NEET Proteins: A New Link Between Iron Metabolism, Reactive Oxygen Species, and Cancer

Emerging mechanisms and applications of ferroptosis in the treatment of resistant cancers

The development of chemotherapy drugs has promoted anticancer treatment, but the effect on tumours is not clear because of treatment resistance; thus, it is necessary to further understand the mechanism of cell death to explore new therapeutic targets. As a new type of programmed cell death, ferroptosis is increasingly being targeted in the treatment of many cancers with clinical drugs and experimental compounds. Ferroptosis is stimulated in tumours with inherently high levels of ferrous ions by a reaction with abundant polyunsaturated fatty acids and the inhibition of antioxidant enzymes, which can overcome treatment resistance in cancers mainly through GPX4. In this review, we focus on the intrinsic cellular regulators against ferroptosis in cancer resistance, such as GPX4, NRF2 and the thioredoxin system. We summarize the application of novel compounds and drugs to circumvent treatment resistance. We also introduce the application of nanoparticles for the treatment of resistant cancers. In conclusion, targeting ferroptosis represents a considerable strategy for resistant cancer treatment.

Exploring the FMN binding site in the mitochondrial outer membrane protein mitoNEET

MitoNEET is a mitochondrial outer membrane protein that hosts a redox active [2Fe-2S] cluster in the C-terminal cytosolic domain. Increasing evidence has shown that mitoNEET has an essential role in regulating energy metabolism in human cells. Previously, we reported that the [2Fe-2S] clusters in mitoNEET can be reduced by the reduced flavin mononucleotide (FMNH₂) and oxidized by oxygen or ubiquinone-2, suggesting that mitoNEET may act as a novel redox enzyme catalyzing electron transfer from FMNH₂ to oxygen or ubiquinone. Here, we explore the FMN binding site in mitoNEET by using FMN analogs and find that lumiflavin, like FMN, at nanomolar concentrations can mediate the redox transition of the mitoNEET [2Fe-2S] clusters in the presence of flavin reductase and NADH (100 μM) under aerobic conditions. The electron paramagnetic resonance (EPR) measurements show that both FMN and lumiflavin can dramatically change the EPR spectrum of the reduced mitoNEET [2Fe-2S] clusters and form a covalently bound complex with mitoNEET under blue light exposure, suggesting that FMN/lumiflavin has specific interactions with the [2Fe-2S] clusters in mitoNEET. In contrast, lumichrome, another FMN analog, fails to mediate the redox transition of the mitoNEET [2Fe-2S] clusters and has no effect on the EPR spectrum of the reduced mitoNEET [2Fe-2S] clusters under blue light exposure. Instead, lumichrome can effectively inhibit the FMNH₂-mediated reduction of the mitoNEET [2Fe-2S] clusters, indicating that lumichrome may act as a potential inhibitor to block the electron transfer activity of mitoNEET.

Ferroptosis and Cancer: Mitochondria Meet the "Iron Maiden" Cell Death

Ferroptosis is a new type of oxidative regulated cell death (RCD) driven by iron-dependent lipid peroxidation. As major sites of iron utilization and master regulators of oxidative metabolism, mitochondria are the main source of reactive oxygen species (ROS) and, thus, play a role in this type of RCD. Ferroptosis is, indeed, associated with severe damage in mitochondrial morphology, bioenergetics, and metabolism. Furthermore, dysregulation of mitochondrial metabolism is considered a biochemical feature of neurodegenerative diseases linked to ferroptosis. Whether mitochondrial dysfunction can, per se, initiate ferroptosis and whether mitochondrial function in ferroptosis is context-dependent are still under debate. Cancer cells accumulate high levels of iron and ROS to promote their metabolic activity and growth. Of note, cancer cell metabolic rewiring is often associated with acquired sensitivity to ferroptosis. This strongly suggests that ferroptosis may act as an adaptive response to metabolic imbalance and, thus, may constitute a new promising way to eradicate malignant cells. Here, we review the current literature on the role of mitochondria in ferroptosis, and we discuss opportunities to potentially use mitochondria-mediated ferroptosis as a new strategy for cancer therapy.

