A regulatory module comprising G3BP1-FBXL5-IRP2 axis determines sodium arsenite-induced ferroptosis

Arsenic contamination is extremely threatening to the global public health. It was reported that sodium arsenite exposure induces serious kidney injury. However, the underlying mechanism is unclear. Ferroptosis is a newly characterized form of iron-dependent programmed cell death, which is implicated in the pathogenesis of various human diseases, including kidney injury. The lethal accumulation of iron-catalyzed lipid peroxidation is the fundamental biochemical characteristic of ferroptosis. Herein we report that sodium arsenite exposure initiates ferroptosis in mammalian HEK293, MEF and HT1080 cells, and induces ferroptosis-associated acute kidney injury in mice. RNA-binding protein G3BP1, the switch component of stress granules, is indispensable for sodium arsenite-induced ferroptosis in a stress granule-independent manner. Mechanistically, G3BP1 stabilizes IRP2, the master regulator of cellular iron homeostasis, through binding to and suppressing the translation of FBXL5 mRNA, which encodes the E3 ligase component to mediate IRP2 ubiquitination and proteasomal degradation. Sodium arsenite intoxication expedites this G3BP1-FBXL5-IRP2 axis and elevates cellular labile free iron, which is responsible for sodium arsenite exposure-induced lipid peroxidation and ferroptotic cell death. In summary, this study highlights a regulatory module comprising G3BP1-FBXL5-IRP2 axis in determining sodium arsenite-induced ferroptosis and ferroptosis-associated acute kidney injury in mice.

FBXL5 promotes lipid accumulation in alcoholic fatty liver disease by promoting the ubiquitination and degradation of TFEB

BACKGROUND

Alcoholic fatty liver disease (AFLD) is characterized by abnormal lipid droplet accumulation in liver. Epigenetic regulation plays an important role in the pathogenesis of AFLD. Comprehensive bioinformatics analysis revealed that an E3 ubiquitin ligase, F-box and leucine-rich repeats protein 5 (FBXL5), was significantly upregulated in AFLD mice.

METHODS

The mouse model of AFLD was established by feeding Lieber-DeCarli liquid diet containing ethanol. An in vitro model of AFLD was established by treating HepG2 cells with ethanol (EtOH). The FBXL5 expression was assessed by quantitative real-time PCR (qRT-PCR) and western blotting assays. The levels of triglyceride (TG), alanine aminotransferase (ALT), aspartate aminotransferase (AST), and lipid accumulation were analyzed by enzyme-linked immunosorbent assay (ELISA) and Nile red staining.

RESULTS

The FBXL5 expression was markedly up-regulated in in vivo and in vitro models of AFLD compared with controls. Functionally, FBXL5 knockdown alleviated lipid accumulation in EtOH-treated HepG2 cells. Mechanistically, FBXL5 directly interacted with transcription factor EB (TFEB) and accelerated its ubiquitination-mediated degradation. TFEB knockdown reversed the effect of FBXL5 inhibition on decreasing EtOH-induced lipid accumulation.

CONCLUSION

Our data suggest that FBXL5 promotes lipid accumulation in AFLD by promoting the ubiquitination and degradation of TFEB.

Distinct functions between ferrous and ferric iron in lung cancer cell growth

Accumulating evidence suggests an association between iron metabolism and lung cancer progression. In biological systems, iron is present in either reduced (Fe2+ ; ferrous) or oxidized (Fe3+ ; ferric) states. However, ferrous and ferric iron exhibit distinct chemical and biological properties, the role of ferrous and ferric iron in lung cancer cell growth has not been clearly distinguished. In this study, we manipulated the balance between cellular ferrous and ferric iron status by inducing gene mutations involving the FBXL5-IRP2 axis, a ubiquitin-dependent regulatory system for cellular iron homeostasis, and determined its effects on lung cancer cell growth. FBXL5 depletion (ferrous iron accumulation) was found to suppress lung cancer cell growth, whereas IRP2 depletion (ferric iron accumulation) did not suppress such growth, suggesting that ferrous iron but not ferric iron plays a suppressive role in cell growth. Mechanistically, the depletion of FBXL5 impaired the degradation of the cyclin-dependent kinase inhibitor, p27, resulting in a delay in the cell cycle at the G1/S phase. FBXL5 depletion in lung cancer cells also improved the survival of tumor-bearing mice. Overall, this study highlights the important function of ferrous iron in cell cycle progression and lung cancer cell growth.

