Mitoferrin-2-dependent mitochondrial iron uptake sensitizes human head and neck squamous carcinoma cells to photodynamic therapy

STARD7 maintains intestinal epithelial mitochondria architecture, barrier integrity, and protection from colitis

Susceptibility to inflammatory bowel diseases (IBDs), Crohn's disease (CD), and ulcerative colitis (UC) is linked with loss of intestinal epithelial barrier integrity and mitochondria dysfunction. Steroidogenic acute regulatory (StAR) protein-related lipid transfer (START) domain-containing protein 7 (STARD7) is a phosphatidylcholine-specific (PC-specific) lipid transfer protein that transports PC from the ER to the mitochondria, facilitating mitochondria membrane stabilization and respiration function. The aim of this study was to define the contribution of STARD7 in the regulation of the intestinal epithelial mitochondrial function and susceptibility to colitis. In silico analyses identified significantly reduced expression of STARD7 in patients with UC, which was associated with downregulation of metabolic function and a more severe disease phenotype. STARD7 was expressed in intestinal epithelial cells, and STARD7 knockdown resulted in deformed mitochondria and diminished aerobic respiration. Loss of mitochondria function was associated with reduced expression of tight junction proteins and loss of intestinal epithelial barrier integrity that could be recovered by AMPK activation. Stard7+/- mice were more susceptible to the development of DSS-induced and Il10-/- spontaneous models of colitis. STARD7 is critical for intestinal epithelial mitochondrial function and barrier integrity, and loss of STARD7 function increases susceptibility to IBD.

Role of ferroptosis in the pathogenesis of heart disease

Ferroptosis is a new form of regulated necrosis characterized by iron-dependent lipid peroxidation, leading to irreparable lipid damage, membrane permeabilization, and necrotic cell death. