mRNA. Moreover, the translational control of HIF2α mRNA in kidney by IRP1 coordinates erythropoietin synthesis with iron and oxygen supply. Besides IRPs, body iron absorption is negatively regulated by hepcidin. This peptide hormone, synthesized and secreted by the liver in response to high serum iron, downregulates ferroportin at the protein level and thereby limits iron absorption from the diet. Hepcidin will not be discussed in further detail here.

Iron misregulation and neurodegenerative disease in mouse models that lack iron regulatory proteins

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) are two cytosolic proteins that maintain cellular iron homeostasis by binding to RNA stem loops known as iron responsive elements (IREs) that are found in the untranslated regions of target mRNAs that encode proteins involved in iron metabolism. IRPs modify the expression of iron metabolism genes, and global and tissue-specific knockout mice have been made to evaluate the physiological significance of these iron regulatory proteins (Irps). Here, we will discuss the results of the studies that have been performed with mice engineered to lack the expression of one or both Irps and made in different strains using different methodologies. Both Irp1 and Irp2 knockout mice are viable, but the double knockout (Irp1(-/-)Irp2(-/-)) mice die before birth, indicating that these Irps play a crucial role in maintaining iron homeostasis. Irp1(-/-) mice develop polycythemia and pulmonary hypertension, and when these mice are challenged with a low iron diet, they die early of abdominal hemorrhages, suggesting that Irp1 plays an essential role in erythropoiesis and in the pulmonary and cardiovascular systems. Irp2(-/-) mice develop microcytic anemia, erythropoietic protoporphyria and a progressive neurological disorder, indicating that Irp2 has important functions in the nervous system and erythropoietic homeostasis. Several excellent review articles have recently been published on Irp knockout mice that mainly focus on Irp1(-/-) mice (referenced in the introduction). In this review, we will briefly describe the phenotypes and physiological implications of Irp1(-/-) mice and discuss the phenotypes observed for Irp2(-/-) mice in detail with a particular emphasis on the neurological problems of these mice.

MtDNA mutagenesis impairs elimination of mitochondria during erythroid maturation leading to enhanced erythrocyte destruction

Haematopoietic progenitor cells show special sensitivity to mitochondrial DNA (mtDNA) mutagenesis, which suggests that increased mtDNA mutagenesis could underlie anemias. Here we show that elevated mtDNA mutagenesis in mice with a proof-reading deficient mtDNA polymerase (PolG) leads to incomplete mitochondrial clearance, with asynchronized iron loading in erythroid precursors, and increased total and free cellular iron content. The resulting Fenton chemistry leads to oxidative damage and premature destruction of erythrocytes by splenic macrophages. Our data indicate that mitochondria actively contribute to their own elimination in reticulocytes and modulate iron loading. Asynchrony of this sequence of events causes severe mitochondrial anaemia by depleting the organism of red blood cells and the bone marrow of iron. Our findings account for the anaemia development in a progeroid mouse model and may have direct relevance to the anemias associated with human mitochondrial disease and ageing.

Electrophoretic mobility shift assay (EMSA) for the study of RNA-protein interactions: the IRE/IRP example

RNA/protein interactions are critical for post-transcriptional regulatory pathways. Among the best-characterized cytosolic RNA-binding proteins are iron regulatory proteins, IRP1 and IRP2. They bind to iron responsive elements (IREs) within the untranslated regions (UTRs) of several target mRNAs, thereby controlling the mRNAs translation or stability. IRE/IRP interactions have been widely studied by EMSA. Here, we describe the EMSA protocol for analyzing the IRE-binding activity of IRP1 and IRP2, which can be generalized to assess the activity of other RNA-binding proteins as well. A crude protein lysate containing an RNA-binding protein, or a purified preparation of this protein, is incubated with an excess of(32) P-labeled RNA probe, allowing for complex formation. Heparin is added to preclude non-specific protein to probe binding. Subsequently, the mixture is analyzed by non-denaturing electrophoresis on a polyacrylamide gel. The free probe migrates fast, while the RNA/protein complex exhibits retarded mobility; hence, the procedure is also called "gel retardation" or "bandshift" assay. After completion of the electrophoresis, the gel is dried and RNA/protein complexes, as well as free probe, are detected by autoradiography. The overall goal of the protocol is to detect and quantify IRE/IRP and other RNA/protein interactions. Moreover, EMSA can also be used to determine specificity, binding affinity, and stoichiometry of the RNA/protein interaction under investigation.