Nrf2 and Ferroptosis: A New Research Direction for Neurodegenerative Diseases

Ferroptosis is a kind of regulated cell death (RCD) caused by the redox state disorder of intracellular microenvironment controlled by glutathione (GSH) peroxidase 4 (GPX4), which is inhibited by iron chelators and lipophilic antioxidants. In addition to classical regulatory mechanisms, new regulatory factors for ferroptosis have been discovered in recent years, such as the P53 pathway, the activating transcription factor (ATF)3/4 pathway, Beclin 1 (BECN1) pathway, and some non-coding RNA. Ferroptosis is closely related to cancer treatment, neurodegenerative diseases, ischemia–reperfusion of organ, neurotoxicity, and others, in particular, in the field of neurodegenerative diseases treatment has aroused people’s interest. The nuclear factor E2 related factor 2 (Nrf2/NFE2L2) has been proved to play a key role in neurodegenerative disease treatment and ferroptosis regulation. Ferroptosis promotes the progression of neurodegenerative diseases, while the expression of Nrf2 and its target genes (Ho-1, Nqo-1, and Trx) has been declined with aging; therefore, there is still insufficient evidence for ferroptosis and Nrf2 regulatory networks in the field of neurodegenerative diseases. In this review, we will provide a brief overview of ferroptosis regulatory mechanisms, as well as an emphasis on the mechanism of Nrf2 regulating ferroptosis. We also highlight the role of ferroptosis and Nrf2 during the process of neurodegenerative diseases and investigate a theoretical basis for further research on the relationship between Nrf2 and ferroptosis in the process of neurodegenerative diseases treatment.

The Role of Selective Protein Degradation in the Regulation of Iron and Sulfur Homeostasis in Plants

Plants are able to synthesize all essential metabolites from minerals, water, and light to complete their life cycle. This plasticity comes at a high energy cost, and therefore, plants need to tightly allocate resources in order to control their economy. Being sessile, plants can only adapt to fluctuating environmental conditions, relying on quality control mechanisms. The remodeling of cellular components plays a crucial role, not only in response to stress, but also in normal plant development. Dynamic protein turnover is ensured through regulated protein synthesis and degradation processes. To effectively target a wide range of proteins for degradation, plants utilize two mechanistically-distinct, but largely complementary systems: the 26S proteasome and the autophagy. As both proteasomal- and autophagy-mediated protein degradation use ubiquitin as an essential signal of substrate recognition, they share ubiquitin conjugation machinery and downstream ubiquitin recognition modules. Recent progress has been made in understanding the cellular homeostasis of iron and sulfur metabolisms individually, and growing evidence indicates that complex crosstalk exists between iron and sulfur networks. In this review, we highlight the latest publications elucidating the role of selective protein degradation in the control of iron and sulfur metabolism during plant development, as well as environmental stresses.

Nanoparticle formulation and in vitro efficacy testing of the mitoNEET ligand NL-1 for drug delivery in a brain endothelial model of ischemic reperfusion-injury

Ischemic reperfusion injury after a stroke is a leading cause of mortality and disability due to neuronal loss and tissue damage. Mitochondrial dysfunction plays a major role in the reperfusion-injury sequelae, and offers an attractive drug target. Mitochondrial derived reactive oxygen species (ROS) and resultant apoptotic cascade are among the primary mechanisms of neuronal death following ischemia and reperfusion injury. Here we optimized a nanoparticle formulation for the mitoNEET ligand NL-1, to target mitochondrial dysfunction post ischemic reperfusion (IR) injury. NL-1, a hydrophobic drug, was formulated using PLGA polymers with a particle size and entrapment efficiency of 123.9 ± 17.1 nm and 59.7 ± 10.1%, respectively. The formulation was characterized for physical state of NL-1, in vitro release, uptake and nanoparticle localization. A near complete uptake of nanoparticles was found to occur by three hours, with the process being energy-dependent and occurring via caveolar mediated endocytosis. The fluorescent nanoparticles were found to localize in the cytoplasm of the endothelial cells. An in vitro oxygen glucose deprivation (OGD) model to mimic IR was employed for in vitro efficacy testing in murine brain vascular endothelium cells (bEND.3 cells). Efficacy studies showed that both NL-1 and the nanoparticles loaded with NL-1 had a protective activity against peroxide generation, and displayed improved cellular viability, as seen via reduction in cellular apoptosis. Finally, PLGA nanoparticles were found to have a non-toxic profile in vitro, and were found to be safe for intravenous administration. This study lays the preliminary work for potential use of mitoNEET as a target and NL-1 as a therapeutic for the treatment of cerebral ischemia and reperfusion injury.

Expression of a dominant-negative AtNEET-H89C protein disrupts iron-sulfur metabolism and iron homeostasis in Arabidopsis

