Histological detection of catalytic ferrous iron with the selective turn-on fluorescent probe RhoNox-1 in a Fenton reaction-based rat renal carcinogenesis model

Essential roles of ferric reductase-like proteins in growth, development, stress response, and virulence of the filamentous entomopathogenic fungus Beauveria bassiana

In yeasts, ferric reductase catalyzes reduction of ferric ion to ferrous form, which is essential for the reductive iron assimilation system. However, the physiological roles of ferric reductases remain largely unknown in the filamentous fungi. In this study, genome-wide annotation revealed thirteen ferric reductase-like (Fre) proteins in the filamentous insect pathogenic fungus Beauveria bassiana, and all their functions were genetically characterized. Ferric reductase family proteins exhibit different sub-cellular distributions (e.g., cell periphery and vacuole), which was due to divergent domain architectures. Fre proteins had a synergistic effect on fungal virulence, which was ascribed to their distinct functions in different physiologies. Ten Fre proteins were not involved in reduction of ferric ion in submerged mycelia, but most proteins contributed to blastospore development. Only two Fre proteins significantly contributed to B. bassiana vegetative growth under the chemical-induced iron starvation, but most Fre proteins were involved in resistance to osmotic and oxidative stresses. Notably, a bZIP-type transcription factor HapX bound to the promoter regions of all FRE genes in B. bassiana, and displayed varying roles in the transcription activation of these genes. This study reveals the important role of BbFre family proteins in development, stress response, and insect pathogenicity, as well as their distinctive role in the absorption of ferric iron from the environment.

Retinal ferroptosis as a critical mechanism for the induction of retinochoroiditis during ocular toxoplasmosis

Toxoplasmosis is a major infectious disease, affecting approximately one-third of the world's population; its main clinical manifestation, ocular toxoplasmosis (OT), is a severe sight-threatening disease. Nevertheless, the diagnosis of OT is based on clinical findings, which needs improvement, even with biochemical tests, such as polymerase chain reaction and antibody detections. Furthermore, the efficacy of OT-targeted treatment is limited; thus, additional measures for diagnosis and treatments are needed. Here, we for the first time report a significantly reduced iron concentration in the vitreous humor (VH) of human patients infected with OT. To obtain further insights into molecular mechanisms, we established a mouse model of T. gondii infection, in which intravitreally injected tracer 57Fe, was accumulated in the neurosensory retina. T. gondii-infected eyes showed increased lipid peroxidation, reduction of glutathione peroxidase-4 expression and mitochondrial deformity in the photoreceptor as cristae loss. These findings strongly suggest the involvement of ferroptotic process in the photoreceptor of OT. In addition, deferiprone, an FDA-approved iron chelator, reduced the iron uptake but also ameliorated toxoplasma-induced retinochoroiditis by reducing retinal inflammation. In conclusion, the iron levels in the VH could serve as diagnostic markers and iron chelators as potential treatments for OT.

Three-Dimensional Regulation of Ferroptosis at the Intersection of Iron, Sulfur, and Oxygen Executing Scrap and Build Toward Evolution

Significance: Iron is an essential element for every life on earth as a primary media for electron flow. Sulfur compounds as sulfhydryls counteract catalytic activity of iron whereas sulfur overdose is also toxic. In aerobic organisms, oxygen is the major media for electron transfer with higher intracellular mobility, which cooperates with the iron system. Based on the importance of iron, there is no active pathway to excrete iron outside the body in higher species. Whereas bacterial infection causes a scramble for iron in situ, cancer can be the outcome of the side effects of long use of iron and oxygen. Recent Advances: Ferroptosis is a recently coined cell death, defined as catalytic Fe(II)-dependent regulated necrosis accompanied by lipid peroxidation. Researchers recently recognized that ferroptosis is involved in a variety of physiological and pathological contexts, including embryonic erythropoiesis, aging, neurodegeneration and cancer cell death. Alternatively, carcinogenesis is a process to obtain iron addiction with ferroptosis-resistance, based on rodent animal studies. Critical Issues: Here we propose that ferroptosis is three-dimensionally regulated by iron, sulfur and oxygen, which correspond to oxidants, antioxidants and membrane fluidity with susceptibility to lipid peroxidation, respectively. Future Directions: Whereas life attempts to prevent ferroptosis, ferroptotic cells eventually emit iron-loaded ferritin as extracellular vesicles to maintain monopoly of iron. Antioxid. Redox Signal. 39, 807-815.

Iron-Induced Oxidative Stress in Human Diseases

Iron is responsible for the regulation of several cell functions. However, iron ions are catalytic and dangerous for cells, so the cells sequester such redox-active irons in the transport and storage proteins. In systemic iron overload and local pathological conditions, redox-active iron increases in the human body and induces oxidative stress through the formation of reactive oxygen species. Non-transferrin bound iron is a candidate for the redox-active iron in extracellular space. Cells take iron by the uptake machinery such as transferrin receptor and divalent metal transporter 1. These irons are delivered to places where they are needed by poly(rC)-binding proteins 1/2 and excess irons are stored in ferritin or released out of the cell by ferroportin 1. We can imagine transit iron pool in the cell from iron import to the export. Since the iron in the transit pool is another candidate for the redox-active iron, the size of the pool may be kept minimally. When a large amount of iron enters cells and overflows the capacity of iron binding proteins, the iron behaves as a redox-active iron in the cell. This review focuses on redox-active iron in extracellular and intracellular spaces through a biophysical and chemical point of view.

Methionine Deprivation Reveals the Pivotal Roles of Cell Cycle Progression in Ferroptosis That Is Induced by Cysteine Starvation

Ferroptosis, a type of iron-dependent necrotic cell death, is triggered by the accumulation of excessive lipid peroxides in cells. Glutathione (GSH), a tripeptide redox molecule that contains a cysteine (Cys) unit in the center, plays a pivotal role in protection against ferroptosis. When the transsulfuration pathway is activated, the sulfur atom of methionine (Met) is utilized to generate Cys, which can then suppress Cys-starvation-induced ferroptosis. In the current study, we cultured HeLa cells in Met- and/or cystine (an oxidized Cys dimer)- deprived medium and investigated the roles of Met in ferroptosis execution. The results indicate that, in the absence of cystine or Met, ferroptosis or cell cycle arrest, respectively, occurred. Contrary to our expectations, however, the simultaneous deprivation of both Met and cystine failed to induce ferroptosis, although the intracellular levels of Cys and GSH were maintained at low levels. Supplementation with S-adenosylmethionine (SAM), a methyl group donor that is produced during the metabolism of Met, caused the cell cycle progression to resume and lipid peroxidation and the subsequent induction of ferroptosis was also restored under conditions of Met/cystine double deprivation. DNA methylation appeared to be involved in the resumption in the SAM-mediated cell cycle because its downstream metabolite S-adenosylhomocysteine failed to cause either cell cycle progression or ferroptosis to be induced. Taken together, our results suggest that elevated lipid peroxidation products that are produced during cell cycle progression are involved in the execution of ferroptosis under conditions of Cys starvation.

Persistent ferroptosis promotes cervical squamous intraepithelial lesion development and oncogenesis by regulating KRAS expression in patients with high risk-HPV infection

Cervical squamous cell carcinoma (CSCC) is a type of female cancer that affects millions of families worldwide. Human papillomavirus (HPV) infection is the main reason for CSCC formation, and squamous intraepithelial lesions (SILs) induced by high-risk HPV (HR-HPV) infection are considered precancerous lesions. A previous study reported that HPV-infected cancer cells were able to counteract lipid peroxidation for survival. Recent research has reported that ferroptosis acts in an iron-dependent lipid peroxidation manner to kill cancer cells, and it is proposed as a new approach for female cancer therapy. Here, we investigated the role of ferroptosis throughout SIL development into CSCC. We found that ferroptosis occurred in SIL, but anti-ferroptosis emerged in CSCC. Our data further indicated that an antiferroptotic effect was formed in response to persistent ferroptosis and then promoted oncogenesis. Altogether, we provide novel insight into ferroptosis in cervical SIL development and suggest a potential therapeutic target for the treatment of CSCC.