Role of FBXL5 in redox homeostasis and spindle assembly during oocyte maturation in mice

As an E3 ubiquitin ligase, F-box and leucine-rich repeat protein 5 (FBXL5) participates in diverse biologic processes. However, the role of Fbxl5 in mouse oocyte meiotic maturation has not yet been fully elucidated. The present study revealed that mouse oocytes depleted of Fbxl5 were unable to complete meiosis, as Fbxl5 silencing led to oocyte meiotic failure with reduced rates of GVBD and polar body extrusion. In addition, Fbxl5 depletion induced aberrant mitochondrial dynamics as we noted the overproduction of reactive oxygen species (ROS) and the accumulation of phosphorylated γH2AX with Fbxl5 knockdown. We also found that Fbxl5-KD led to the abnormal accumulation of CITED2 proteins in mouse oocytes. Our in vitro ubiquitination assay showed that FBXL5 interacted with CITED2 and that it mediated the degradation of CITED2 protein through the ubiquitination-proteasome pathway. Collectively, our data revealed critical functions of FBXL5 in redox hemostasis and spindle assembly during mouse oocyte maturation.

Loss of F-Box and Leucine Rich Repeat Protein 5 (FBXL5) Expression Is Associated With Poor Survival in Patients With Hepatocellular Carcinoma After Curative Resection: A Two-institute Study

BACKGROUND/AIM

Alteration of F-box and leucine-rich repeat protein 5 (FBXL5), an iron-sensing ubiquitin ligase, might be related with carcinogenesis of hepatocellular carcinoma (HCC), by disturbing cellular iron homeostasis. However, the clinical implications of FBXL5 expression using patient samples need to be elucidated.

PATIENTS AND METHODS

We collected HCC tissue samples from two institutes: Samsung Medical Center (n=259) and Hallym University Sacred Heart Hospital (n=115) and evaluated FBXL5 expression using immunohistochemistry. Using cut-off values determined by X-tile software, association between FBXL5 expression and several clinicopathological parameters was investigated. For external validation, the Cancer Genome Atlas (TCGA) cohort was used.

RESULTS

The best cutoff value for FBXL5 IHC expression associated with recurrence-free survival (RFS) was 5%. Low FBXL5 expression was found in 18.7% of the total 374 HCCs and was associated with non-viral etiology (p=0.019). Low FBXL5 expression was related with inferior disease-specific survival (DSS, p=0.002) and RFS (p=0.001) and also was an independent prognostic factor for DSS and RFS. In addition, cases with low FBLX5 mRNA levels showed inferior DSS and RFS (p<0.001 and p=0.002, respectively) compared to high FBLX5 mRNA levels in the TCGA cohort.

CONCLUSION

Down-regulation of FBXL5 expression in HCCs might be associated with poor prognosis. FBXL5 might be a prognostic biomarker of HCCs and a potential therapeutic target in conjunction with iron homeostasis.

RNA m6A Demethylase ALKBH5 Protects Against Pancreatic Ductal Adenocarcinoma via Targeting Regulators of Iron Metabolism

Although RNA m⁶A regulators have been implicated in the tumorigenesis of several different types of tumors, including pancreatic cancer, their clinical relevance and intrinsic regulatory mechanism remain elusive. This study analyzed eight m⁶A regulators (METTL3, METTL14, WTAP, FTO, ALKBH5, and YTHDF1-3) in pancreatic ductal adenocarcinoma (PDAC) and found that only RNA m⁶A demethylase ALKBH5 serves as an independent favorable prognostic marker for this tumor. To better understand the molecular mechanism underlying the protective effect conferred by ALKBH5 against pancreatic tumorigenesis, we performed a transcriptome-wide analysis of m⁶A methylation, gene expression, and alternative splicing (AS) using the MIA PaCa-2 stable cell line with ALKBH5 overexpression. We demonstrated that ALKBH5 overexpression induced a reduction in RNA m⁶A levels globally. Furthermore, mRNAs encoding ubiquitin ligase FBXL5, and mitochondrial iron importers SLC25A28 and SLC25A37, were identified as substrates of ALKBH5. Mechanistically, the RNA stabilities of FBXL5 and SLC25A28, and the AS of SLC25A37 were affected, which led to their upregulation in pancreatic cancer cell line. Particularly, we observed that downregulation of FBXL5 in tumor samples correlated with shorter survival time of patients. Owing to FBXL5-mediated degradation, ALKBH5 overexpression incurred a significant reduction in iron-regulatory protein IRP2 and the modulator of epithelial-mesenchymal transition (EMT) SNAI1. Notably, ALKBH5 overexpression led to a significant reduction in intracellular iron levels as well as cell migratory and invasive abilities, which could be rescued by knocking down FBXL5. Overall, our results reveal a previously uncharacterized mechanism of ALKBH5 in protecting against PDAC through modulating regulators of iron metabolism and underscore the multifaceted role of m⁶A in pancreatic cancer.