Impaired TLR4 and HIF expression in cystic fibrosis bronchial epithelial cells downregulates hemeoxygenase-1 and alters iron homeostasis in vitro

Hemeoxygenase-1 (HO-1), an inducible heat shock protein, is upregulated in response to multiple cellular insults via oxidative stress, lipopolysaccharides (LPS), and hypoxia. In this study, we investigated in vitro the role of Toll-like receptor 4 (TLR4), hypoxia-inducible factor 1α (HIF-1α), and iron on HO-1 expression in cystic fibrosis (CF). Immunohistochemical analysis of TLR4, HO-1, ferritin, and HIF-1α were performed on lung sections of CFTR−/− and wild-type mice. CFBE41o- and 16HBE14o- cell lines were employed for in vitro analysis via immunoblotting, immunofluorescence, real-time PCR, luciferase reporter gene analysis, and iron quantification. We observed a reduced TLR4, HIF-1α, HO-1, and ferritin in CFBE41o- cell line and CF mice. Knockdown studies using TLR4-siRNA in 16HBE14o- revealed significant decrease of HO-1, confirming the role of TLR4 in HO-1 downregulation. Inhibition of HO-1 using tin protoporphyrin in 16HBE14o- cells resulted in increased iron levels, suggesting a probable role of HO-1 in iron accumulation. Additionally, sequestration of excess iron using iron chelators resulted in increased hypoxia response element response in CFBE41o- and 16HBE14o-, implicating a role of iron in HIF-1α stabilization and HO-1. To conclude, our in vitro results demonstrate that multiple regulatory factors, such as impaired TLR4 surface expression, increased intracellular iron, and decreased HIF-1α, downregulate HO-1 expression in CFBE41o- cells.

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

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

SIRT3 regulates cellular iron metabolism and cancer growth by repressing iron regulatory protein 1

Iron metabolism is essential for many cellular processes including oxygen transport, respiration and DNA synthesis, and many cancer cells exhibit dysregulation in iron metabolism. Maintenance of cellular iron homeostasis is regulated by iron regulatory proteins (IRPs), which control the expression of iron-related genes by binding iron-responsive elements (IREs) of target mRNAs. Here, we report that mitochondrial SIRT3 regulates cellular iron metabolism by modulating IRP1 activity. SIRT3 loss increases reactive oxygen species production, leading to elevated IRP1 binding to IREs. As a consequence, IRP1 target genes, such as the transferrin receptor (TfR1), a membrane-associated glycoprotein critical for iron uptake and cell proliferation, are controlled by SIRT3. Importantly, SIRT3 deficiency results in a defect in cellular iron homeostasis. SIRT3 null cells contain high levels of iron and lose iron-dependent TfR1 regulation. Moreover, SIRT3 null mice exhibit higher levels of iron and TfR1 expression in the pancreas. We found that the regulation of iron uptake and TfR1 expression contribute to the tumor suppressive activity of SIRT3. Indeed, SIRT3 expression is negatively correlated with TfR1 expression in human pancreatic cancers. SIRT3 overexpression decreases TfR1 expression by inhibiting IRP1 and represses proliferation in pancreatic cancer cells. Our data uncover a novel role of SIRT3 in cellular iron metabolism through IRP1 regulation, and suggest that SIRT3 functions as a tumor suppressor, in part, by modulating cellular iron metabolism.

Molecular mechanisms of action and therapeutic uses of pharmacological inhibitors of HIF-prolyl 4-hydroxylases for treatment of ischemic diseases

SIGNIFICANCE

In this review, we have discussed the efficacy and effect of small molecules that act as prolyl hydroxylase domain inhibitors (PHDIs). The use of these compounds causes upregulation of the pro-angiogenic factors and hypoxia inducible factor-1α and -2α (HIF-1α and HIF-2α) to enhance angiogenic, glycolytic, erythropoietic, and anti-apoptotic pathways in the treatment of various ischemic diseases responsible for significant morbidity and mortality in humans.