Selective Metal Chelation by a Thiosemicarbazone Derivative Interferes with Mitochondrial Respiration and Ribosome Biogenesis in Candida albicans

Metal chelation is generally considered as a promising antifungal approach but its specific mechanisms are unclear. Here, we identify 13 thiosemicarbazone derivatives that exert broad-spectrum antifungal activity with potency comparable or superior to that of fluconazole in vitro by screening a small compound library comprising 89 thiosemicarbazone derivatives as iron chelators. Among the hits, 19ak exhibits minimal cytotoxicity and potent activity against either azole-sensitive or azole-resistant fungal pathogens. Mechanism investigations reveal that 19ak inhibits mitochondrial respiration mainly by retarding mitochondrial respiratory chain complex I activity through iron chelation, and further reduces mitochondrial membrane potential and ATP synthesis in Candida albicans. In addition, 19ak inhibits fungal ribosome biogenesis mainly by disrupting intracellular zinc homeostasis. 19ak also stimulates the activities of antioxidant enzymes and decreases reactive oxygen species formation in C. albicans, resulting in an increase in detrimental intracellular reductive stress. However, 19ak has minor effects on mammalian cells in depleting intracellular iron and zinc. Moreover, 19ak exhibits low capacity to induce drug resistance and in vivo efficacy in a Galleria mellonella infection model. These findings uncover retarded fungal mitochondrial respiration and ribosome biogenesis as downstream effects of disruption of iron and zinc homeostasis in C. albicans and provide a basis for the thiosemicarbazone 19ak in antifungal application. IMPORTANCE The increasing incidence of fungal infections and resistance to existing antifungals call for the development of broad-spectrum antifungals with novel mechanisms of action. In this study, we demonstrate that a thiosemicarbazone derivative 19ak selectively inhibits mitochondrial respiration mainly by retarding mitochondrial respiratory chain complex I activity through iron chelation and inhibits ribosome biogenesis mainly by disrupting intracellular zinc homeostasis in C. albicans. In addition, 19ak exhibits low capacity to induce fungal resistance, minimal cytotoxicity, and in vivo antifungal efficacy. This study provides the basis of thiosemicarbazone derivative 19ak as a metal chelator for the treatment of fungal infections.

Heterogeneity of ferrous iron-containing endolysosomes and effects of endolysosome iron on endolysosome numbers, sizes, and localization patterns

Endolysosomes are key regulators of iron metabolism and are central to iron trafficking and redox signaling. Iron homeostasis is linked to endolysosome acidity and inhibition of endolysosome acidity triggers iron dysregulation. Because of the physiological importance and pathological relevance of ferrous iron (Fe2+ ), we determined levels of Fe2+ specifically and quantitatively in endolysosomes as well as the effects of Fe2+ on endolysosome morphology, distribution patterns, and function. The fluorescence dye FeRhoNox-1 was specific for Fe2+ and localized to endolysosomes in U87MG astrocytoma cells and primary rat cortical neurons; in U87MG cells the endolysosome concentration of Fe2+ ([Fe2+ ]el ) was 50.4 μM in control cells, 73.6 μM in ferric ammonium citrate (FAC) treated cells, and 12.4 μM in cells treated with the iron chelator deferoxamine (DFO). Under control conditions, in primary rat cortical neurons, [Fe2+ ]el was 32.7 μM. Endolysosomes containing the highest levels of Fe2+ were located perinuclearly. Treatment of cells with FAC resulted in endolysosomes that were less acidic, increased in numbers and sizes, and located further from the nucleus; opposite effects were observed for treatments with DFO. Thus, FeRhoNox-1 is a useful probe for the study of endolysosome Fe2+ , and much more work is needed to understand better the physiological significance and pathological relevance of endolysosomes classified according to their heterogeneous iron content Cover Image for this issue: https://doi.org/10.1111/jnc.15396.

Ferristatin II, an Iron Uptake Inhibitor, Exerts Neuroprotection against Traumatic Brain Injury via Suppressing Ferroptosis

As a specific ferroptosis marker, transferrin receptor 1 (TfR1) expression is increased following traumatic brain injury (TBI), but the precise role of TfR1 in TBI-induced ferroptosis and neurodegeneration remains to be determined. To further identify more potent ferroptosis inhibitors and effective targets for treating TBI, our study aims at investigating the effects of TfR1 on ferroptosis in a mouse TBI model using ferristatin II (an iron uptake and TfR1 inhibitor). The effect of ferristatin II was first verified in the HT-22 cell line in vitro and showed antiferroptotic action when exposed to ferric citrate (FAC), which is in parallel with the results obtained from the positive controls, including deferoxamine (DFO) and liproxstatin-1 (Lip-1). In vivo, ferristatin II administration reduced the expression of TfR1 at 12 h after TBI, and immunofluorescence experiments further confirmed that this decreased TfR1-positive cells were neurons. Importantly, ferristatin II suppressed TBI-induced iron homeostatic imbalance by decreasing the content of Fe (III) and iron-positive deposits and reversed the expression of iron homeostasis-related proteins. Moreover, ferristatin II attenuated TBI-induced lipid peroxidation by reversing the expression of lipid peroxidative genes and proteins, as well as the increase in malondialdehyde (MDA) level following TBI. Finally, ferristatin II alleviated TBI-induced neuronal injury and neurodegeneration, as detected by staining with Nissl and Fluoro-Jade B, thereby exerting a neuroprotective effect. In summary, these data indicated that ferristatin II might be a potential strategy to restrain ferroptosis and develop novel therapeutic agents against TBI.

PCBP2 knockdown promotes ferroptosis in malignant mesothelioma

Malignant mesothelioma (MM) is still increasing worldwide. The pathogenesis depends on asbestos-induced iron accumulation, which eventually leads to ferroptosis-resistance of mesothelial cells via somatic mutations. Poly (rC)-binding proteins 1 and 2 (PCBP1/2) are recently recognized cytosolic Fe(II) chaperones. Here we studied the role of PCBP1/2 in rat/human mesothelial and MM cells as well as rat/human MM specimens. Normal peritoneal mesothelial cells in rats exhibited PCBP1 but not PCBP2 immunopositivity whereas primary/immortalized mesothelial cells showed PCBP1/2 immunopositivity. Rat MM specimens induced by intraperitoneal injection of chrysotile, including in situ lesion, revealed PCBP1/2 immunopositivity (90% for both) in the nucleus and cytoplasm with a tendency of higher expression in epithelioid subtype. Knockdown of PCBP2 but not PCBP1 significantly decreased both TfR1 and FTH expression in MM cells with inhibition of proliferation, indicating stagnation of intracellular iron transport. Erastin, a cysteine-deprivation type ferroptosis inducer, decreased the expression of both PCBP1/2 in MM cells. Furthermore, PCBP2 knockdown significantly increased the sensitivity of MM cells to erastin-induced ferroptosis with increased catalytic Fe(II). In conclusion, PCBP2 works for ferroptosis-resistance not only during mesothelial carcinogenesis but also in MM, which warrants further investigation as a novel therapeutic target.

Polydatin Attenuates Cisplatin-Induced Acute Kidney Injury by Inhibiting Ferroptosis

Cisplatin is widely used in the treatment of solid tumors, but its application is greatly limited due to its nephrotoxicity; thus, there is still no effective medicine for the treatment of cisplatin-induced acute kidney injury (Cis-AKI). We previously identified that polydatin (PD) exerts nephroprotective effects by antioxidative stress in AKI models. Recent evidence suggests that oxidative stress-induced molecular events overlap with the process of ferroptosis and that there are common molecular targets, such as glutathione (GSH) depletion and lipid peroxidation. Nevertheless, whether the nephroprotective effect of PD is related to anti-ferroptosis remains unclear. In this study, the inhibitory effect of PD on ferroptosis was observed in both cisplatin-treated HK-2 cells (20 μM) in vitro and a Cis-AKI mouse model (20 mg/kg, intraperitoneally) in vivo, characterized by the reversion of excessive intracellular free iron accumulation and reactive oxygen species (ROS) generation, a decrease in malondialdehyde (MDA) content and GSH depletion, and an increase in glutathione peroxidase-4 (GPx4) activity. Remarkably, PD dose-dependently alleviated cell death induced by the system Xc⁻ inhibitor erastin (10 μM), and the effect of the 40 μM dose of PD was more obvious than that of ferrostatin-1 (1 μM) and deferoxamine (DFO, 100 μM), classical ferroptosis inhibitors. Our results provide insight into nephroprotection with PD in Cis-AKI by inhibiting ferroptosis via maintenance of the system Xc⁻-GSH-GPx4 axis and iron metabolism.