Iron-sulfur (Fe-S) clusters play an essential role in plants as protein cofactors mediating diverse electron transfer reactions. Because they can react with oxygen to form reactive oxygen species (ROS) and inflict cellular damage, the biogenesis of Fe-S clusters is highly regulated. A recently discovered group of 2Fe-2S proteins, termed NEET proteins, was proposed to coordinate Fe-S, Fe and ROS homeostasis in mammalian cells. Here we report that disrupting the function of AtNEET, the sole member of the NEET protein family in Arabidopsis thaliana, triggers leaf-associated Fe-S- and Fe-deficiency responses, elevated Fe content in chloroplasts (1.2-1.5-fold), chlorosis, structural damage to chloroplasts and a high seedling mortality rate. Our findings suggest that disrupting AtNEET function disrupts the transfer of 2Fe-2S clusters from the chloroplastic 2Fe-2S biogenesis pathway to different cytosolic and chloroplastic Fe-S proteins, as well as to the cytosolic Fe-S biogenesis system, and that uncoupling this process triggers leaf-associated Fe-S- and Fe-deficiency responses that result in Fe over-accumulation in chloroplasts and enhanced ROS accumulation. We further show that AtNEET transfers its 2Fe-2S clusters to DRE2, a key protein of the cytosolic Fe-S biogenesis system, and propose that the availability of 2Fe-2S clusters in the chloroplast and cytosol is linked to Fe homeostasis in plants.

Defects in CISD-1, a mitochondrial iron-sulfur protein, lower glucose level and ATP production in Caenorhabditis elegans

Background

CDGSH iron sulfur domain-containing protein 1 (CISD-1) belongs to the CISD protein family that is evolutionary conserved across different species. In mammals, CISD-1 protein has been implicated in diseases such as cancers and diabetes. As a tractable model organism to study disease-associated proteins, we employed Caenorhabditis elegans in this study with an aim to establish a model for interrogating the functional relevance of CISD-1 in human metabolic conditions.

Methods

We first bioinformatically identified the human Cisd-1 homologue in worms. We then employed N2 wild-type and cisd-1(tm4993) mutant to investigate the consequences of CISD-1 loss-of-function on: 1) the expression pattern of CISD-1, 2) mitochondrial morphology pattern, 3) mitochondrial function and bioenergetics, and 4) the effects of anti-diabetes drugs.

Results

We first identified C. elegans W02B12.15 gene as the human Cisd-1 homologous gene, and pinpointed the localization of CISD-1 to the outer membrane of mitochondria. As compared with the N2 wild-type worm, cisd-1(tm4993) mutant exhibited a higher proportion of hyperfused form of mitochondria. This structural abnormality was associated with the generation of higher levels of ROS and mitochondrial superoxide but lower ATP. These physiological changes in mutants did not result in discernable effects on animal motility and lifespan. Moreover, the amount of glucose in N2 wild-type worms treated with troglitazone and pioglitazone, derivatives of TZD, was reduced to a comparable level as in the mutant animals.

Conclusions

By focusing on the Cisd-1 gene, our study established a C. elegans genetic system suitable for modeling human diabetes-related diseases.

Iron metabolic pathways in the processes of sponge plasticity

The ability to regulate oxygen consumption evolved in ancestral animals and is intrinsically linked to iron metabolism. The iron pathways have been intensively studied in mammals, whereas data on distant invertebrates are limited. Sea sponges represent the oldest animal phylum and have unique structural plasticity and capacity to reaggregate after complete dissociation. We studied iron metabolic factors and their expression during reaggregation in the White Sea cold-water sponges Halichondria panicea and Halisarca dujardini. De novo transcriptomes were assembled using RNA-Seq data, and evolutionary trends were analyzed with bioinformatic tools. Differential expression during reaggregation was studied for H. dujardini. Enzymes of the heme biosynthesis pathway and transport globins, neuroglobin (NGB) and androglobin (ADGB), were identified in sponges. The globins mutate at higher evolutionary rates than the heme synthesis enzymes. Highly conserved iron-regulatory protein 1 (IRP1) presumably interacts with the iron-responsive elements (IREs) found in mRNAs of ferritin (FTH1) and a putative transferrin receptor NAALAD2. The reaggregation process is accompanied by increased expression of IRP1, the antiapoptotic factor BCL2, the inflammation factor NFκB (p65), FTH1 and NGB, as well as by an increase in mitochondrial density. Our data indicate a complex mechanism of iron regulation in sponge structural plasticity and help to better understand general mechanisms of morphogenetic processes in multicellular species.

Deregulated High Affinity Copper Transport Alters Iron Homeostasis in Arabidopsis

The present work describes the effects on iron homeostasis when copper transport was deregulated in Arabidopsis thaliana by overexpressing high affinity copper transporters COPT1 and COPT3 (COPT^OE ). A genome-wide analysis conducted on COPT1^OE plants, highlighted that iron homeostasis gene expression was affected under both copper deficiency and excess. Among the altered genes were those encoding the iron uptake machinery and their transcriptional regulators. Subsequently, COPT^OE seedlings contained less iron and were more sensitive than controls to iron deficiency. The deregulation of copper (I) uptake hindered the transcriptional activation of the subgroup Ib of basic helix-loop-helix (bHLH-Ib) factors under copper deficiency. Oppositely, copper excess inhibited the expression of the master regulator FIT but activated bHLH-Ib expression in COPT^OE plants, in both cases leading to the lack of an adequate iron uptake response. As copper increased in the media, iron (III) was accumulated in roots, and the ratio iron (III)/iron (II) was increased in COPT^OE plants. Thus, iron (III) overloading in COPT^OE roots inhibited local iron deficiency responses, aimed to metal uptake from soil, leading to a general lower iron content in the COPT^OE seedlings. These results emphasized the importance of appropriate spatiotemporal copper uptake for iron homeostasis under non-optimal copper supply. The understanding of the role of copper uptake in iron metabolism could be applied for increasing crops resistance to iron deficiency.