RECENT ADVANCES

Sprouting of new blood vessels from the existing vasculature and surgical intervention, such as coronary bypass and stent insertion, have been shown to be effective in attenuating ischemia. However, the initial reentry of oxygen leads to the formation of reactive oxygen species that cause oxidative stress and result in ischemia/reperfusion (IR) injury. This apparent "oxygen paradox" must be resolved to combat IR injury. During hypoxia, decreased activity of PHDs initiates the accumulation and activation of HIF-1α, wherein the modulation of both PHD and HIF-1α appears as promising therapeutic targets for the pharmacological treatment of ischemic diseases.

CRITICAL ISSUES

Research on PHDs and HIFs has shown that these molecules can serve as therapeutic targets for ischemic diseases by modulating glycolysis, erythropoiesis, apoptosis, and angiogenesis. Efforts are underway to identify and synthesize safer small-molecule inhibitors of PHDs that can be administered in vivo as therapy against ischemic diseases.

FUTURE DIRECTIONS

This review presents a comprehensive and current account of the existing small-molecule PHDIs and their use in the treatment of ischemic diseases with a focus on the molecular mechanisms of therapeutic action in animal models.

Human CIA2A-FAM96A and CIA2B-FAM96B integrate iron homeostasis and maturation of different subsets of cytosolic-nuclear iron-sulfur proteins

Numerous cytosolic and nuclear proteins involved in metabolism, DNA maintenance, protein translation, or iron homeostasis depend on iron-sulfur (Fe/S) cofactors, yet their assembly is poorly defined. Here, we identify and characterize human CIA2A (FAM96A), CIA2B (FAM96B), and CIA1 (CIAO1) as components of the cytosolic Fe/S protein assembly (CIA) machinery. CIA1 associates with either CIA2A or CIA2B and the CIA-targeting factor MMS19. The CIA2B-CIA1-MMS19 complex binds to and facilitates assembly of most cytosolic-nuclear Fe/S proteins. In contrast, CIA2A specifically matures iron regulatory protein 1 (IRP1), which is critical for cellular iron homeostasis. Surprisingly, a second layer of iron regulation involves the stabilization of IRP2 by CIA2A binding or upon depletion of CIA2B or MMS19, even though IRP2 lacks an Fe/S cluster. In summary, CIA2B-CIA1-MMS19 and CIA2A-CIA1 assist different branches of Fe/S protein assembly and intimately link this process to cellular iron regulation via IRP1 Fe/S cluster maturation and IRP2 stabilization.

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

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

Prolyl 4-hydroxylases, master regulators of the hypoxia response

A decrease in oxygenation is a life-threatening situation for most organisms. An evolutionarily conserved efficient and rapid hypoxia response mechanism activated by a hypoxia-inducible transcription factor (HIF) is present in animals ranging from the simplest multicellular phylum Placozoa to humans. In humans, HIF induces the expression of more than 100 genes that are required to increase oxygen delivery and to reduce oxygen consumption. As its name indicates HIF is found at protein level only in hypoxic cells, whereas in normoxia, it is degraded by the proteasome pathway. Prolyl 4-hydroxylases, enzymes that require oxygen in their reaction, are the cellular oxygen sensors regulating the stability of HIF. In normoxia, 4-hydroxyproline residues formed in the α-subunit of HIF by these enzymes lead to its ubiquitination by the von Hippel-Lindau E3 ubiquitin ligase and immediate destruction in proteasomes thus preventing the formation of a functional HIF αβ dimer. Prolyl 4-hydroxylation is inhibited in hypoxia, facilitating the formation of the HIF dimer and activation of its target genes, such as those for erythropoietin and vascular endothelial growth factor. This review starts with a summary of the molecular and catalytic properties and individual functions of the four HIF prolyl 4-hydroxylase isoenzymes. Induction of the hypoxia response via inhibition of the HIF prolyl 4-hydroxylases may provide a novel therapeutic target in the treatment of hypoxia-associated diseases. The current status of studies aiming at such therapeutic approaches is introduced in the final part of this review.