Double-edged Sword Role of Iron-loaded Ferritin in Extracellular Vesicles

Human epidemiological and animal studies have demonstrated that excess iron is a risk for cancer. The responsible mechanisms are: 1) increased intracellular iron catalyzes the Fenton reaction to generate hydroxyl radicals, leading to mutagenic oxidative DNA lesions; 2) iron is necessary for cellular proliferation as cofactors of many enzymes. Thus, iron-excess milieu promotes selecting cellular evolution to ferroptosis-resistance, a major basis for carcinogenesis. Ferritin is a 24-subunit nanocage protein required for iron storage under the regulation of the iron-regulatory protein (IRP)/iron-responsive element (IRE) system. Ferritin is a serum marker, representing total body iron storage. However, how ferritin is secreted extracellularly has been unelucidated. We recently discovered that an exosomal marker CD63 is regulated by the IRP/IRE system and that iron-loaded ferritin is secreted as extracellular vesicles under the guidance of nuclear receptor coactivator 4 (NCOA4). On the other hand, we found that macrophages under asbestos-induced ferroptosis emit ferroptosis-dependent extracellular vesicles (FedEVs), which are received by nearby mesothelial cells, resulting in significant mutagenic DNA damage. Therefore, cells, including macrophages, can share excess iron with other cells, via iron-loaded ferritin packaged in extracellular vesicles as safe non-catalytic iron. However, similar process, such as one involving FedEVs, may cause accumulation of excess iron in other specific cells, which may eventually promote carcinogenesis.

Ferroptosis resistance determines high susceptibility of murine A/J strain to iron-induced renal carcinogenesis

Cancer susceptibility is a critical factor in the understanding of carcinogenesis. Intraperitoneal (i.p.) injection of an iron chelate, ferric nitrilotriacetate (Fe‐NTA), produces hydroxyl radicals via Fenton reaction to induce ferroptosis in renal proximal tubules. Rats or mice subjected to repeated i.p. injections of Fe‐NTA develop renal cell carcinoma (RCC). To elucidate the molecular mechanisms that cause susceptibility to renal carcinogenesis, we first established an inter‐strain difference in the susceptibility to Fe‐NTA‐induced renal carcinogenesis in mice. Based on a previous observation of a low incidence of RCC with this model in C57BL/6J strain mice, we investigated A/J strain mice here, which demonstrated significantly higher susceptibility to Fe‐NTA‐induced renal carcinogenesis. Homozygous deletion of the Cdkn2a/2b tumor suppressor locus was detected for the first time in A/J strain mice. Focusing on ferroptosis and iron metabolism, we explored the mechanisms involved that lead to the difference in RCC development. We compared the protective responses in the kidney of A/J and C57BL/6J strains after Fe‐NTA treatment. After 3‐week Fe‐NTA treatment, A/J mice maintained higher levels of expression of glutathione peroxidase 4 and xCT (SLC7A11), leading to a lower level of lipid peroxidation. Simultaneously, A/J mice had decreased expression of transferrin receptor and increased expression of ferritin to greater degrees than C57BL/6 mice. After a single Fe‐NTA injection, higher levels of oxidative cell damage and cytosolic catalytic Fe(II) were observed in C57BL/6J mice, accompanied by a greater increase in lipocalin‐2. Lipocalin‐2 deficiency significantly decreased oxidative renal damage. Our results suggest that a genetic trait favoring ferroptosis resistance contributes to high susceptibility to Fe‐NTA‐induced RCC in A/J strain.

Novel insights on targeting ferroptosis in cancer therapy

Ferroptosis belongs to a novel form of regulated cell death. It is characterized by iron dependence, destruction of intracellular redox balance and non-apoptosis. And cellular structure and molecules level changes also occur abnormally during ferroptosis. It has been proved that ferroptosis exist widespreadly in many diseases, such as heart disease, brain damage or alzheimer disease. At the same time, the role of ferroptosis in cancer cannot be underestimated. More and more indications have told that ferroptosis is becoming a powerful weapon against cancer. In addition, therapies rely on ferroptosis have been applied to the clinic. Therefore, it is necessary to understand this newly discovered form of cell death and its connection with cancer. This review summarizes the mechanism of ferroptosis, ferroptosis inducers based on different targets and inspection methods. At last, we analyzed the relationship between ferroptosis and malignancies, in order to provide a novel theory basis for cancer treatment.

Nrf2-regulated redox signaling in brain endothelial cells adapted to physiological oxygen levels: Consequences for sulforaphane mediated protection against hypoxia-reoxygenation

Ischemic stroke is associated with a surge in reactive oxygen species generation during reperfusion. The narrow therapeutic window for the delivery of intravenous thrombolysis and endovascular thrombectomy limits therapeutic options for patients. Thus, understanding the mechanisms regulating neurovascular redox defenses are key for improved clinical translation. Our previous studies in a rodent model of ischemic stroke established that activation of Nrf2 defense enzymes by pretreatment with sulforaphane (SFN) affords protection against neurovascular and neurological deficits. We here further investigate SFN mediated protection in mouse brain microvascular endothelial cells (bEnd.3) adapted long-term (5 days) to hyperoxic (18 kPa) and normoxic (5 kPa) O₂ levels. Using an O₂-sensitive phosphorescent nanoparticle probe, we measured an intracellular O₂ level of 3.4 ± 0.1 kPa in bEnd 3 cells cultured under 5 kPa O₂. Induction of HO-1 and GCLM by SFN (2.5 μM) was significantly attenuated in cells adapted to 5 kPa O₂, despite nuclear accumulation of Nrf2. To simulate ischemic stroke, bEnd.3 cells were adapted to 18 or 5 kPa O₂ and subjected to hypoxia (1 kPa O₂, 1 h) and reoxygenation. In cells adapted to 18 kPa O₂, reoxygenation induced free radical generation was abrogated by PEG-SOD and significantly attenuated by pretreatment with SFN (2.5 μM). Silencing Nrf2 transcription abrogated HO-1 and NQO1 induction and led to a significant increase in reoxygenation induced free radical generation. Notably, reoxygenation induced oxidative stress, assayed using the luminescence probe L-012 and fluorescence probes MitoSOX™ Red and FeRhoNox™-1, was diminished in cells cultured under 5 kPa O₂, indicating an altered redox phenotype in brain microvascular cells adapted to physiological normoxia. As redox and other intracellular signaling pathways are critically affected by O₂, the development of antioxidant therapies targeting the Keap1-Nrf2 defense pathway in treatment of ischemia-reperfusion injury in stroke, coronary and renal disease will require in vitro studies conducted under well-defined O₂ levels.

•

Physiological normoxia alters the redox phenotype of murine microvascular brain endothelial cells.

•

Intracellular GSH levels are lower in bEnd.3 cells adapted to 5 kPa versus 18 kPa O₂.

•

Nrf2 activated HO-1 and GCLM expression is attenuated under physiological normoxia.

•

Sulforaphane protects against reoxygenation induced reactive oxygen species generation via Nrf2.