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

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

An Oxygen-Dependent Interaction between FBXL5 and the CIA-Targeting Complex Regulates Iron Homeostasis

The iron-sensing protein FBXL5 is the substrate adaptor for a SKP1-CUL1-RBX1 E3 ubiquitin ligase complex that regulates the degradation of iron regulatory proteins (IRPs). Here, we describe a mechanism of FBXL5 regulation involving its interaction with the cytosolic Fe-S cluster assembly (CIA) targeting complex composed of MMS19, FAM96B, and CIAO1. We demonstrate that the CIA-targeting complex promotes the ability of FBXL5 to degrade IRPs. In addition, the FBXL5-CIA-targeting complex interaction is regulated by oxygen (O₂) tension displaying a robust association in 21% O₂ that is severely diminished in 1% O₂ and contributes to O₂-dependent regulation of IRP degradation. Together, these data identify a novel oxygen-dependent signaling axis that links IRP-dependent iron homeostasis with the Fe-S cluster assembly machinery.

Regulation of cellular iron metabolism: Iron-dependent degradation of IRP by SCFFBXL5 ubiquitin ligase

Because of essentiality and toxicity of iron in our body, iron metabolism is tightly regulated in cells. In mammalian cells, iron regulatory protein 1 and 2 (IRP1 and IRP2) are the central regulators of cellular iron metabolism. IRPs regulate iron metabolism by interacting with the RNA stem-loop structures, iron-responsive elements (IREs), found on the transcripts encoding proteins involved in iron metabolism only in iron depleted condition. It is also well-known that the ubiquitin system plays central roles in cellular iron regulation because both IRPs having the IRE binding activity are recognized and ubiquitinated by the SCF^FBXL5 ubiquitin ligase in condition of iron-replete. FBXL5, which is a substrate recognition subunit of SCF^FBXL5, senses iron availability via its hemerythrin-like domain. In this small article, current understanding of the roles of SCF^FBXL5-mediated degradation of IRPs played in cellular iron metabolism is discussed.

Oxidative Stress Regulated Iron Regulatory Protein IRP2 Through FBXL5-Mediated Ubiquitination-Proteasome Way in SH-SY5Y Cells

Iron regulatory protein 2 (IRP2) plays a key role in the cellular iron homeostasis and could be regulated by a variety of factors, such as oxidative stress, hypoxia and iron, etc. IRP2 depletion results in neurodegenerative movement disorder with the loss of neurons and accumulations of iron. Since oxidative stress extensively exists in several neurodegenerative diseases where iron accumulation also exists, it is important to clarify the mechanisms underlying the effects of oxidative stress on IRP2 expression and its consequence. 200 and 300 μM H₂O₂ could result in the reduced cell viability in SH-SY5Y cells. The intracellular levels of reactive oxygen species (ROS) were increased by 52.2 and 87.3% with 200 and 300 μM H₂O₂ treatments, respectively. The decreased levels of mitochondrial transmembrane potential (ΔΨm) were only observed in 300 μM H₂O₂-treated group. The protein levels of IRP2, but not for its mRNA levels, were observed decreased in both groups, which resulted in the lower TfR1 expression and decreased iron uptake in these cells. Pretreatment with MG132, the decreased IRP2 levels caused by H₂O₂ treatment could be antagonized. The protein levels of F box and leucine-rich repeat protein 5 (FBXL5), the only E3 ligase of IRP2, were observed decreased accordingly. When knockdown the intracellular FBXL5 levels by si-FBXL5, the protein levels of IRP2 were found increased with H₂O₂ treatment. Our results suggest that FBXL5 is involved in the degradation of IRP2 under oxidative stress in dopaminergic-like neuroblastoma cells, which implies that its role in the neuronal regulation of IRP2 in neurodegenerative diseases.