Mammalian iron metabolism

ABSTRACT Iron is an essential transition metal for mammalian cellular and tissue viability. It is critical to supplying oxygen through heme, the mitochondrial respiratory chain, and enzymes such as ribonucleotide reductase. Mammalian organisms have evolved with the means of regulating the metabolism of iron, because if left unregulated, the resulting excess amounts of iron may induce chronic toxicities affecting multiple organ systems. Several homeostatic mechanisms exist to control the amount of intestinal dietary iron uptake, cellular iron uptake, distribution, and export. Within these processes, numerous molecular participants have been identified because of advancements in basic cell biology and efforts in disease-based research of iron storage abnormalities. For example, dietary iron uptake across the intestinal duodenal mucosa is mediated by an intramembrane divalent metal transporter 1 (DMT1), and cellular iron efflux involves ferroportin, the only known iron exporter. In addition to duodenal enterocytes, ferroportin is present in other cell types, and exports iron into plasma. Ferroportin was recently discovered to be regulated by the expression of the circulating hormone hepcidin, a small peptide synthesized in hepatocytes. These recent studies on the role of hepcidin in the regulation of dietary, cellular, and extracellular iron have led to a better understanding of the pathways by which iron balance in humans is influenced, especially its involvement in human genetic diseases of iron overload. Other important molecular pathways include iron binding to transferrin in the bloodstream for cellular delivery through the plasma membrane transferrin receptor (TfR1). In the cytosol, iron regulatory proteins 1 and 2 (IRP1 and IRP2) play a prominent role in sensing the presence of iron in order to posttranscriptionally regulate the expression of TfR1 and ferritin, two important participants in iron metabolism. From a toxicological standpoint, posttranscriptional regulation of these genes aids in the sequestration, control, and hence prevention of cytotoxic effects from free-floating nontransferrin-bound iron. Given the importance of dietary iron in normal physiology, its potential to induce chronic toxicity, and recent discoveries in the regulation of human iron metabolism by hepcidin, this review will address the regulatory mechanisms of normal iron metabolism in mammals with emphasis on dietary exposure. It is the goal of this review that this information may provide in a concise format our current understanding of major pathways and mechanisms involved in mammalian iron metabolism, which is a basis for control of iron toxicity. Such a discussion is intended to facilitate the identification of deficiencies so that future metabolic or toxicological studies may be appropriately focused. A better knowledge of iron metabolism from normal to pathophysiological conditions will ultimately broaden the spectrum of the usefulness of this information in biomedical and toxicological sciences for improving and protecting human health.

Mammalian iron metabolism and its control by iron regulatory proteins

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

Protein degradation and iron homeostasis

Regulation of both systemic and cellular iron homeostasis requires the capacity to sense iron levels and appropriately modify the expression of iron metabolism genes. These responses are coordinated through the efforts of several key regulatory factors including F-box and Leucine-rich Repeat Protein 5 (FBXL5), Iron Regulatory Proteins (IRPs), Hypoxia Inducible Factor (HIF), and ferroportin. Notably, the stability of each of these proteins is regulated in response to iron. Recent discoveries have greatly advanced our understanding of the molecular mechanisms governing iron-sensing and protein degradation within these pathways. It has become clear that iron's privileged roles in both enzyme catalysis and protein structure contribute to its regulation of protein stability. Moreover, these multiple pathways intersect with one another in larger regulatory networks to maintain iron homeostasis. This article is part of a Special Issue entitled: Cell Biology of Metals.

Exposure to nitric oxide increases the nitrosyl-iron complexes content in sorghum embryonic axes

This work was aimed to investigate nitrosyl-Fe complexes formation by reaction of endogenous ligands and Fe, in sorghum embryonic axes exposed to NO-donors. Electron paramagnetic resonance (EPR) was employed to detect the presence of nitrosyl-Fe complexes in plant embryos, as well as changes in labile iron pool (LIP). Nitrosyl-Fe complexes formation was detected in sorghum embryonic axes homogenates incubated in vitro in the presence of 1 mM of NO donors: diethylenetriamine NONOate (DETA NONOate), S-nitrosoglutathione (GSNO) and sodium nitroprusside (SNP). In axes isolated from seeds incubated in vivo in the presence of 1 mM SNP for 24 h, the content of NO was increased by 2-fold, and the EPR spectrum from mononitrosyl-Fe complexes (MNIC) was observed with a concomitant increase in the fresh weight of sorghum axes. The simultaneous exposure to deferoxamine and the NO donor precluded the increase in fresh weight observed in the presence of excess NO. While total Fe content in the axes isolated from seeds exposed to 1mM SNP was not significantly affected as compared to control axes, the LIP was increased by over 2-fold.The data reported suggest a critical role for the generation of complexes between Fe and NO when cells faced a situation leading to a significant increase in NO content. Moreover, demonstrate the presence of MNICs as one of the important components of the LIP, which could actively participate in Fe cellular mobilization.