UVA irradiation increases ferrous iron release from human skin fibroblast and endothelial cell ferritin: Consequences for cell senescence and aging

UVA irradiation of human dermal fibroblasts and endothelial cells induces an immediate transient increase in cytosolic Fe(II), as monitored by the fluorescence Fe(II) reporters, FeRhonox1 in cytosol and MitoFerroGreen in mitochondria. Both superoxide dismutase (SOD) inhibition by tetrathiomolybdate (ATM) and catalase inhibition by 3-amino-1, 2, 4-triazole (ATZ) increase and prolong the cytosolic Fe(II) signal after UVA irradiation. SOD inhibition with ATM also increases mitochondrial Fe(II). Thus, mitochondria do not source the UV-dependent increase in cytosolic Fe(II), but instead reflect and amplify raised cytosolic labile Fe(II) concentration. Hence control of cytosolic ferritin iron release is key to preventing UVA-induced inflammation. UVA irradiation also increases dermal endothelial cell H₂O₂, as monitored by the adenovirus vector Hyper-DAAO-NES(HyPer). These UVA-dependent changes in intracellular Fe(II) and H₂O₂ are mirrored by increases in cell superoxide, monitored with the luminescence probe L-012. UV-dependent increases in cytosolic Fe(II), H₂O₂ and L-012 chemiluminescence are prevented by ZnCl₂ (10 μM), an effective inhibitor of Fe(II) transport via ferritin's 3-fold channels. Quercetin (10 μM), a potent membrane permeable Fe(II) chelator, abolishes the cytosolic UVA-dependent FeRhonox1, Fe(II) and HyPer, H₂O₂ and increase in MitoFerroGreen Fe(II) signals. The time course of the quercetin-dependent decrease in endothelial H₂O₂ correlates with the decrease in FeRhox1 signal and both signals are fully suppressed by preloading cells with ZnCl₂. These results confirm that antioxidant enzyme activity is the key factor in controlling intracellular iron levels, and hence maintenance of cell antioxidant capacity is vitally important in prevention of skin aging and inflammation initiated by labile iron and UVA.

Ferroptosis at the crossroads of infection, aging and cancer

Despite significant developments and persistent efforts by scientists, cancer is one of the primary causes of human death worldwide. No form of life on Earth can survive without iron, although some species can live without oxygen. Iron presents a double‐edged sword. Excess iron is a risk for carcinogenesis, while its deficiency causes anemia, leading to oxygen shortage. Every cell is eventually destined to death, either through apoptosis or necrosis. Regulated necrosis is recognized in distinct forms. Ferroptosis is defined as catalytic Fe(II)‐dependent regulated necrosis accompanied by lipid peroxidation. The main observation was necrosis of fibrosarcoma cells through inhibition of cystine/glutamate antiporter with erastin, which reduced intracellular cysteine and, thus, glutathione levels. Our current understanding of ferroptosis is relative abundance of iron (catalytic Fe[II]) in comparison with sulfur (sulfhydryls). Thus, either excess iron or sulfur deficiency causes ferroptosis. Cell proliferation inevitably requires iron for DNA synthesis and energy production. Carcinogenesis is a process toward iron addiction with ferroptosis resistance. Conversely, ferroptosis is associated with aging and neurodegeneration. Ferroptosis of immune cells during infection is advantageous for infectious agents, whereas ferroptosis resistance incubates carcinogenic soil as excess iron. Cancer cells are rich in catalytic Fe(II). Directing established cancer cells to ferroptosis is a novel strategy for discovering cancer therapies. Appropriate iron regulation could be a tactic to reduce and delay carcinogenesis.

Organelle-specific analysis of labile Fe(ii) during ferroptosis by using a cocktail of various colour organelle-targeted fluorescent probes

Ferroptosis is an emerging type of cell death mode that is dependent on iron. Unfortunately, the detailed analysis of the function of organelle labile Fe(ii) in oxidative damage and lethality of the cells has not been demonstrated so far, mainly due to the lack of efficient methods to visualize labile Fe(ii) at the targeted organelles. We have recently reported a series of Fe(ii)-selective fluorescent probes, i.e., Ac-MtFluNox, Lyso-RhoNox, and ER-SiRhoNox, which can detect Fe(ii) specifically in the mitochondria, lysosomes, and endoplasmic reticulum (ER), respectively. These probes demonstrate similar reaction rates and off/on contrasts with various colours and intracellular distributions, enabling simultaneous multi-colour imaging that allows the monitoring of labile Fe(ii) levels at each targeted organelle. In this paper, by using a cocktail of these probes, we successfully visualised the aberrant elevation of labile Fe(ii) in the lysosomes and ER prior to HT1080 cell death induced by erastin, which is an inducer of ferroptosis.

Recent Progress in Ferroptosis Inducers for Cancer Therapy

Ferroptosis is a newly discovered form of regulated cell death that is the nexus between metabolism, redox biology, and human health. Emerging evidence shows the potential of triggering ferroptosis for cancer therapy, particularly for eradicating aggressive malignancies that are resistant to traditional therapies. Recently, there has been a great deal of effort to design and develop anticancer drugs based on ferroptosis induction. Recent advances of ferroptosis-inducing agents at the intersection of chemistry, materials science, and cancer biology are presented. The basis of ferroptosis is summarized first to highlight the feasibility and characteristics of triggering ferroptosis for cancer therapy. A literature review of ferroptosis inducers (including small molecules and nanomaterials) is then presented to delineate their design, action mechanisms, and anticancer applications. Finally, some considerations for research on ferroptosis inducers are spotlighted, followed by a discussion on the challenges and future development directions of this burgeoning field.

Edaravone, a free radical scavenger, protects against ferroptotic cell death in vitro

Ferroptosis is characterized by an iron-dependent cell death with increased lipid peroxidation and is typically induced by either a decrease in glutathione (GSH) levels due to an insufficient supply of cysteine (Cys) or the inhibition of phospholipid hydroperoxide glutathione peroxidase (Gpx4). While lipid peroxides are the direct trigger for ferroptosis, the issue of how radical species involve in the cytocidal process remains unclear. To gain insights into this issue, we employed edaravone, a free radical scavenger that is clinically approved for the treatment of acute ischemic strokes and amyotrophic lateral sclerosis (ALS), against ferroptotic cell death caused by various situations, notably under cystine deprivation. We initially investigated the effects of edaravone on ferroptosis in mouse hepatoma Hepa 1-6 cells cultivated in cystine-free medium and found that edaravone largely suppressed ferroptosis. Ferroptosis that was induced in the cells by the use of inhibitors for xCT or Gpx4 was also suppressed by edaravone. Moreover, edaravone also suppressed ferroptosis in xCT-knockout mouse-derived embryonic fibroblasts, which usually die in normal cultivating conditions due to the depletion of intracellular Cys and GSH. Although the edaravone treatment had no effects on the intracellular levels of Cys and GSH, both of which remained low in Hepa 1-6 cells under conditions of cystine deprivation, the causative factors for ferroptosis, including ferrous iron and lipid peroxide levels, were significantly suppressed. Collectively, these results indicate that radical species produced at the initial stage of the cytocidal process under Cys-deprived conditions trigger ferroptosis and scavenging these radicals by edaravone represents a promising treatment.

Induction of ferroptosis by singlet oxygen generated from naphthalene endoperoxide

Singlet oxygen causes a cytotoxic process in tumor cells in photodynamic therapy (PDT) and skin photoaging. The mechanism responsible for this cytotoxicity is, however, not fully understood. 1-Methylnaphthalene-4-propionate endoperoxide (MNPE) is a cell-permeable endoperoxide that generates pure singlet oxygen. We previously reported that cell death induced by MNPE did not show the typical profile of apoptosis, and the cause of this cell death remains elusive. We report herein on an investigation of the mechanism for MNPE-induced cell death from the view point of ferroptosis. The findings indicate that the MNPE treatment decreased the viabilities of mouse hepatoma Hepa 1-6 cells in vitro, and that this decrease was accompanied by increases in the concentrations of both intracellular ferrous iron and the level of lipid peroxidation, but that the caspase-mediated apoptotic pathway was not activated. The intracellular levels of cysteine and glutathione were not affected by the MNPE treatment. Importantly, an assay of lactate dehydrogenase activity revealed that the cell death caused by MNPE was suppressed by ferrostatin-1, a ferroptosis-specific inhibitor. Collectively, these results strongly indicate that ferroptosis is the main cell death pathway induced by singlet oxygen.

