The labile iron pool attenuates peroxynitrite-dependent damage and can no longer be considered solely a pro-oxidative cellular iron source

Insights on the endogenous labile iron pool binding properties

Under normal physiological conditions, the endogenous Labile Iron Pool (LIP) constitutes a ubiquitous, dynamic, tightly regulated reservoir of cellular ferrous iron. Furthermore, LIP is loaded into new apo-iron proteins, a process akin to the activity of metallochaperones. Despite such importance on iron metabolism, the LIP identity and binding properties have remained elusive. We hypothesized that LIP binds to cell constituents (generically denoted C) and forms an iron complex termed CLIP. Combining this binding model with the established Calcein (CA) methodology for assessing cytosolic LIP, we have formulated an equation featuring two experimentally quantifiable parameters (the concentrations of the cytosolic free CA and CA and LIP complex termed CALIP) and three unknown parameters (the total concentrations of LIP and C and their thermodynamic affinity constant Kd). The fittings of cytosolic CALIP × CA concentrations data encompassing a few cellular models to this equation with floating unknown parameters were successful. The computed adjusted total LIP (LIPT) and C (CT) concentrations fall within the sub-to-low micromolar range while the computed Kd was in the 10-2 µM range for all cell types. Thus, LIP binds and has high affinity to cellular constituents found in low concentrations and has remarkably similar properties across different cell types, shedding fresh light on the properties of endogenous LIP within cells.

Intracellular peroxynitrite perturbs redox balance, bioenergetics, and Fe-S cluster homeostasis in Mycobacterium tuberculosis

The ability of Mycobacterium tuberculosis (Mtb) to tolerate nitric oxide (•NO) and superoxide (O2•-) produced by phagocytes contributes to its success as a human pathogen. Recombination of •NO and O2•- generates peroxynitrite (ONOO-), a potent oxidant produced inside activated macrophages causing lethality in diverse organisms. While the response of Mtb toward •NO and O2•- is well established, how Mtb responds to ONOO- remains unclear. Filling this knowledge gap is important to understand the persistence mechanisms of Mtb during infection. We synthesized a series of compounds that generate both •NO and O2•-, which should combine to produce ONOO-. From this library, we identified CJ067 that permeates Mtb to reliably enhance intracellular ONOO- levels. CJ067-exposed Mtb strains, including multidrug-resistant (MDR) and extensively drug-resistant (XDR) clinical isolates, exhibited dose-dependent, long-lasting oxidative stress and growth inhibition. In contrast, Mycobacterium smegmatis (Msm), a fast-growing, non-pathogenic mycobacterial species, maintained redox balance and growth in response to intracellular ONOO-. RNA-sequencing with Mtb revealed that CJ067 induces antioxidant machinery, sulphur metabolism, metal homeostasis, and a 4Fe-4S cluster repair pathway (suf operon). CJ067 impaired the activity of the 4Fe-4S cluster-containing TCA cycle enzyme, aconitase, and diminished bioenergetics of Mtb. Work with Mtb strains defective in SUF and IscS involved in Fe-S cluster biogenesis pathways showed that both systems cooperatively protect Mtb from intracellular ONOO- in vitro and inducible nitric oxide synthase (iNOS)-dependent growth inhibition during macrophage infection. Thus, Mtb is uniquely sensitive to intracellular ONOO- and targeting Fe-S cluster homeostasis is expected to promote iNOS-dependent host immunity against tuberculosis (TB).

The Ubiquity of the Reaction of the Labile Iron Pool That Attenuates Peroxynitrite-Dependent Oxidation Intracellularly

Although the labile iron pool (LIP) biochemical identity remains a topic of debate, it serves as a universal homeostatically regulated and essential cellular iron source. The LIP plays crucial cellular roles, being the source of iron that is loaded into nascent apo-iron proteins, a process akin to protein post-translational modification, and implicated in the programmed cell death mechanism known as ferroptosis. The LIP is also recognized for its reactivity with chelators, nitric oxide, and peroxides. Our recent investigations in a macrophage cell line revealed a reaction of the LIP with the oxidant peroxynitrite. In contrast to the LIP's pro-oxidant interaction with hydrogen peroxide, this reaction is rapid and attenuates the peroxynitrite oxidative impact. In this study, we demonstrate the existence and antioxidant characteristic of the LIP and peroxynitrite reaction in various cell types. Beyond its potential role as a ubiquitous complementary or substitute protection system against peroxynitrite for cells, the LIP and peroxynitrite reaction may influence cellular iron homeostasis and ferroptosis by changing the LIP redox state and LIP binding properties and reactivity.