Iron regulatory protein 2 turnover through a nonproteasomal pathway

Iron regulatory protein 2 (IRP2) controls the synthesis of many proteins involved in iron metabolism, and the level of IRP2 itself is regulated by varying the rate of its degradation. The proteasome is known to mediate degradation, with specificity conferred by an iron-sensing E3 ligase. Most studies on the degradation of IRP2 have employed cells overexpressing IRP2 and also rendered iron deficient to further increase IRP2 levels. We utilized a sensitive, quantitative assay for IRP2, which allowed study of endogenous IRP2 degradation in HEK293A cells under more physiologic conditions. We found that under these conditions, the proteasome plays only a minor role in the degradation of IRP2, with almost all the IRP2 being degraded by a nonproteasomal pathway. This new pathway is calcium-dependent but is not mediated by calpain. Elevating the cellular level of IRP2 by inducing iron deficiency or by transfection causes the proteasomal pathway to account for the major fraction of IRP2 degradation. We conclude that under physiological, iron-sufficient conditions, the steady-state level of IRP2 in HEK293A cells is regulated by the nonproteasomal pathway.

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.

Regulation of iron pathways in response to hypoxia

Constituting an integral part of a heme's porphyrin ring, iron is essential for supplying cells and tissues with oxygen. Given tight links between oxygen delivery and iron availability, it is not surprising that iron deprivation and oxygen deprivation (hypoxia) have very similar consequences at the molecular level. Under hypoxia, the expression of major iron homeostasis genes including transferrin, transferrin receptor, ceruloplasmin, and heme oxygenase-1 is activated by hypoxia-inducible factors to provide increased iron availability for erythropoiesis in an attempt to enhance oxygen uptake and delivery to hypoxic cells. Iron-response proteins (IRP1 and IRP2) and "cap-n-collar" bZIP transcriptional factors (NE-F2 p45; Nrf1, 2, and 3; Bach1 and 2) also control gene and protein expression of the key iron homeostasis proteins. In this article, we give an overview of the mechanisms by which iron pathways are regulated by hypoxia at multiple levels. In addition, potential clinical benefits of manipulating iron pathways in the hypoxia-related conditions anemia and ischemia are discussed.

Redox control of iron regulatory protein 2 stability

Iron regulatory protein 2 (IRP2) is a critical switch for cellular and systemic iron homeostasis. In iron-deficient or hypoxic cells, IRP2 binds to mRNAs containing iron responsive elements (IREs) and regulates their expression. Iron promotes proteasomal degradation of IRP2 via the F-box protein FBXL5. Here, we explored the effects of oxygen and cellular redox status on IRP2 stability. We show that iron-dependent decay of tetracycline-inducible IRP2 proceeds efficiently under mild hypoxic conditions (3% oxygen) but is compromised in severe hypoxia (0.1% oxygen). A treatment of cells with exogenous H(2)O(2) protects IRP2 against iron and increases its IRE-binding activity. IRP2 is also stabilized during menadione-induced oxidative stress. These data demonstrate that the degradation of IRP2 in iron-replete cells is not only oxygen-dependent but also sensitive to redox perturbations.

Iron regulatory proteins: from molecular mechanisms to drug development

Eukaryotic cells require iron for survival but, as an excess of poorly liganded iron can lead to the catalytic production of toxic radicals that can damage cell structures, regulatory mechanisms have been developed to maintain appropriate cell and body iron levels. The interactions of iron responsive elements (IREs) with iron regulatory proteins (IRPs) coordinately regulate the expression of the genes involved in iron uptake, use, storage, and export at the post-transcriptional level, and represent the main regulatory network controlling cell iron homeostasis. IRP1 and IRP2 are similar (but not identical) proteins with partially overlapping and complementary functions, and control cell iron metabolism by binding to IREs (i.e., conserved RNA stem-loops located in the untranslated regions of a dozen mRNAs directly or indirectly related to iron metabolism). The discovery of the presence of IREs in a number of other mRNAs has extended our knowledge of the influence of the IRE/IRP regulatory network to new metabolic pathways, and it has been recently learned that an increasing number of agents and physiopathological conditions impinge on the IRE/IRP system. This review focuses on recent findings concerning the IRP-mediated regulation of iron homeostasis, its alterations in disease, and new research directions to be explored in the near future.