Hemerythrin E3 Ubiquitin Ligases as Negative Regulators of Iron Homeostasis in Plants

Iron (Fe) is an essential nutrient for plants, but at the same time its redox properties can make it a dangerous toxin inside living cells. Homeostasis between uptake, use and storage of Fe must be maintained at all times. A small family of unique hemerythrin E3 ubiquitin ligases found in green algae and plants play an important role in avoiding toxic Fe overload, acting as negative regulators of Fe homeostasis. Protein interaction data showed that they target specific transcription factors for degradation by the 26S proteasome. It is thought that the activity of the E3 ubiquitin ligases is controlled by Fe binding to the N-terminal hemerythrin motifs. Here, we discuss what we have learned so far from studies on the HRZ (Hemerythrin RING Zinc finger) proteins in rice, the homologous BTS (BRUTUS) and root-specific BTSL (BRUTUS-LIKE) in Arabidopsis. A mechanistic model is proposed to help focus future research questions towards a full understanding of the regulatory role of these proteins in Fe homeostasis in plants.

miR-1306-3p targets FBXL5 to promote metastasis of hepatocellular carcinoma through suppressing snail degradation

This study aimed to elucidate the effect of miR-1306-3p on metastasis of hepatocellular carcinoma (HCC) and potential mechanism involved. miR-1306-3p promoted migration and invasion of HCC in vivo and in vitro. Moreover, miR-1306-3p inhibited snail to enhance its expression via directly targeting FBXL5, thus inducing the epithelial-mesenchymal transition (EMT) in HCC. Intriguingly, miR-1306-3p expression was transcriptionally enhanced by FoxM1. Consistently, miR-1306-3p was upregulated in HCC compared with paracarcinoma and correlated with poor prognosis of HCC patients. Our researches suggest that miR-1306-3p is a tumor enhancer in regulating of HCC metastasis, and miR-1306-3p may be clinically utilized as a factor for the clinical diagnosis and prognosis of HCC.

F-box and leucine-rich repeat protein 5 promotes colon cancer progression by modulating PTEN/PI3K/AKT signaling pathway

The hyper-activation of PI3K/AKT signaling is common in many kinds of malignant tumors and promotes cell growth. Moreover, FBXL5 is reported to play an important role in the progression of gastric cancer and cervical cancer. In this view, this study aims to explore the function of FBXL5 in the progression of colon cancer and determine if PI3K/AKT signaling pathway involves in this process. Western blotting, RT-PCR, and immunohistochemistry were used to detect the expression pattern of FBXL5 in colon cancer tissues and cell lines. Immunofluorescence, Duolink, and immunoprecipitation (IP) assays were performed to evaluate the interaction between FBXL5 and PI3K/AKT signaling. Results showed that FBXL5 was elevated in colon cancer tissues and cells, which had physical interaction with PTEN protein and negatively regulated its expression, whereas positively modulated PI3K, AKT and mTOR expression and their phosphorylation. Besides, FBXL5 promoted cell proliferation and tumorigenesis and inhibited apoptosis by modulating PTEN/PI3K/AKT signaling. In conclusion, this study demonstrated that FBXL5 functioned as an oncogene in the progression of colon cancer through regulating PTEN/PI3K/AKT signaling.

A synergistic role of IRP1 and FBXL5 proteins in coordinating iron metabolism during cell proliferation

Iron-regulatory protein 1 (IRP1) belongs to a family of RNA-binding proteins that modulate metazoan iron metabolism. Multiple mechanisms are employed to control the action of IRP1 in dictating changes in the uptake and metabolic fate of iron. Inactivation of IRP1 RNA binding by iron primarily involves insertion of a [4Fe-4S] cluster by the cytosolic iron-sulfur cluster assembly (CIA) system, converting it into cytosolic aconitase (c-acon), but can also involve iron-mediated degradation of IRP1 by the E3 ligase FBXL5 that also targets IRP2. How CIA and FBXL5 collaborate to maintain cellular iron homeostasis through IRP1 and other pathways is poorly understood. Because impaired Fe-S cluster biogenesis associates with human disease, we determined the importance of FBXL5 for regulating IRP1 when CIA is impaired. Suppression of FBXL5 expression coupled with induction of an IRP1 mutant (IRP1^3C>3S) that cannot insert the Fe-S cluster, or along with knockdown of the CIA factors NUBP2 or FAM96A, reduced cell viability. Iron supplementation reversed this growth defect and was associated with FBXL5-dependent polyubiquitination of IRP1. Phosphorylation of IRP1 at Ser-138 increased when CIA was inhibited and was required for iron rescue. Impaired CIA activity, as noted by reduced c-acon activity, was associated with enhanced FBXL5 expression and a concomitant reduction in IRP1 and IRP2 protein level and RNA-binding activity. Conversely, expression of either IRP induced FBXL5 protein level, demonstrating a negative feedback loop limiting excessive accumulation of iron-response element RNA-binding activity, whose disruption reduces cell growth. We conclude that a regulatory circuit involving FBXL5 and CIA acts through both IRPs to control iron metabolism and promote optimal cell growth.