Carbonic anhydrase 9 confers resistance to ferroptosis/apoptosis in malignant mesothelioma under hypoxia

Hypoxia and acidity provide microenvironment for selection under evolutionary pressure and proliferation in cancer cells. Carbonic anhydrases (CAs) are a superfamily of metalloenzymes present in all life kingdoms, equilibrating the reactions among CO₂, bicarbonate and H⁺. CA9, a membrane-associated α-CA, has been a drug target for various cancers. Whereas iron is essential not only for cancer cells but also for all the lives on earth, little is known on the association among hypoxia, iron metabolism, extracellular acidity and redox regulation. Malignant mesothelioma (MM), an aggressive tumor with poor prognosis, is an intriguing model in that asbestos-associated pathogenesis includes excess iron environment during carcinogenesis. Re-analysis of rat asbestos-induced MM model revealed an inverse association between high CA9 expression and survival. Here we used human MMs to identify the molecular events surrounding CA9 from the viewpoint of iron metabolism. CA9 expression was significantly higher in MM cells than in MeT-5A mesothelial cells, which was further amplified under hypoxia (1%O₂) with increased catalytic Fe(II). CA9 suppression by inhibitors (S4 and U104) decreased viability and migration of MM cells, accompanied by overexpression of TFRC, IREB1/2 and FPN1(SLC40A1) and by downregulation of FTH/FTL. This expressional pattern was similar to that of erastin-induced ferroptosis in the same cells. Furthermore, we observed mitochondrial fission and enhanced autophagy with increased catalytic Fe(II) in both mitochondria and lysosomes after CA9 inhibition, accompanied by increased peroxides, mitochondrial O₂⁻ and lipid peroxidation. The eventual cell death was significantly inhibited by deferoxamine, ferrostatin-1 and Z-VAD-FMK, suggesting a mixed cell death of ferroptosis and apoptosis. Therefore, CA9 plays a role in equilibrating among hypoxia, iron metabolism and redox regulation in MM cells.

Morphine-Induced Modulation of Endolysosomal Iron Mediates Upregulation of Ferritin Heavy Chain in Cortical Neurons

HIV-associated neurocognitive disorders (HAND) remain prevalent and are aggravated by µ-opioid use. We have previously shown that morphine and other µ-opioids may contribute to HAND by inhibiting the homeostatic and neuroprotective chemokine receptor CXCR4 in cortical neurons, and this novel mechanism depends on upregulation of the protein ferritin heavy chain (FHC). Here, we examined the cellular events and potential mechanisms involved in morphine-mediated FHC upregulation using rat cortical neurons of either sex in vitro and in vivo. Morphine dose dependently increased FHC protein levels in primary neurons through µ-opioid receptor (µOR) and Gαi-protein signaling. Cytoplasmic FHC levels were significantly elevated, but nuclear FHC levels and FHC gene expression were unchanged. Morphine-treated rats also displayed increased FHC levels in layer 2/3 neurons of the prefrontal cortex. Importantly, both in vitro and in vivo FHC upregulation was accompanied by loss of mature dendritic spines, which was also dependent on µOR and Gαi-protein signaling. Moreover, morphine upregulated ferritin light chain (FLC), a component of the ferritin iron storage complex, suggesting that morphine altered neuronal iron metabolism. Indeed, prior to FHC upregulation, morphine increased cytoplasmic labile iron levels as a function of decreased endolysosomal iron. In line with this, chelation of endolysosomal iron (but not extracellular iron) blocked morphine-induced FHC upregulation and dendritic spine reduction, whereas iron overloading mimicked the effect of morphine on FHC and dendritic spines. Overall, these data demonstrate that iron mediates morphine-induced FHC upregulation and consequent dendritic spine deficits and implicate endolysosomal iron efflux to the cytoplasm in these effects.

Suppressive effects of iron chelation in clear cell renal cell carcinoma and their dependency on VHL inactivation

Increasing data implicate iron accumulation in tumorigenesis of the kidney, particularly the clear cell renal cell carcinoma (ccRCC) subtype. The von Hippel Lindau (VHL)/hypoxia inducible factor-α (HIF-α) axis is uniquely dysregulated in ccRCC and is a major regulator and regulatory target of iron metabolism, yet the role of iron in ccRCC tumorigenesis and its potential interplay with VHL inactivation remains unclear. We investigated whether ccRCC iron accumulation occurs due to increased cell dependency on iron for growth and survival as a result of VHL inactivation. Free iron levels were compared between four VHL-mutant ccRCC cell lines (786-0, A704, 769-P, RCC4) and two benign renal tubule epithelial cell lines (RPTEC, HRCEp) using the Phen Green SK fluorescent iron stain. Intracellular iron deprivation was achieved using two clinical iron chelator drugs, deferasirox (DFX) and deferoxamine (DFO), and chelator effects were measured on cell line growth, cell cycle phase, apoptosis, HIF-1α and HIF-2α protein levels and HIF-α transcriptional activity based on expression of target genes CA9, OCT4/POU5F1 and PDGFβ/PDGFB. Similar assays were performed in VHL-mutant ccRCC cells with and without ectopic wild-type VHL expression. Baseline free iron levels were significantly higher in ccRCC cell lines than benign renal cell lines. DFX depleted cellular free iron more rapidly than DFO and led to greater growth suppression of ccRCC cell lines (>90% at ~30-150 µM) than benign renal cell lines (~10-50% at up to 250 µM). Similar growth responses were observed using DFO, with the exception that a prolonged treatment duration was necessary to deplete cellular iron adequately for differential growth suppression of the less susceptible A704 ccRCC cell line relative to benign renal cell lines. Apoptosis and G1-phase cell cycle arrest were identified as potential mechanisms of chelator growth suppression based on their induction in ccRCC cell lines but not benign renal cell lines. Iron chelation in ccRCC cells but not benign renal cells suppressed HIF-1α and HIF-2α protein levels and transcriptional activity, and the degree and timing of HIF-2α suppression correlated with the onset of apoptosis. Restoration of wild-type VHL function in ccRCC cells was sufficient to prevent chelator-induced apoptosis and G1 cell cycle arrest, indicating that ccRCC susceptibility to iron deprivation is VHL inactivation-dependent. In conclusion, ccRCC cells are characterized by high free iron levels and a cancer-specific dependency on iron for HIF-α overexpression, cell cycle progression and apoptotic escape. This iron dependency is introduced by VHL inactivation, revealing a novel interplay between VHL/HIF-α dysregulation and ccRCC iron metabolism. Future study is warranted to determine if iron deprivation using chelator drugs provides an effective therapeutic strategy for targeting HIF-2α and suppressing tumor progression in ccRCC patients.

Fluorescent probes for the detection of catalytic Fe(II) ion

Iron (Fe) is the most abundant redox-active metal ion in the human body, and its redox-active inter-convertible multiple oxidation states contributes to numerous essential biological processes. Moreover, iron overload can potentially cause cellular damage and death, as wel as numerous diseases through the aberrant production of highly reactive oxidative species (hROS). Protein-free or weakly-protein-bound Fe ions play a pivotal role as catalytic reactants of the Fenton reaction. In this reaction, hROS, such as hydroxyl radicals and high valent-iron-oxo species, are generated by a reaction between hydrogen peroxide and Fe(II), which is re-generated through reduction using abundant intracellular reductants, such as glutathione. This results in the catalytic evolution of hROS. Thus, selective detection of the catalytic Fe(II) in the living systems can explain both of the pathological and physiological functions of Fe(II). Written from the perspective of their modes of actions, this paper presents recent advances in the development of fluorescent and bioluminescent probes that can selectively detect catalytic Fe(II) together with their biological applications.