Iron-sensing proteins that regulate hepcidin and enteric iron absorption

The human body cannot actively excrete excess iron. As a consequence, iron absorption must be strictly regulated to ensure adequate iron uptake and prevent toxic iron accumulation. Iron absorption is controlled chiefly by hepcidin, the iron-regulatory hormone. Produced by the liver and secreted into the circulation, hepcidin regulates iron metabolism by inhibiting iron release from cells, including duodenal enterocytes, which mediate the absorption of dietary iron. Hepcidin production increases in response to iron loading and decreases in iron deficiency. Such regulation of hepcidin expression serves to modulate iron absorption to meet body iron demand. This review discusses the proteins that orchestrate hepatic hepcidin production and iron absorption by the intestine. Emphasis is placed on the proteins that directly sense iron and how they coordinate and fine-tune the molecular, cellular, and physiologic responses to iron deficiency and overload.

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.

Iron homeostasis and its interaction with prolyl hydroxylases

The ability of iron to accept or donate electrons, coupled with the ability of oxygen to act as an electron acceptor, renders both elements essential to normal cellular biology. However, these same chemical properties allow free iron in solution to generate toxic free radicals, particularly in combination with oxygen. Thus, closely interwoven homeostatic mechanisms have evolved to regulate both iron and oxygen concentrations at the systemic and the cellular levels. Systemically, iron levels are regulated through hepcidin-mediated uptake of iron in the duodenum, whereas intracellular free-iron levels are controlled through iron-regulatory proteins (IRPs). Cardiorespiratory changes increase systemic oxygen delivery, whereas at a cellular level, many responses to altered oxygen levels are coordinated by hypoxia-inducible factor (HIF). However, the mechanisms of iron homeostasis also are regulated by oxygen availability, with alterations in both hepcidin and IRP activity. In addition, many genes involved in iron homeostasis are direct targets of HIF. Furthermore, HIF activation is modulated by intracellular iron, through regulation of hydroxylase activity, which requires iron as a cofactor. In addition, HIF-2alpha translation is controlled by IRP activity, providing another level of interdependence between iron and oxygen homeostasis.

Up-regulation of divalent metal transporter 1 in 6-hydroxydopamine intoxication is IRE/IRP dependent

Iron plays a key role in Parkinson's disease (PD). Increased iron content of the substantia nigra (SN) has been found in PD patients, and divalent metal transporter 1 (DMT1) has been shown to be up-regulated in the SN of both MPTP-induced PD models and PD patients. However, the mechanisms underlying DMT1 up-regulation are largely unknown. In the present study, we observed that in the SN of 6-hydroxydopamine (6-OHDA)-induced PD rats, DMT1 with the iron responsive element (IRE, DMT1+IRE), but not DMT1 without IRE (DMT1-IRE), was up-regulated, suggesting that increased DMT1+IRE expression might account for nigral iron accumulation in PD rats. This possibility was further assessed in an in vitro study using 6-OHDA-treated and DMT1+IRE-over-expressing MES23.5 cells. In 6-OHDA-treated MES23.5 cells, increased iron regulatory protein (IRP) 1 and IRP2 expression was observed, while silencing of IRPs dramatically diminished 6-OHDA-induced DMT1+IRE up-regulation. Pretreatment with N-acetyl-L-cysteine fully suppressed IRPs up-regulation by inhibition of 6-OHDA-induced oxidative stress. Increased DMT1+IRE expression resulted in increased iron influx by MES23.5 cells. Our data provide direct evidence that DMT1+IRE up-regulation can account for IRE/IRP-dependent 6-OHDA-induced iron accumulation initiated by 6-OHDA-induced intracellular oxidative stress and that increased levels of intracellular iron result in aggravated oxidative stress. The results of this study provide novel evidence supporting the use of anti-oxidants in the treatment of PD, with the goal of inhibiting iron accumulation by regulation of DMT1 expression.