Glycogen branching enzyme controls cellular iron homeostasis via Iron Regulatory Protein 1 and mitoNEET

Iron Regulatory Protein 1 (IRP1) is a bifunctional cytosolic iron sensor. When iron levels are normal, IRP1 harbours an iron-sulphur cluster (holo-IRP1), an enzyme with aconitase activity. When iron levels fall, IRP1 loses the cluster (apo-IRP1) and binds to iron-responsive elements (IREs) in messenger RNAs (mRNAs) encoding proteins involved in cellular iron uptake, distribution, and storage. Here we show that mutations in the Drosophila 1,4-Alpha-Glucan Branching Enzyme (AGBE) gene cause porphyria. AGBE was hitherto only linked to glycogen metabolism and a fatal human disorder known as glycogen storage disease type IV. AGBE binds specifically to holo-IRP1 and to mitoNEET, a protein capable of repairing IRP1 iron-sulphur clusters. This interaction ensures nuclear translocation of holo-IRP1 and downregulation of iron-dependent processes, demonstrating that holo-IRP1 functions not just as an aconitase, but throttles target gene expression in anticipation of declining iron requirements.

Higher organisms regulate cellular iron concentrations through Iron Regulatory Proteins (IRPs), which regulate specific messenger RNAs. Here Huynh et al. show that IRP1 requires a Glycogen Branching Enzyme for proper function, and that IRP1 has additional regulatory roles in cell nuclei.

Mitochondria regulation in ferroptosis

Ferroptosis is recognized as a new form of regulated cell death which is initiated by severe lipid peroxidation relying on reactive oxygen species (ROS) generation and iron overload. This iron-dependent cell death manifests evident morphological, biochemical and genetic differences from other forms of regulated cell death, such as apoptosis, autophagy, necrosis and pyroptosis. Ferroptosis was primarily characterized by condensed mitochondrial membrane densities and smaller volume than normal mitochondria, as well as the diminished or vanished of mitochondria crista and outer membrane ruptured. Mitochondria take the center role in iron metabolism, as well as substance and energy metabolism as it's the major organelle in iron utilization, catabolic and anabolic pathways. Interference of key regulators of mitochondrial lipid metabolism (e.g., ASCF2 and CS), iron homeostasis (e.g., ferritin, mitoferrin1/2 and NEET proteins), glutamine metabolism and other signaling pathways make a difference to ferroptotic sensitivity. Targeted induction of ferroptosis was also considered as a potential therapeutic strategy to some oxidative stress diseases, including neurodegenerative disorders, ischemia-reperfusion injury, traumatic spinal cord injury. However, the pertinence between mitochondria and ferroptosis is still in dispute. Here we systematic elucidate the morphological characteristics and metabolic regulation of mitochondria in the regulation of ferroptosis.

Intestinal SIRT1 Deficiency Protects Mice from Ethanol-Induced Liver Injury by Mitigating Ferroptosis

Aberrant liver sirtuin 1 (SIRT1), a mammalian NAD⁺-dependent protein deacetylase, is implicated in the pathogenesis of alcoholic liver disease (ALD). However, the role of intestinal SIRT1 in ALD is presently unknown. This study investigated the involvement of intestine-specific SIRT1 in ethanol-induced liver dysfunction in mice. Ethanol feeding studies were performed on knockout mice with intestinal-specific SIRT1 deletion [SIRT1i knockout (KO)] and flox control [wild-type (WT)] mice with a chronic-plus-binge ethanol feeding protocol. After ethanol administration, hepatic inflammation and liver injury were substantially attenuated in the SIRT1iKO mice compared with the WT mice, suggesting that intestinal SIRT1 played a detrimental role in the ethanol-induced liver injury. Mechanistically, the hepatic protective effect of intestinal SIRT1 deficiency was attributable to ameliorated dysfunctional iron metabolism, increased hepatic glutathione contents, and attenuated lipid peroxidation, along with inhibition of a panel of genes implicated in the ferroptosis process in the livers of ethanol-fed mice. This study demonstrates that ablation of intestinal SIRT1 protected mice from the ethanol-induced inflammation and liver damage. The protective effects of intestinal SIRT1 deficiency are mediated, at least partially, by mitigating hepatic ferroptosis. Targeting intestinal SIRT1 or dampening hepatic ferroptosis signaling may have therapeutic potential for ALD in humans.