[Development and applications of Fe(II)-selective fluorescent probes]

Iron is the most abundant transition metal in our body and plays various pivotal roles in our lives including oxygen transport, energy production, and metabolic reactions. At the same time, an excess amount of iron may cause cellular damages through undesired oxidative reactions due to the high redox activity of Fe ion. We have developed the several fluorescent probes which can detect Fe(II) ion selectively with fluorescence enhancement to understand both the physiological and pathological contributions of Fe ion in living systems. These fluorescent probes worked in an aqueous buffer, living cells, and histochemical-stained samples (Chem Sci. 2013;4:1250, Chem Sci. 2017;8:4858, Free Radic Res. 2014;48:990, Sci Rep. 2017;7:10621). We established a color series of Fe(II)-selective fluorescent probes from blue to deep-red, which were applied to organelle-targeted fluorescent probes for mitochondria, lysosome, and endoplasmic reticulum. Herein, I would like to focus on fluorescence imaging study about the alteration of labile Fe(II) level in each organelle during ferroptosis, iron-dependent cell death, by using the various organelle-targeted fluorescent probes of Fe(II).

Development of Chemical Tools for Imaging of Fe(II) Ions in Living Cells: A Review

Intracellular labile iron is an iron species which is not or weakly bound to proteins and depicts an important effect on homeostatic regulation in cells. An excess or deficiency of iron can cause oxidative damage to key cellular biomolecules. The behavior and concentrations of labile iron are difficult to monitor, but the specific redox state of the Fe ions is relevant to the physiological and pathological properties that we would like to study. We have developed a series of turn-on type fluorescent probes that are highly selective to the labile Fe(II) ions, and we have tested their applications to cellular level imaging. These probes are based on N-oxide chemistry with a range of fluorophores that depict optimal performance for specific applications. Herein, I review the recent progress of our research and discuss prospects for future work to understand the relation between intracellular ion and oxidative stress.

Fe(II) Ion Release during Endocytotic Uptake of Iron Visualized by a Membrane-Anchoring Fe(II) Fluorescent Probe

Iron is an essential transition metal species for all living organisms and plays various physiologically important roles on the basis of its redox activity; accordingly, the disruption of iron homeostasis triggers oxidative stress and cellular damage. Therefore, cells have developed sophisticated iron-uptake machinery to acquire iron while protecting cells from uncontrolled oxidative damage during the uptake process. To examine the detailed mechanism of iron uptake while controlling the redox status, it is necessary to develop useful methods with redox state selectivity, sensitivity, and organelle specificity to monitor labile iron, which is weakly bound to subcellular ligands. Here, we report the development of Mem-RhoNox to monitor local Fe(II) at the surface of the plasma membrane of living cells. The redox state-selective fluorescence response of the probe relies on our recently developed N-oxide strategy, which is applicable to fluorophores with dialkylarylamine in their π-conjugation systems. Mem-RhoNox consists of the N-oxygenated rhodamine scaffold, which has two arms, both of which are tethered with palmitoyl groups as membrane-anchoring domains. In an aqueous buffer, Ac-RhoNox, a model compound of Mem-RhoNox, shows a fluorescence turn-on response to the Fe(II) redox state-selectively. An imaging study with Mem-RhoNox and its derivatives reveals that labile Fe(II) is transiently generated during the major iron-uptake pathways: endocytotic uptake and direct transport. Furthermore, Mem-RhoNox is capable of monitoring endosomal Fe(II) in primary cultured neurons during endocytotic uptake. This report is the first example that identifies the generation of Fe(II) over the course of cellular iron-uptake processes.

Contribution of macrophages in the contrast loss in iron oxide-based MRI cancer cell tracking studies

Magnetic resonance imaging (MRI) cell tracking of cancer cells labeled with superparamagnetic iron oxides (SPIO) allows visualizing metastatic cells in preclinical models. However, previous works showed that the signal void induced by SPIO on T₂(*)-weighted images decreased over time. Here, we aim at characterizing the fate of iron oxide nanoparticles used in cell tracking studies and the role of macrophages in SPIO metabolism.

In vivo MRI cell tracking of SPIO positive 4T1 breast cancer cells revealed a quick loss of T₂* contrast after injection. We next took advantage of electron paramagnetic resonance (EPR) spectroscopy and inductively coupled plasma mass spectroscopy (ICP-MS) for characterizing the evolution of superparamagnetic and non-superparamagnetic iron pools in 4T1 breast cancer cells and J774 macrophages after SPIO labeling. These in vitro experiments and histology studies performed on 4T1 tumors highlighted the quick degradation of iron oxides by macrophages in SPIO-based cell tracking experiments.

In conclusion, the release of SPIO by dying cancer cells and the subsequent uptake of iron oxides by tumor macrophages are limiting factors in MRI cell tracking experiments that plead for the use of (MR) reporter-gene based imaging methods for the long-term tracking of metastatic cells.

Iron Overload Accelerates the Progression of Diabetic Retinopathy in Association with Increased Retinal Renin Expression

Diabetic retinopathy (DR) is a leading cause of blindness among working-age adults. Increased iron accumulation is associated with several degenerative diseases. However, there are no reports on the status of retinal iron or its implications in the pathogenesis of DR. In the present study, we found that retinas of type-1 and type-2 mouse models of diabetes have increased iron accumulation compared to non-diabetic retinas. We found similar iron accumulation in postmortem retinal samples from human diabetic patients. Further, we induced diabetes in HFE knockout (KO) mice model of genetic iron overload to understand the role of iron in the pathogenesis of DR. We found increased neuronal cell death, vascular alterations and loss of retinal barrier integrity in diabetic HFE KO mice compared to diabetic wildtype mice. Diabetic HFE KO mouse retinas also exhibited increased expression of inflammation and oxidative stress markers. Severity in the pathogenesis of DR in HFE KO mice was accompanied by increase in retinal renin expression mediated by G-protein-coupled succinate receptor GPR91. In light of previous reports implicating retinal renin-angiotensin system in DR pathogenesis, our results reveal a novel relationship between diabetes, iron and renin-angiotensin system, thereby unraveling new therapeutic targets for the treatment of DR.

The iron chelator deferasirox induces apoptosis by targeting oncogenic Pyk2/β-catenin signaling in human multiple myeloma

Deregulated iron metabolism underlies the pathogenesis of many human cancers. Recently, low expression of ferroportin, which is the only identified non-heme iron exporter, has been associated with significantly reduced overall survival in multiple myeloma (MM); however, the altered iron metabolism in MM biology remains unclear. In this study we demonstrated, by live cell imaging, that MM cells have increased intracellular iron levels as compared with normal cells. In experiments to test the effect of iron chelation on the growth of MM cells, we found that deferasirox (DFX), an oral iron chelator used to treat iron overload in clinical practice, inhibits MM cell growth both in vivo and in vitro. Mechanistically, DFX was found to induce apoptosis of MM cells via the inhibition of proline-rich tyrosine kinase 2 (Pyk2), which is known to promote tumor growth in MM. Inhibition of Pyk2 is caused by the suppression of reactive oxygen species, and leads to downregulation of the Wnt/β-catenin signaling pathway. Taken together, our findings indicate that high levels of intracellular iron, which might be due to low ferroportin expression, play a role in MM pathophysiology. Therefore, DFX may provide a therapeutic option for MM that is driven by deregulated iron homeostasis and/or Pyk2/Wnt signaling.