Production of Peroxymonocarbonate by Steady-State Micromolar H2O2 and Activated Macrophages in the Presence of CO2/HCO3- Evidenced by Boronate Probes

Peroxymonocarbonate (HCO4-/HOOCO2-) is produced by the reversible reaction of CO2/HCO3- with H2O2 (K = 0.33 M-1, pH 7.0). Although produced in low yields at physiological pHs and H2O2 and CO2/HCO3- concentrations, HCO4- oxidizes most nucleophiles with rate constants 10 to 100 times higher than those of H2O2. Boronate probes are known examples because HCO4- reacts with coumarin-7-boronic acid pinacolate ester (CBE) with a rate constant that is approximately 100 times higher than that of H2O2 and the same holds for fluorescein-boronate (Fl-B) as reported here. Therefore, we tested whether boronate probes could provide evidence for HCO4- formation under biologically relevant conditions. Glucose/glucose oxidase/catalase were adjusted to produce low steady-state H2O2 concentrations (2-18 μM) in Pi buffer at pH 7.4 and 37 °C. Then, CBE (100 μM) was added and fluorescence increase was monitored with time. The results showed that each steady-state H2O2 concentration reacted more rapidly (∼30%) in the presence of CO2/HCO3- (25 mM) than in its absence, and the data permitted the calculation of consistent rate constants. Also, RAW 264.7 macrophages were activated with phorbol 12-myristate 13-acetate (PMA) (1 μg/mL) at pH 7.4 and 37 °C to produce a time-dependent H2O2 concentration (8.0 ± 2.5 μM after 60 min). The media contained 0, 21.6, or 42.2 mM HCO3- equilibrated with 0, 5, or 10% CO2, respectively. In the presence of CBE or Fl-B (30 μM), a time-dependent increase in the fluorescence of the bulk solution was observed, which was higher in the presence of CO2/HCO3- in a concentration-dependent manner. The Fl-B samples were also examined by fluorescence microscopy. Our results demonstrated that mammalian cells produce HCO4- and boronate probes can evidence and distinguish it from H2O2 under biologically relevant concentrations of H2O2 and CO2/HCO3-.

Insight into the relevance of dinitrosyl iron complex (DNIC) formation in the absence of thiols in aqueous media

DNIC can be formed in aqueous media in the absence of thiols via mechanisms that depend exclusively on Fe(II) and NO. However, these reactions do not take place at intracellular concentrations of Fe(II) and NO, reinforcing the relevance of thiols to assist Fe(II) to Fe(I) reduction during DNIC formation in biological media.

Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide-Dioxygen Interconversion

Through nitrosylation of [Fe-S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2-). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2- by a series of DNICs [(NO)2Fe(μ-MePyr)2Fe(NO)2] (1) and [(NO)2Fe(μ-SEt)2Fe(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)][Fe(μ-MePyr)4(μ-O)2(Fe(NO)2)4] (2-K-crown) and a [Fe3+(MePyr)x(NO2)y(O)z]n adduct, stoichiometric NO monooxygenation yielding NO2- occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe(NO)2}9-{Fe(NO)2}10 electronic structure, [K-18-crown-6-ether][(NO)2Fe(μ-MePyr)2Fe(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO-)2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether][(NO)2Fe(μ-SEt)2Fe(NO)2] (3-red) display a reversible equilibrium of "3 + O2- ⇋ 3-red + O2(g)", which is ascribed to the covalent [Fe(μ-SEt)2Fe] core and redox-active [Fe(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2- and O2 during reaction with O2- (or O2).

Iron therapy mitigates chronic kidney disease progression by regulating intracellular iron status of kidney macrophages

Systemic iron metabolism is disrupted in chronic kidney disease (CKD). However, little is known about local kidney iron homeostasis and its role in kidney fibrosis. Kidney-specific effects of iron therapy in CKD also remain elusive. Here, we elucidate the role of macrophage iron status in kidney fibrosis and demonstrate that it is a potential therapeutic target. In CKD, kidney macrophages exhibited depletion of labile iron pool (LIP) and induction of transferrin receptor 1, indicating intracellular iron deficiency. Low LIP in kidney macrophages was associated with their defective antioxidant response and proinflammatory polarization. Repletion of LIP in kidney macrophages through knockout of ferritin heavy chain (Fth1) reduced oxidative stress and mitigated fibrosis. Similar to Fth1 knockout, iron dextran therapy, through replenishing macrophage LIP, reduced oxidative stress, decreased the production of proinflammatory cytokines, and alleviated kidney fibrosis. Interestingly, iron markedly decreased TGF-β expression and suppressed TGF-β-driven fibrotic response of macrophages. Iron dextran therapy and FtH suppression had an additive protective effect against fibrosis. Adoptive transfer of iron-loaded macrophages alleviated kidney fibrosis, validating the protective effect of iron-replete macrophages in CKD. Thus, targeting intracellular iron deficiency of kidney macrophages in CKD can serve as a therapeutic opportunity to mitigate disease progression.