Linking Cancer Metabolic Dysfunction and Genetic Instability through the Lens of Iron Metabolism

Iron (Fe) is an essential element that plays a fundamental role in a wide range of cellular functions, including cellular proliferation, DNA synthesis, as well as DNA damage and repair. Because of these connections, iron has been strongly implicated in cancer development. Cancer cells frequently have changes in the expression of iron regulatory proteins. For example, cancer cells frequently upregulate transferrin (increasing uptake of iron) and down regulate ferroportin (decreasing efflux of intracellular iron). These changes increase the steady-state level of intracellular redox active iron, known as the labile iron pool (LIP). The LIP typically contains approximately 2% intracellular iron, which primarily exists as ferrous iron (Fe²⁺). The LIP can readily contribute to oxidative distress within the cell through Fe²⁺-dioxygen and Fenton chemistries, generating the highly reactive hydroxyl radical (HO^•). Due to the reactive nature of the LIP, it can contribute to increased DNA damage. Mitochondrial dysfunction in cancer cells results in increased steady-state levels of hydrogen peroxide and superoxide along with other downstream reactive oxygen species. The increased presence of H₂O₂ and O₂^•- can increase the LIP, contributing to increased mitochondrial uptake of iron as well as genetic instability. Thus, iron metabolism and labile iron pools may play a central role connecting the genetic mutational theories of cancer to the metabolic theories of cancer.

Crystal structure of the mitochondrial protein mitoNEET bound to a benze-sulfonide ligand

MitoNEET (gene cisd1) is a mitochondrial outer membrane [2Fe-2S] protein and is a potential drug target in several metabolic diseases. Previous studies have demonstrated that mitoNEET functions as a redox-active and pH-sensing protein that regulates mitochondrial metabolism, although the structural basis of the potential drug binding site(s) remains elusive. Here we report the crystal structure of the soluble domain of human mitoNEET with a sulfonamide ligand, furosemide. Exploration of the high-resolution crystal structure is used to design mitoNEET binding molecules in a pilot study of molecular probes for use in future development of mitochondrial targeted therapies for a wide variety of metabolic diseases, including obesity, diabetes and neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease.

The MitoNEET Ligand NL-1 Mediates Antileukemic Activity in Drug-Resistant B-Cell Acute Lymphoblastic Leukemia

Disease relapse in B-cell acute lymphoblastic leukemia (ALL), either due to development of acquired resistance after therapy or because of de novo resistance, remains a therapeutic challenge. In the present study, we have developed a cytarabine (Ara-C)-resistant REH cell line (REH/Ara-C) as a chemoresistance model. REH/Ara-C 1) was not crossresistant to vincristine or methotrexate; 2) showed a similar proliferation rate and cell surface marker expression as parental REH; 3) demonstrated decreased chemotaxis toward bone marrow stromal cells; and 4) expressed higher transcript levels of cytidine deaminase (CDA) and mitoNEET (CISD1) than the parental REH cell line. Based on these findings, we tested NL-1, a mitoNEET inhibitor, which induced a concentration-dependent decrease in cell viability with a comparable IC50 value in REH and REH/Ara-C. Furthermore, NL-1 decreased cell viability in six different ALL cell lines and showed inhibitory activity in a hemosphere assay. NL-1 also impaired the migratory ability of leukemic cells, irrespective of the chemoattractant used, in a chemotaxis assay. More importantly, NL-1 showed specific activity in inducing death in a drug-resistant population of leukemic cells within a coculture model that mimicked the acquired resistance and de novo resistance observed in the bone marrow of relapsed patients. Subsequent studies indicated that NL-1 mediates autophagy, and inhibition of autophagy partially decreased NL-1-induced tumor cell death. Finally, NL-1 showed antileukemic activity in an in vivo mouse ALL model. Taken together, our study demonstrates that mitoNEET has potential as a novel antileukemic drug target in treatment refractory or relapsed ALL.

4-Hydroxynonenal and 4-Oxononenal Differentially Bind to the Redox Sensor MitoNEET

MitoNEET is a CDGSH iron-sulfur protein that has been a target for drug development for diseases such as type-2 diabetes, cancer, and Parkinson's disease. Functions proposed for mitoNEET are as a redox sensor and regulator of free iron in the mitochondria. We have investigated the reactivity of mitoNEET toward the reactive electrophiles 4-hydroxynonenal (HNE) and 4-oxononenal (ONE) that are produced from the oxidation of polyunsaturated fatty acid during oxidative stress. Proteomic, electrophoretic, and spectroscopic analysis has shown that HNE and ONE react in a sequence selective manner that was unexpected considering the structure similarity of these two reactive electrophiles.