Role of catalytic iron and oxidative stress in nitrofen-induced congenital diaphragmatic hernia and its amelioration by Saireito (TJ-114)

Congenital diaphragmatic hernia (CDH) is a life-threatening neonatal disease that leads to lung hypoplasia and pulmonary hypertension. We recently found that maternal prenatal administration of Saireito (TJ-114) ameliorates fetal CDH in a nitrofen-induced rat model. Here, we studied the role of iron and oxidative stress in neonates of this model and in lung fibroblasts IMR90-SV in association with nitrofen and Saireito. We observed increased immunostaining of 8-hydroxy-2'-deoxyguanosine in the lungs of neonates with CDH, which was ameliorated by maternal Saireito intake. Pulmonary transferrin receptor expression was significantly decreased in both CDH and CDH after Saireito in comparison to normal controls, indicating functional lung immaturity, whereas catalytic Fe(II) and pulmonary DMT1/ferroportin expression remained constant among the three groups. Saireito revealed a dose-dependent scavenging capacity with electron spin resonance spin trapping in vitro against hydroxyl radicals but not against superoxide. Finally, nitrofen revealed dose-dependent cytotoxicity to IMR90-SV cells, accompanied by an increase in oxidative stress, as seen by 5(6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate and catalytic Fe(II). Saireito ameliorated all of these in IMR90-SV cells. In conclusion, catalytic Fe(II)-dependent oxidative stress by nitrofen may be the pathogenic cause of CDH, and the antioxidative activity of Saireito is at least partially responsible for improving nitrofen-induced CDH.

Iron and thiol redox signaling in cancer: An exquisite balance to escape ferroptosis

Epidemiological data indicate a constant worldwide increase in cancer mortality, although the age of onset is increasing. Recent accumulation of genomic data on human cancer via next-generation sequencing confirmed that cancer is a disease of genome alteration. In many cancers, the Nrf2 transcription system is activated via mutations either in Nrf2 or Keap1 ubiquitin ligase, leading to persistent activation of the genes with antioxidative functions. Furthermore, deep sequencing of passenger mutations is clarifying responsible cancer causative agent(s) in each case, including aging, APOBEC activation, smoking and UV. Therefore, it is most likely that oxidative stress is the principal initiating factor in carcinogenesis, with the involvement of two essential molecules for life, iron and oxygen. There is evidence based on epidemiological and animal studies that excess iron is a major risk for carcinogenesis, suggesting the importance of ferroptosis-resistance. Microscopic visualization of catalytic Fe(II) has recently become available. Although catalytic Fe(II) is largely present in lysosomes, proliferating cells harbor catalytic Fe(II) also in the cytosol and mitochondria. Oxidative stress catalyzed by Fe(II) is counteracted by thiol systems at different functional levels. Nitric oxide, carbon monoxide and hydrogen (per)sulfide modulate these reactions. Mitochondria generate not only energy but also heme/iron sulfur cluster cofactors and remain mostly dysfunctional in cancer cells, leading to Warburg effects. Cancer cells are under persistent oxidative stress with a delicate balance between catalytic iron and thiols, thereby escaping ferroptosis. Thus, high-dose L-ascorbate and non-thermal plasma as well as glucose/glutamine deprivation may provide additional benefits as cancer therapies over preexisting therapeutics.

D-penicillamine combined with inhibitors of hydroperoxide metabolism enhances lung and breast cancer cell responses to radiation and carboplatin via H2O2-mediated oxidative stress

D-penicillamine (DPEN), a copper chelator, has been used in the treatment of Wilson's disease, cystinuria, and rheumatoid arthritis. Recent evidence suggests that DPEN in combination with biologically relevant copper (Cu) concentrations generates H2O2 in cancer cell cultures, but the effects of this on cancer cell responses to ionizing radiation and chemotherapy are unknown. Increased steady-state levels of H2O2 were detected in MB231 breast and H1299 lung cancer cells following treatment with DPEN (100µM) and copper sulfate (15µM). Clonogenic survival demonstrated that DPEN-induced cancer cell toxicity was dependent on Cu and was significantly enhanced by depletion of glutathione [using buthionine sulfoximine (BSO)] as well as inhibition of thioredoxin reductase [using Auranofin (Au)] prior to exposure. Treatment with catalase inhibited DPEN toxicity confirming H2O2 as the toxic species. Furthermore, pretreating cancer cells with iron sucrose enhanced DPEN toxicity while treating with deferoxamine, an Fe chelator that inhibits redox cycling, inhibited DPEN toxicity. Importantly, DPEN also demonstrated selective toxicity in human breast and lung cancer cells, relative to normal untransformed human lung or mammary epithelial cells and enhanced cancer cell killing when combined with ionizing radiation or carboplatin. Consistent with the selective cancer cell toxicity, normal untransformed human lung epithelial cells had significantly lower labile iron pools than lung cancer cells. These results support the hypothesis that DPEN mediates selective cancer cell killing as well as radio-chemo-sensitization by a mechanism involving metal ion catalyzed H2O2-mediated oxidative stress and suggest that DPEN could be repurposed as an adjuvant in conventional cancer therapy.

Iron suppresses erythropoietin expression via oxidative stress-dependent hypoxia-inducible factor-2 alpha inactivation

Renal anemia is a major complication in chronic kidney disease (CKD). Iron supplementation, as well as erythropoiesis-stimulating agents, are widely used for treatment of renal anemia. However, excess iron causes oxidative stress via the Fenton reaction, and iron supplementation might damage remnant renal function including erythropoietin (EPO) production in CKD. EPO gene expression was suppressed in mice following direct iron treatment. Hypoxia-inducible factor-2 alpha (HIF-2α), a positive regulator of the EPO gene, was also diminished in the kidney of mice following iron treatment. Anemia-induced increase in renal EPO and HIF-2α expression was inhibited by iron treatment. In in vitro experiments using EPO-producing HepG2 cells, iron stimulation reduced the expression of the EPO gene, as well as HIF-2α. Moreover, iron treatment augmented oxidative stress, and iron-induced reduction of EPO and HIF-2α expression was restored by tempol, an antioxidant compound. HIF-2α interaction with the Epo promoter was inhibited by iron treatment, and was restored by tempol. These findings suggested that iron supplementation reduced EPO gene expression via an oxidative stress-HIF-2α-dependent signaling pathway.

Iron regulatory protein 2 in ovarian endometrial cysts

Ovarian endometrial cysts cause some kinds of ovarian cancer, and iron is considered as one factor of carcinogenesis. In contrast, hypoxia is associated with progression, angiogenesis, metastasis, and resistance to therapy in cancer. We investigated hypoxia-induced perturbation of iron homeostasis in terms of labile iron, iron deposition, and iron regulatory protein (IRP) in ovarian endometrial cysts. Iron deposition, expression of IRPs, and a protein marker of hypoxia in human ovarian endometrial cysts were analyzed histologically. The concentration of free iron and the pO₂ level of the cyst fluid of human ovarian cysts (n = 9) were measured. The expression of IRP2 under hypoxia was investigated in vitro by using Ishikawa cells as a model of endometrial cells. Iron deposition and the expression of IRP2 and Carbonic anhydrase 9 (CA9) were strong in endometrial stromal cells in the human ovarian endometrial cysts. The average concentration of free iron in the cyst fluid was 8.1 ± 2.9 mg/L, and the pO₂ was 22.4 ± 5.2 mmHg. A cell-based study using Ishikawa cells revealed that IRP2 expression was decreased by an overload of Fe(II) under normoxia but remained unchanged under hypoxia even in the presence of excess Fe(II). An increase in the expression of IRP2 caused upregulation of intracellular iron as a result of the response to iron deficiency, whereas the protein was degraded under iron-rich conditions. We found that iron-rich regions existed in ovarian endometrial cysts concomitantly with the high level of IRP2 expression, which should generally be decomposed upon an overload of iron. We revealed that an insufficient level of oxygen in the cysts is the main factor for the unusual stabilization of IRP2 against iron-mediated degradation, which provides aberrant uptake of iron in ovarian endometrial stromal cells and can potentially lead to carcinogenesis.

Chemical tools for detecting Fe ions

Owing to its distinctive electrochemical properties with interconvertible multiple oxidation states, iron plays a significant role in various physiologically important functions such as respiration, oxygen transport, energy production, and enzymatic reactions. This redox activity can also potentially produce cellular damage and death, and numerous diseases are related to iron overload resulting from the dysfunction of the iron regulatory system. In this case, “free iron” or “labile iron,” which refers to iron ion weakly bound or not bound to proteins, causes aberrant production of reactive oxygen species. With the aim of elucidating the variation of labile iron involved in pathological processes, some chemical tools that can qualitatively and/or quantitatively monitor iron have been utilized to investigate the distribution, accumulation, and flux of biological iron species. Since iron ions show unique reactivity depending on its redox state, i.e., Fe²⁺ or Fe³⁺ (or transiently higher oxidative states), methods for the separate detection of iron species with different redox states are preferred to understand its physiological and pathological roles more in detail. The scope of this review article covers from classical chromogenic to newly emerging chemical tools for the detection of Fe ions. In particular, chemical tools applicable to biological studies will be presented.