iASPP induces EMT and cisplatin resistance in human cervical cancer through miR-20a-FBXL5/BTG3 signaling

BACKGROUND

Epithelial-mesenchymal transition (EMT) and dysregulated microRNAs (miRNAs) have important roles in driving chemoresistance. We previously reported that iASPP is a key EMT inducer and could increase cisplatin resistance in cervical cancer (CC) cells. Herein, we investigate the downstream mechanisms through which iASPP contributes to EMT and cisplatin resistance in CC.

METHODS

By using a lentiviral system, we investigated the effects of iASPP knockdown on CC cell growth and chemosensitivity of CC cells to cisplatin in vivo. We examined if miR-20a, which was up-regulated following iASPP overexpression, would influence metastatic phenotypes and cisplatin resistance in CC cells, and explored the possible molecular mechanisms involved.

RESULTS

Knockdown of iASPP suppressed CC cell proliferation and sensitized CC cells to cisplatin in vivo. iASPP promotes miR-20a expression in a p53-dependent manner. Upregulation of miR-20a induced EMT and the recovery of CC cell invasion and cisplatin chemoresistance that was repressed by iASPP knockdown. We identified FBXL5 and BTG3 as two direct miR-20a targets. Silencing of FBXL5 and BTG3 restored cell invasion and cisplatin chemoresistance, which was suppressed by iASPP or miR-20a knockdown. Reduced FBXL5 and BTG3 expression was found in CC samples and associated with poor prognosis in CC patients.

CONCLUSIONS

iASPP promotes EMT and confers cisplatin resistance in CC via miR-20a-FBXL5/BTG3 signaling.

Redox sensing molecular mechanism of an iron metabolism regulatory protein FBXL5

FBXL5 is a subunit of the SCF^FBXL5 ubiquitin ligase complex that targets the proteasomal degradation of iron regulatory protein IRP2, which is an important regulator in iron metabolism. The degradation of FBXL5 itself is regulated in an iron- and oxygen-responsive manner through its diiron center containing Hr-like domain. Although the crystal structure of the Hr-like domain of FBXL5 and its degradation based on iron/oxygen sensing has been reported, the redox sensing molecular mechanism is still not clear. Herein the redox properties of FBXL5 were investigated via EPR, direct electrochemistry, SRCD, fluorescence emission spectroscopy, and redox kinetics. The results indicated that the conformation and function of FBXL5 are tuned by the redox states of the diiron center. The redox reactions of the diiron center are accompanied with conformational changes and iron release, which are associated with FBXL5 stability and degradation. These results provide insights into the redox sensing mechanism by which FBXL5 can serve as an iron metabolism regulator within mammalian cells.

FBXL5 modulates HIF-1α transcriptional activity by degradation of CITED2

CITED2 is a ubiquitously expressed nuclear protein exhibiting a high affinity for the cysteine-histidine-rich domain 1 (CH1) of the transcriptional co-activators CBP/p300. CITED2 is particularly efficient in the inhibition of the hypoxia-inducible factor-1α (HIF-1α) dependent transcription by competing with it for the interaction with the CH1 domain. Here we report a direct and specific interaction between CITED2 and the F-box and leucine rich repeat protein 5 (FBXL5), a substrate adaptor protein which is part of E3 ubiquitin ligase complexes mediating protein degradation by the proteasome. We demonstrated that depletion of FBXL5 by RNA interference led to an increase of CITED2 protein levels. Conversely, overexpression of FBXL5 caused the decrease of CITED2 protein levels in a proteasome-dependent manner, and impaired the interaction between CITED2 and the CH1 domain of p300 in living cells. In undifferentiated mouse embryonic stem cells, the overexpression of FBXL5 also reduced Cited2 protein levels. Finally, we evidenced that FBXL5 overexpression and the consequent degradation of CITED2 enabled the transcriptional activity of the N-terminal transactivation domain of HIF-1α. Collectively, our results highlighted a novel molecular interaction between CITED2 and FBXL5, which might regulate the steady state CITED2 protein levels and contribute to the modulation of gene expression by HIF-1α.