Binding of thiazolidinediones to the endoplasmic reticulum protein nutrient-deprivation autophagy factor-1

Nutrient-deprivation autophagy factor-1 (NAF-1, miner1; gene cisd2) is part of the [2Fe-2S]-containing protein family which includes mitoNEET (gene cisd1) and MiNT (miner2; gene cisd3). These proteins are redox active and are thought to play an important role in cellular energy homeostasis with NAF-1 playing a critical role in calcium regulation and aging. To date, no studies have investigated potential ligand interaction with NAF-1. Here we show that the thiazolidinediones pioglitazone and rosiglitazone along with the mitoNEET ligand, NL-1, bind to NAF-1 with low micromolar affinities. Further, we show that overexpression of NAF-1 in hepatocellular carcinoma (HepG2) cells reduces inhibition of mitochondrial respiration by pioglitazone. Our findings support the need for further efforts of the rational design of selective NAF-1 ligands.

Iron Metabolism in Cancer

Demanded as an essential trace element that supports cell growth and basic functions, iron can be harmful and cancerogenic though. By exchanging between its different oxidized forms, iron overload induces free radical formation, lipid peroxidation, DNA, and protein damages, leading to carcinogenesis or ferroptosis. Iron also plays profound roles in modulating tumor microenvironment and metastasis, maintaining genomic stability and controlling epigenetics. in order to meet the high requirement of iron, neoplastic cells have remodeled iron metabolism pathways, including acquisition, storage, and efflux, which makes manipulating iron homeostasis a considerable approach for cancer therapy. Several iron chelators and iron oxide nanoparticles (IONPs) has recently been developed for cancer intervention and presented considerable effects. This review summarizes some latest findings about iron metabolism function and regulation mechanism in cancer and the application of iron chelators and IONPs in cancer diagnosis and therapy.

Characterization of a new N-terminally acetylated extra-mitochondrial isoform of frataxin in human erythrocytes

Frataxin is a highly conserved protein encoded by the frataxin (FXN) gene. The full-length 210-amino acid form of protein frataxin (1-210; isoform A) expressed in the cytosol of cells rapidly translocates to the mitochondria, where it is converted to the mature form (81-210) by mitochondrial processing peptidase. Mature frataxin (81-210) is a critically important protein because it facilitates the assembly of mitochondrial iron-sulfur cluster protein complexes such as aconitase, lipoate synthase, and succinate dehydrogenases. Decreased expression of frataxin protein is responsible for the devastating rare genetic disease of Friedreich's ataxia. The mitochondrial form of frataxin has long been thought to be present in erythrocytes even though paradoxically, erythrocytes lack mitochondria. We have discovered that erythrocyte frataxin is in fact a novel isoform of frataxin (isoform E) with 135-amino acids and an N-terminally acetylated methionine residue. There is three times as much isoform E in erythrocytes (20.9 ± 6.4 ng/mL) from the whole blood of healthy volunteers (n = 10) when compared with the mature mitochondrial frataxin present in other blood cells (7.1 ± 1.0 ng/mL). Isoform E lacks a mitochondrial targeting sequence and so is distributed to both cytosol and the nucleus when expressed in cultured cells. When extra-mitochondrial frataxin isoform E is expressed in HEK 293 cells, it is converted to a shorter isoform identical to the mature frataxin found in mitochondria, which raises the possibility that it is involved in disease etiology. The ability to specifically quantify extra-mitochondrial and mitochondrial isoforms of frataxin in whole blood will make it possible to readily follow the natural history of diseases such as Friedreich's ataxia and monitor the efficacy of therapeutic interventions.

Hydrogen Sulfide-Synthesizing Enzymes Are Altered in a Case of Oral Cavity Mucoepidermoid Carcinoma

Mucoepidermoid carcinoma (MEC) is the most common malignant epithelial neoplasm of the salivary glands. MECs of the mouth floor are rare, with only a few cases reported. Here we report a MEC of the mouth floor in a 55-year-old woman. Since several studies have shown that hydrogen sulfide (H₂S)-synthesizing enzymes are often increased in malignant tumors compared to benign counterpart tissues, we used western blotting to compare the protein levels of cystathionine-β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3-MST) in a mouth floor MEC to adjacent benign oral mucosae. We also used high-performance liquid chromatography to quantify possible differences in tissue sulfur fraction concentrations between the two biopsy types. Last, we used western blotting to examine nicotinamide phosphoribosyl transferase (Nampt), mitoNEET, and phospho-ser727-Stat3 levels in the biopsies. We found that all the proteins and phospho-ser727-Stat3 are increased in the MEC compared to benign mucosae. Interestingly, free H₂S levels, acid-labile, and the sulfane sulfur factions were essentially the same between the MEC and benign tissue. Although limited to a single and unusual tumor type, to our knowledge this is only the third time H₂S concentrations were directly quantified inside a human tumor. Last, our results replicate those of two previous studies where the H₂S-synthesizing enzymes are increased in a malignant tumor, while free H₂S is either not increased or only slightly increased, suggesting that malignant tumors rapidly metabolize H₂S as part of tumor maintenance and growth.