Role of hemoglobin and transferrin in multi-wall carbon nanotube-induced mesothelial injury and carcinogenesis

Multi‐wall carbon nanotubes (MWCNT) are a form of flexible fibrous nanomaterial with high electrical and thermal conductivity. However, 50‐nm MWCNT in diameter causes malignant mesothelioma (MM) in rodents and, thus, the International Agency of Research on Cancer has designated them as a possible human carcinogen. Little is known about the molecular mechanism through which MWCNT causes MM. To elucidate the carcinogenic mechanisms of MWCNT in mesothelial cells, we used a variety of lysates to comprehensively identify proteins specifically adsorbed on pristine MWCNT of different diameters (50 nm, NT50; 100 nm, NT100; 150 nm, NT150; and 15 nm/tangled, NTtngl) using mass spectrometry. We identified >400 proteins, which included hemoglobin, histone, transferrin and various proteins associated with oxidative stress, among which we selected hemoglobin and transferrin for coating MWCNT to further evaluate cytotoxicity, wound healing, intracellular catalytic ferrous iron and oxidative stress in rat peritoneal mesothelial cells (RPMC). Cytotoxicity to RPMC was observed with pristine NT50 but not with NTtngl. Coating NT50 with hemoglobin or transferrin significantly aggravated cytotoxicity to RPMC, with an increase in cellular catalytic ferrous iron and DNA damage also observed. Knockdown of transferrin receptor with ferristatin II decreased not only NT50 uptake but also cellular catalytic ferrous iron. Our results suggest that adsorption of hemoglobin and transferrin on the surface of NT50 play a role in causing mesothelial iron overload, contributing to oxidative damage and possibly subsequent carcinogenesis in mesothelial cells. Uptake of NT50 at least partially depends on transferrin receptor 1. Modifications of NT50 surface may decrease this human risk.

Remodulating effect of doxorubicin on the state of iron-containing proteins, and redox characteristics of tumor with allowance for its sensitivity to cytostatic agents

The study was aimed at determining the changes of metal-containing proteins in blood serum and tumor tissue of animals with parental and doxorubicin-resistant strains of Walker-256 carcinosarcoma before and after the cytostatic administration. It has been shown that upon doxorubicin action the levels of total iron and transferrin in the tissues from the both groups of animals decreased while that of ferritine simultaneously increased with more pronounced pattern in the group of animals with resistant tumor strain. It has been shown that upon the action of doxorubicin in tumor tissue of animals with different sensitivity to the cytostatic there could be observed oppositely directed changes in the redox state of these cells that in turn determined the content of “ free iron” complexes, RO S generation and concentration of active forms of matrix metaloproteinase- 2 and matrix metaloproteinase-9, namely, the increase of these indexes in animals with parental strain and their decrease in animals with the resistant one. So, our study has demonstrated the remodulating effect of doxorubicin on the state of metal-containing proteins and redox characteristics of tumor dependent on its sensitivity to cytostatic, at the levels of the tumor and an organism. These data may serve as a criterion for the development of programs for the correction of malfunction of iron metabolism aimed at elevating tumor sensitivity to cytostatic agents.

Ovarian endometriosis-associated stromal cells reveal persistently high affinity for iron

Ovarian endometriosis is a recognized risk for infertility and epithelial ovarian cancer, presumably due to iron overload resulting from repeated hemorrhage. To find a clue for early detection and prevention of ovarian endometriosis-associated cancer, it is mandatory to evaluate catalytic (labile) ferrous iron (catalytic Fe(II)) and to study iron manipulation in ovarian endometriotic lesions. By the use of tissues from women of ovarian endometriosis as well as endometrial tissue from women with and without endometriosis, we for the first time performed histological analysis and cellular detection of catalytic Fe(II) with a specific fluorescent probe (HMRhoNox-M), and further evaluated iron transport proteins in the human specimens and in co-culture experiments using immortalized human eutopic/ectopic endometrial stromal cells (ESCs) in the presence or absence of epithelial cells (EpCs). The amounts of catalytic Fe(II) were higher in ectopic endometrial stromal cells (ecESCs) than in normal eutopic endometrial stromal cells (n-euESCs) both in the tissues and in the corresponding immortalized ESCs. ecESCs exhibited higher transferrin receptor 1 expression both in vivo and in vitro and lower ferroportin expression in vivo than n-euESCs, leading to sustained iron uptake. In co-culture experiments of ESCs with iron-loaded EpCs, ecESCs received catalytic ferrous iron from EpCs, but n-euESCs did not. These data suggest that ecESC play a protective role for cancer-target epithelial cells by collecting excess iron, and that these characteristics are retained in the immortalized ecESCs.

Catalytic ferrous iron in amniotic fluid as a predictive marker of human maternal-fetal disorders

Amniotic fluid contains numerous biomolecules derived from fetus and mother, thus providing precious information on pregnancy. Here, we evaluated oxidative stress of human amniotic fluid and measured the concentration of catalytic Fe(II). Amniotic fluid samples were collected with consent from a total of 89 subjects in Nagoya University Hospital, under necessary medical interventions: normal pregnancy at term, normal pregnancy at the 2nd trimester, preterm delivery with maternal disorders but without fetal disorders, congenital diaphragmatic hernia, fetal growth restriction, pregnancy-induced hypertension, gestational diabetes mellitus, Down syndrome and trisomy 18. Catalytic Fe(II) and oxidative stress markers (8-hydroxy-2'-deoxyguanosine, 8-OHdG; dityrosine) were determined with RhoNox-1 and specific antibodies, respectively, using plate assays. Levels of 8-OHdG and dityrosine were higher in the 3rd trimester compared with the 2nd trimester in normal subjects, and the abnormal groups generally showed lower levels than the controls, thus suggesting that they represent fetal metabolic activities. In contrast, catalytic Fe(II) was higher in the 2nd trimester than the 3rd trimester in the normal subjects, and overall the abnormal groups showed higher levels than the controls, suggesting that high catalytic Fe(II) at late gestation reflects fetal pathologic alterations. Notably, products of H₂O₂ and catalytic Fe(II) remained almost constant in amniotic fluid.

Iron and thiols as two major players in carcinogenesis: friends or foes?

Iron is the most abundant metal in the human body and mainly works as a cofactor for proteins such as hemoglobin and various enzymes. No independent life forms on earth can survive without iron. However, excess iron is intimately associated with carcinogenesis by increasing oxidative stress via its catalytic activity to generate hydroxyl radicals. Biomolecules with redox-active sulfhydryl function(s) (thiol compounds) are necessary for the maintenance of mildly reductive cellular environments to counteract oxidative stress, and for the execution of redox reactions for metabolism and detoxification. Involvement of glutathione S-transferase and thioredoxin has long attracted the attention of cancer researchers. Here, I update recent findings on the involvement of iron and thiol compounds during carcinogenesis and in cancer cells. It is now recognized that the cystine/glutamate transporter (antiporter) is intimately associated with ferroptosis, an iron-dependent, non-apoptotic form of cell death, observed in cancer cells, and also with cancer stem cells; the former with transporter blockage but the latter with its stabilization. Excess iron in the presence of oxygen appears the most common known mutagen. Ironically, the persistent activation of antioxidant systems via genetic alterations in Nrf2 and Keap1 also contributes to carcinogenesis. Therefore, it is difficult to conclude the role of iron and thiol compounds as friends or foes, which depends on the quantity/distribution and induction/flexibility, respectively. Avoiding further mutation would be the most helpful strategy for cancer prevention, and myriad of efforts are being made to sort out the weaknesses of cancer cells.