F-box and leucine-rich repeat protein 5 (FBXL5): sensing intracellular iron and oxygen

Though essential for many vital biological processes, excess iron results in the formation of damaging reactive oxygen species (ROS). Therefore, iron metabolism must be tightly regulated. F-box and leucine-rich repeat protein 5 (FBXL5), an E3 ubiquitin ligase subunit, regulates cellular and systemic iron homeostasis by facilitating iron regulatory protein 2 (IRP2) degradation. FBXL5 possesses an N-terminal hemerythrin (Hr)-like domain that mediates its own differential stability by switching between two different conformations to communicate cellular iron availability. In addition, the FBXL5-Hr domain also senses O2 availability, albeit by a distinct mechanism. Mice lacking FBXL5 fail to sense intracellular iron levels and die in utero due to iron overload and exposure to damaging levels of oxidative stress. By closely monitoring intracellular levels of iron and oxygen, FBLX5 prevents the formation of conditions that favor ROS formation. These findings suggest that FBXL5 is essential for the maintenance of iron homeostasis and is a key sensor of bioavailable iron. Here, we describe the iron and oxygen sensing mechanisms of the FBXL5 Hr-like domain and its role in mediating ROS biology.

The complex interplay of iron metabolism, reactive oxygen species, and reactive nitrogen species: insights into the potential of various iron therapies to induce oxidative and nitrosative stress

Production of minute concentrations of superoxide (O2(*-)) and nitrogen monoxide (nitric oxide, NO*) plays important roles in several aspects of cellular signaling and metabolic regulation. However, in an inflammatory environment, the concentrations of these radicals can drastically increase and the antioxidant defenses may become overwhelmed. Thus, biological damage may occur owing to redox imbalance-a condition called oxidative and/or nitrosative stress. A complex interplay exists between iron metabolism, O2(*-), hydrogen peroxide (H2O2), and NO*. Iron is involved in both the formation and the scavenging of these species. Iron deficiency (anemia) (ID(A)) is associated with oxidative stress, but its role in the induction of nitrosative stress is largely unclear. Moreover, oral as well as intravenous (iv) iron preparations used for the treatment of ID(A) may also induce oxidative and/or nitrosative stress. Oral administration of ferrous salts may lead to high transferrin saturation levels and, thus, formation of non-transferrin-bound iron, a potentially toxic form of iron with a propensity to induce oxidative stress. One of the factors that determine the likelihood of oxidative and nitrosative stress induced upon administration of an iv iron complex is the amount of labile (or weakly-bound) iron present in the complex. Stable dextran-based iron complexes used for iv therapy, although they contain only negligible amounts of labile iron, can induce oxidative and/or nitrosative stress through so far unknown mechanisms. In this review, after summarizing the main features of iron metabolism and its complex interplay with O2(*-), H2O2, NO*, and other more reactive compounds derived from these species, the potential of various iron therapies to induce oxidative and nitrosative stress is discussed and possible underlying mechanisms are proposed. Understanding the mechanisms, by which various iron formulations may induce oxidative and nitrosative stress, will help us develop better tolerated and more efficient therapies for various dysfunctions of iron metabolism.

Targeting dysregulation of brain iron homeostasis in Parkinson's disease by iron chelators

Brain iron accumulation has been implicated in a host of chronic neurological diseases, including Parkinson's disease (PD). The elevated iron levels observed in the substantia nigra of PD subjects have been suggested to incite the generation of reactive oxygen species and intracellular α-synuclein aggregation, terminating in the oxidative neuronal destruction of this brain area. Thus, elucidation of the molecular mechanisms involved in iron dysregulation and oxidative stress-induced neurodegeneration is a crucial step in deciphering PD pathology and in developing novel iron-complexing compounds aimed at restoring brain iron homeostasis and attenuating neurodegeneration. This review discusses the involvement of dysregulation of brain iron homeostasis in PD pathology, with an emphasis on the potential effectiveness of naturally occurring compounds and novel iron-chelating/antioxidant therapeutic hybrid molecules, exerting a spectrum of neuroprotective interrelated activities: antioxidant/monoamine oxidase inhibition, activation of the hypoxia-inducible factor (HIF)-1 signaling pathway, induction of HIF-1 target iron-regulatory and antioxidative genes, and inhibition of α-synuclein accumulation and aggregation.