Pioglitazone Represents an Effective Therapeutic Target in Preventing Oxidative/Inflammatory Cochlear Damage Induced by Noise Exposure

Recent progress in hearing loss research has provided strong evidence for the imbalance of cellular redox status and inflammation as common predominant mechanisms of damage affecting the organ of Corti including noise induced hearing loss. The discovery of a protective molecule acting on both mechanisms is challenging. The thiazolidinediones, a class of antidiabetic drugs including pioglitazone and rosiglitazone, have demonstrated diverse pleiotrophic effects in many tissues where they exhibit anti-inflammatory, anti-proliferative, tissue protective effects and regulators of redox balance acting as agonist of peroxisome proliferator-activated receptors (PPARs). They are members of the family of ligand regulated nuclear hormone receptors that are also expressed in several cochlear cell types, including the outer hair cells. In this study, we investigated the protective capacity of pioglitazone in a model of noise-induced hearing loss in Wistar rats and the molecular mechanisms underlying this protective effects. Specifically, we employed a formulation of pioglitazone in a biocompatible thermogel providing rapid, uniform and sustained inner ear drug delivery via transtympanic injection. Following noise exposure (120 dB, 10 kHz, 1 h), different time schedules of treatment were employed: we explored the efficacy of pioglitazone given immediately (1 h) or at delayed time points (24 and 48 h) after noise exposure and the time course and extent of hearing recovery were assessed. We found that pioglitazone was able to protect auditory function at the mid-high frequencies and to limit cell death in the cochlear basal/middle turn, damaged by noise exposure. Immunofluorescence and western blot analysis provided evidence that pioglitazone mediates both anti-inflammatory and anti-oxidant effects by decreasing NF-κB and IL-1β expression in the cochlea and opposing the oxidative damage induced by noise insult. These results suggest that intratympanic pioglitazone can be considered a valid therapeutic strategy for attenuating noise-induced hearing loss and cochlear damage, reducing inflammatory signaling and restoring cochlear redox balance.

Hydrogen Sulfide and Hydrogen Sulfide-Synthesizing Enzymes Are Altered in a Case of Oral Adenoid Cystic Carcinoma

Adenoid cystic carcinomas (ACC) constitute 1% of all head and neck malignancies and are very rare in the oral cavity. With < 60 oral ACCs described, their pathobiology is incompletely understood. Here, we report a case of oral cavity ACC in a 54-year-old woman. Since recent studies have demonstrated that several human tumors overexpress the hydrogen sulfide (H₂S)-synthesizing enzymes cystathionine-β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3-MST), and also show dysregulated H₂S levels, we examined these biomarkers in the oral ACC and compared the results to those of adjacent benign oral epithelium. Western blotting was used to compare the protein expression of CBS, CSE, 3-MST, nicotinamide phosphoribosyl transferase, and mitoNEET in ACC and adjacent benign oral mucosae. High-performance liquid chromatography was used to quantify the differences in tissue H₂S concentrations between the two biopsy types. We found that all the proteins examined here were increased in the ACC compared to adjacent benign oral mucosae. Interestingly, H₂S concentrations were decreased approximately 30% in ACC compared to benign mucosae. Thus, in one example of this rare tumor type, the enzymes that synthesize H₂S are increased, while tissue H₂S levels are lower than those found in adjacent benign oral mucosae. Although limited to a single rare tumor type, to our knowledge this is the second time H₂S concentrations have been directly quantified inside a human tumor. Last, our results may indicate that alterations in H₂S synthesis and metabolism may be important in the pathobiology of ACC.

Targeting Nrf2 to Suppress Ferroptosis and Mitochondrial Dysfunction in Neurodegeneration