How iron controls iron

Cells regulate iron homeostasis by posttranscriptional regulation of proteins responsible for iron uptake and storage. This requires RNA-binding activity of iron-regulatory proteins, IRP1 and IRP2. Two studies recently published in Science by Vashisht et al. (2009) and Salahudeen et al. (2009) reveal how cells adjust IRP2 activity.

Evidence for a lack of a direct transcriptional suppression of the iron regulatory peptide hepcidin by hypoxia-inducible factors

Background

Hepcidin is a major regulator of iron metabolism and plays a key role in anemia of chronic disease, reducing intestinal iron uptake and release from body iron stores. Hypoxia and chemical stabilizers of the hypoxia-inducible transcription factor (HIF) have been shown to suppress hepcidin expression. We therefore investigated the role of HIF in hepcidin regulation.

Methodology/Principal Findings

Hepcidin mRNA was down-regulated in hepatoma cells by chemical HIF stabilizers and iron chelators, respectively. In contrast, the response to hypoxia was variable. The decrease in hepcidin mRNA was not reversed by HIF-1α or HIF-2α knock-down or by depletion of the HIF and iron regulatory protein (IRP) target transferrin receptor 1 (TfR1). However, the response of hepcidin to hypoxia and chemical HIF inducers paralleled the regulation of transferrin receptor 2 (TfR2), one of the genes critical to hepcidin expression. Hepcidin expression was also markedly and rapidly decreased by serum deprivation, independent of transferrin-bound iron, and by the phosphatidylinositol 3 (PI3) kinase inhibitor LY294002, indicating that growth factors are required for hepcidin expression in vitro. Hepcidin promoter constructs mirrored the response of mRNA levels to interleukin-6 and bone morphogenetic proteins, but not consistently to hypoxia or HIF stabilizers, and deletion of the putative HIF binding motifs did not alter the response to different hypoxic stimuli. In mice exposed to carbon monoxide, hypoxia or the chemical HIF inducer N-oxalylglycine, liver hepcidin 1 mRNA was elevated rather than decreased.

Conclusions/Significance

Taken together, these data indicate that hepcidin is neither a direct target of HIF, nor indirectly regulated by HIF through induction of TfR1 expression. Hepcidin mRNA expression in vitro is highly sensitive to the presence of serum factors and PI3 kinase inhibition and parallels TfR2 expression.

Erythropoiesis: model systems, molecular regulators, and developmental programs

Human erythropoiesis is a complex multistep developmental process that begins at the level of pluripotent hematopoietic stem cells (HSCs) at bone marrow microenvironment (HSCs niche) and terminates with the production of erythrocytes (RBCs). This review covers the basic and contemporary aspects of erythropoiesis. These include the: (a) cell-lineage restricted pathways of differentiation originated from HSCs and going downward toward the blood cell development; (b) model systems employed to study erythropoiesis in culture (erythroleukemia cell lines and embryonic stem cells) and in vivo (knockout animals: avian, mice, zebrafish, and xenopus); (c) key regulators of erythropoiesis (iron, hypoxia, stress, and growth factors); (d) signaling pathways operating at hematopoietic stem cell niche for homeostatic regulation of self renewal (SCF/c-kit receptor, Wnt, Notch, and Hox) and for erythroid differentiation (HIF and EpoR). Furthermore, this review presents the mechanisms through which transcriptional factors (GATA-1, FOG-1, TAL-1/SCL/MO2/Ldb1/E2A, EKLF, Gfi-1b, and BCL11A) and miRNAs regulate gene pattern expression during erythroid differentiation. New insights regarding the transcriptional regulation of alpha- and beta-globin gene clusters were also presented. Emphasis was also given on (i) the developmental program of erythropoiesis, which consists of commitment to terminal erythroid maturation and hemoglobin production, (two closely coordinated events of erythropoieis) and (ii) the capacity of human embryonic and umbilical cord blood (UCB) stem cells to differentiate and produce RBCs in culture with highly selective media. These most recent developments will eventually permit customized red blood cell production needed for transfusion.