Probing Subcellular Iron Availability with Genetically Encoded Nitric Oxide Biosensors

Cellular iron supply is required for various biochemical processes. Measuring bioavailable iron in cells aids in obtaining a better understanding of its biochemical activities but is technically challenging. Existing techniques have several constraints that make precise localization difficult, and the lack of a functional readout makes it unclear whether the tested labile iron is available for metalloproteins. Here, we use geNOps; a ferrous iron-dependent genetically encoded fluorescent nitric oxide (NO) biosensor, to measure available iron in cellular locales. We exploited the nitrosylation-dependent fluorescence quenching of geNOps as a direct readout for cellular iron absorption, distribution, and availability. Our findings show that, in addition to ferrous iron salts, the complex of iron (III) with N,N'-bis (2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED) can activate the iron (II)-dependent NO probe within intact cells. Cell treatment for only 20 min with iron sucrose was also sufficient to activate the biosensor in the cytosol and mitochondria significantly; however, ferric carboxymaltose failed to functionalize the probe, even after 2 h of cell treatment. Our findings show that the geNOps approach detects available iron (II) in cultured cells and can be applied to assay functional iron (II) at the (sub)cellular level.

Hydroxyl Radical Overproduction in the Envelope: an Achilles' Heel in Peptidoglycan Synthesis

While many mechanisms governing bacterial envelope homeostasis have been identified, others remain poorly understood. To decipher these processes, we previously developed an assay in the Gram-negative model Escherichia coli to identify genes involved in maintenance of envelope integrity. One such gene was ElyC, which was shown to be required for envelope integrity and peptidoglycan synthesis at room temperature. ElyC is predicted to be an integral inner membrane protein with a highly conserved domain of unknown function (DUF218). In this study, and stemming from a further characterization of the role of ElyC in maintaining cell envelope integrity, we serendipitously discovered an unappreciated form of oxidative stress in the bacterial envelope. We found that cells lacking ElyC overproduce hydroxyl radicals (HO^•) in their envelope compartment and that HO^• overproduction is directly or indirectly responsible for the peptidoglycan synthesis arrest, cell envelope integrity defects, and cell lysis of the ΔelyC mutant. Consistent with these observations, we show that the ΔelyC mutant defect is suppressed during anaerobiosis. HO^• is known to cause DNA damage but to our knowledge has not been shown to interfere with peptidoglycan synthesis. Thus, our work implicates oxidative stress as an important stressor in the bacterial cell envelope and opens the door to future studies deciphering the mechanisms that render peptidoglycan synthesis sensitive to oxidative stress. IMPORTANCE Oxidative stress is caused by the production and excessive accumulation of oxygen reactive species. In bacterial cells, oxidative stress mediated by hydroxyl radicals is typically associated with DNA damage in the cytoplasm. Here, we reveal the existence of a pathway for oxidative stress in the envelope of Gram-negative bacteria. Stemming from the characterization of a poorly characterized gene, we found that HO^• overproduction specifically in the envelope compartment causes inhibition of peptidoglycan synthesis and eventually bacterial cell lysis.

Protective Effect of Dinitrosyl Iron Complexes Bound with Hemoglobin on Oxidative Modification by Peroxynitrite

Dinitrosyl iron complexes (DNICs) are a physiological form of nitric oxide (•NO) in an organism. They are able not only to deposit and transport •NO, but are also to act as antioxidant and antiradical agents. However, the mechanics of hemoglobin-bound DNICs (Hb-DNICs) protecting Hb against peroxynitrite-caused, mediated oxidative modification have not yet been scrutinized. Through EPR spectroscopy we show that Hb-DNICs are destroyed under the peroxynitrite action in a dose-dependent manner. At the same time, DNICs inhibit the oxidation of tryptophan and tyrosine residues and formation of carbonyl derivatives. They also prevent the formation of covalent crosslinks between Hb subunits and degradation of a heme group. These effects can arise from the oxoferryl heme form being reduced, and they can be connected with the ability of DNICs to directly intercept peroxynitrite and free radicals, which emerge due to its homolysis. These data show that DNICs may ensure protection from myocardial ischemia.