Ferroptosis is a newly described form of regulated cell death, distinct from apoptosis, necroptosis and other forms of cell death. Ferroptosis is induced by disruption of glutathione synthesis or inhibition of glutathione peroxidase 4, exacerbated by iron, and prevented by radical scavengers such as ferrostatin-1, liproxstatin-1, and endogenous vitamin E. Ferroptosis terminates with mitochondrial dysfunction and toxic lipid peroxidation. Although conclusive identification of ferroptosis in vivo is challenging, several salient and very well established features of neurodegenerative diseases are consistent with ferroptosis, including lipid peroxidation, mitochondrial disruption and iron dysregulation. Accordingly, interest in the role of ferroptosis in neurodegeneration is escalating and specific evidence is rapidly emerging. One aspect that has thus far received little attention is the antioxidant transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2). This transcription factor regulates hundreds of genes, of which many are either directly or indirectly involved in modulating ferroptosis, including metabolism of glutathione, iron and lipids, and mitochondrial function. This potentially positions Nrf2 as a key deterministic component modulating the onset and outcomes of ferroptotic stress. The minimal direct evidence currently available is consistent with this and indicates that Nrf2 may be critical for protection against ferroptosis. In contrast, abundant evidence demonstrates that enhancing Nrf2 signaling is potently neuroprotective in models of neurodegeneration, although the exact mechanism by which this is achieved is unclear. Further studies are required to determine to extent to which the neuroprotective effects of Nrf2 activation involve the prevention of ferroptosis.

Hydrogen Sulfide Is Increased in Oral Squamous Cell Carcinoma Compared to Adjacent Benign Oral Mucosae

BACKGROUND/AIM

Hydrogen sulfide (H2S) and the enzymes that synthesize it, cystathionine-b-synthase, cystathionine γ-lyase, and 3-mercaptopyruvate, are increased in different human malignancies. Due to its short half-life, H2S concentrations have not been directly measured in a human malignancy. Here we directly measured in vivo H2S levels within oral squamous cell carcinoma (OSCC).

PATIENTS AND METHODS

Punch biopsies of OSCC and benign mucosae from 15 patients were analyzed by HPLC, western blotting, and tissue microarray analyses.

RESULTS

H2S concentrations were significantly higher in OSCC compared to adjacent benign oral mucosae. Western blot and tissue microarray studies revealed significantly increased cystathionine-b-synthase, cystathionine γ-lyase, and 3-mercaptopyruvate, phopho-Stat3, mitoNEET, hTERT, and MAPK protein levels in OSCC.

CONCLUSION

H2S concentrations and the enzymes that synthesize it are significantly increased in OSCC. Here, for the first time H2S concentrations within a living human malignancy were measured and compared to adjacent counterpart benign tissue.

Phylogenetic analysis of the CDGSH iron-sulfur binding domain reveals its ancient origin

The iron-sulfur (2Fe-2S) binding motif CDGSH appears in many important plant and animal proteins that regulate iron and reactive oxygen metabolism. In human it is found in CISD1-3 proteins involved in diabetes, obesity, cancer, aging, cardiovascular disease and neurodegeneration. Despite the important biological role of the CDGSH domain, its origin, evolution and diversification, are largely unknown. Here, we report that: (1) the CDGSH domain appeared early in evolution, perhaps linked to the heavy use of iron-sulfur driven metabolism by early organisms; (2) a CISD3-like protein with two CDGSH domains on the same polypeptide appears to represent the ancient archetype of CDGSH proteins; (3) the origin of the human CISD3 protein is linked to the mitochondrial endosymbiotic event; (4) the CISD1/2 type proteins that contain only one CDGSH domain, but function as homodimers, originated after the divergence of bacteria and archaea/eukaryotes from their common ancestor; and (5) the human CISD1 and CISD2 proteins diverged about 650–720 million years ago, and CISD3 and CISD1/2 share their descent from an ancestral CISD about 1–1.1 billion years ago. Our findings reveal that the CDGSH domain is ancient in its origin and shed light on the complex evolutionary path of modern CDGSH proteins.

Structure of the human monomeric NEET protein MiNT and its role in regulating iron and reactive oxygen species in cancer cells

The NEET family is a relatively new class of three related [2Fe-2S] proteins (CISD1-3), important in human health and disease. While there has been growing interest in the homodimeric gene products of CISD1 (mitoNEET) and CISD2 (NAF-1), the importance of the inner mitochondrial CISD3 protein has only recently been recognized in cancer. The CISD3 gene encodes for a monomeric protein that contains two [2Fe-2S] CDGSH motifs, which we term mitochondrial inner NEET protein (MiNT). It folds with a pseudosymmetrical fold that provides a hydrophobic motif on one side and a relatively hydrophilic surface on the diametrically opposed surface. Interestingly, as shown by molecular dynamics simulation, the protein displays distinct asymmetrical backbone motions, unlike its homodimeric counterparts that face the cytosolic side of the outer mitochondrial membrane/endoplasmic reticulum (ER). However, like its counterparts, our biological studies indicate that knockdown of MiNT leads to increased accumulation of mitochondrial labile iron, as well as increased mitochondrial reactive oxygen production. Taken together, our study suggests that the MiNT protein functions in the same pathway as its homodimeric counterparts (mitoNEET and NAF-1), and could be a key player in this pathway within the mitochondria. As such, it represents a target for anticancer or antidiabetic drug development.