Dinitrosyl Iron Complexes (DNICs). From Spontaneous Assembly to Biological Roles

Dinitrosyl iron complexes (DNICs) are spontaneously and rapidly generated in cells. Their assembly requires nitric oxide (NO), biothiols, and nonheme iron, either labile iron or iron-sulfur clusters. Despite ubiquitous detection by electron paramagnetic resonance in NO-producing cells, the DNIC's chemical biology remains only partially understood. In this Forum Article, we address the reaction mechanisms for endogenous DNIC formation, with a focus on a labile iron pool as the iron source. The capability of DNICs to promote S-nitrosation is discussed in terms of S-nitrosothiol generation associated with the formation and chemical reactivity of DNICs. We also highlight how elucidation of the chemical reactivity and the dynamics of DNICs combined with the development of detection/quantification methods can provide further information regarding their participation in physiological and pathological processes.

The Labile Iron Pool Reacts Rapidly and Catalytically with Peroxynitrite

While investigating peroxynitrite-dependent oxidation in murine RAW 264.7 macrophage cells, we observed that removal of the Labile Iron Pool (LIP) by chelation increases the intracellular oxidation of the fluorescent indicator H2DCF, so we concluded that the LIP reacts with peroxynitrite and decreases the yield of peroxynitrite-derived oxidants. This was a paradigm-shifting finding in LIP biochemistry and raised many questions. In this follow-up study, we address fundamental properties of the interaction between the LIP and peroxynitrite by using the same cellular model and fluorescence methodology. We have identified that the reaction between the LIP and peroxynitrite has catalytic characteristics, and we have estimated that the rate constant of the reaction is in the range of 106 to 107 M-1s-1. Together, these observations suggest that the LIP represents a constitutive peroxynitrite reductase system in RAW 264.7 cells.

[Possible mechanism of antioxidant action of dinitrosyl iron complexes]

The antioxidant effect of dinitrosyl iron complexes (DNICs) was studied in various model systems. DNICs with glutathione ligands effectively inhibited Cu2+-induced peroxidation of low density lipoproteins (LDL). The antioxidant effect of DNICs with phosphate ligands and free reduced glutathione (GSH) was less pronounced. In addition, DNICs with glutathione suppressed the formation of reactive oxygen species during co-oxidation of lecithin liposomes and glucose. Free radical oxidation in this system was induced with a lipophilic azo initiator and evaluated by luminol-dependent chemiluminescence. NO sharply stimulated chemiluminescence during co-oxidation of glucose and liposomes, thus suggesting the formation of potent oxidants under these conditions. Glutathione DNICs scavenge the superoxide radical anion generated in the xanthine-xanthine oxidase system. Superoxide production was assessed by lucigenin-dependent chemiluminescence and electron paramagnetic resonance (EPR) spectroscopy. Chemiluminescence revealed the dose-dependent character of antiradical effect of glutathione DNICs; moreover, these complexes turned out to be more efficient than GSH. EPR spectra of the adducts of the DEPMPO spin trap with free radicals suggest that the interaction of glutathione DNICs and superoxide does not result in the formation of the thiyl radical of glutathione. Here we propose a mechanism of the antioxidant action of glutathione DNICs, suggesting that unstable intermediate complexes are formed upon their interaction with superoxide or lipid radicals. Further, as a result of intramolecular rearrangement, these intermediates decompose without the free radical as the by-products.

Iron(II/III) Halide Complexes Promote the Interconversion of Nitric Oxide and S-Nitrosothiols through Reversible Fe-S Interaction

Heme and non-heme iron in biology mediate the storage/release of NO^• from S-nitrosothiols as a means to control the biological concentration of NO^•. Despite their importance in many physiological processes, the mechanisms of N-S bond formation/cleavage at Fe centers have been controversial. Herein, we report the interconversion of NO^• and S-nitrosothiols mediated by Fe^II/Fe^III chloride complexes. The reaction of 2 equiv of S-nitrosothiol (Ph₃CSNO) with [Cl₆Fe^II₂]^2- results in facile release of NO^• and formation of iron(III) halothiolate. Detailed spectroscopic studies, including in situ UV-vis, IR, and Mössbauer spectroscopy, support the interaction of the S atom with the Fe^II center. This is in contrast to the proposed mechanism of NO^• release from the well-studied "red product" κ¹-N bound S-nitrosothiol Fe^II complex, [(CN)₅Fe(κ¹-N-RSNO)]^3-. Additionally, Fe^III chloride can mediate NO^• storage through the formation of S-nitrosothiols. Treatment of iron(III) halothiolate with 2 equiv of NO^• regenerates Ph₃CSNO with the Fe^II source trapped as the S = 3/2 {FeNO}⁷ species [Cl₃FeNO]^-, which is inert toward further coordination and activation of S-nitrosothiols. Our work demonstrates how labile iron can mediate the interconversion of NO^•/thiolate and S-nitrosothiol, which has important implications toward how Nature manages the biological concentration of free NO^•.

Iron regulates myeloma cell/macrophage interaction and drives resistance to bortezomib

Iron plays a major role in multiple processes involved in cell homeostasis such as metabolism, respiration and DNA synthesis. Cancer cells exhibit pronounced iron retention as compared to healthy counterpart. This phenomenon also occurs in multiple myeloma (MM), a hematological malignancy characterized by terminally differentiated plasma cells (PCs), in which serum ferritin levels have been reported as a negative prognostic marker. The aim of current study is to evaluate the potential role of iron metabolism in promoting drug resistance in myeloma cancer cells with particular regard to the interactions between PCs and tumor-associated macrophages (TAMs) as a source of iron. Our data showed that myeloma cell lines are able to intake and accumulate iron and thus, increasing their scavenger antioxidant-related genes and mitochondrial mass. We further demonstrated that PCs pre-treated with ferric ammonium citrate (FAC) decreased bortezomib (BTZ)-induced apoptosis in vitro and successfully engrafted in zebrafish larvae treated with BTZ. Treating human macrophages with FAC, we observed a switch toward a M2-like phenotype associated with an increased expression of anti-inflammatory markers such as ARG1, suggesting the establishment of an iron-mediated immune suppressive tumor microenvironment favouring myeloma growth. Using mfap4:tomato mutant zebrafish larvae, we further confirmed the increase of PCs-monocytes interactions after FAC treatment which favour BTZ-resistance. Taken together our data support the hypothesis that targeting iron trafficking in myeloma microenvironment may represent a promising strategy to counteract a tumor-supporting milieu and drug resistance.

Iron related changes in MS lesions and their validity to characterize MS lesion types and dynamics with Ultra-high field magnetic resonance imaging

Iron accumulates with age in the normal human brain. This process is altered at several levels in the brain of multiple sclerosis (MS) patients. Since iron is mainly stored in oligodendrocytes and myelin in the normal brain, its liberation in demyelinating lesions may amplify tissue damage in demyelinating lesions and its uptake in macrophages and microglia may help to more precisely define activity stages of the lesions. In addition, glia cells change their iron import, export and storage properties in MS lesions, which is reflected by alterations in the expression of iron transport molecules. Changes of iron distribution in the brain can be reliably detected by MRI, particularly upon application of Ultra‐high magnetic field (7 Tesla). Iron‐sensitive MRI allows to more accurately distinguish the lesions in MS from those in other inflammatory brain diseases, to visualize a subset of slowly expanding lesions in the progressive stage of MS and to increase the sensitivity for lesion detection in the gray matter, such as the cerebral cortex or deep gray matter nuclei.

Noxious Iron-Calcium Connections in Neurodegeneration

Iron and calcium share the common feature of being essential for normal neuronal function. Iron is required for mitochondrial function, synaptic plasticity, and the development of cognitive functions whereas cellular calcium signals mediate neurotransmitter exocytosis, axonal growth and synaptic plasticity, and control the expression of genes involved in learning and memory processes. Recent studies have revealed that cellular iron stimulates calcium signaling, leading to downstream activation of kinase cascades engaged in synaptic plasticity. The relationship between calcium and iron is Janus-faced, however. While under physiological conditions iron-mediated reactive oxygen species generation boosts normal calcium-dependent signaling pathways, excessive iron levels promote oxidative stress leading to the upsurge of unrestrained calcium signals that damage mitochondrial function, among other downstream targets. Similarly, increases in mitochondrial calcium to non-physiological levels result in mitochondrial dysfunction and a predicted loss of iron homeostasis. Hence, if uncontrolled, the iron/calcium self-feeding cycle becomes deleterious to neuronal function, leading eventually to neuronal death. Here, we review the multiple cell-damaging responses generated by the unregulated iron/calcium self-feeding cycle, such as excitotoxicity, free radical-mediated lipid peroxidation, and the oxidative modification of crucial components of iron and calcium homeostasis/signaling: the iron transporter DMT1, plasma membrane, and intracellular calcium channels and pumps. We discuss also how iron-induced dysregulation of mitochondrial calcium contributes to the generation of neurodegenerative conditions, including Alzheimer’s disease (AD) and Parkinson’s disease (PD).

Thiyl radicals are co-products of dinitrosyl iron complex (DNIC) formation

Thiyl radicals are detected by EPR as co-products of dinitrosyl iron complex (DNIC) formation.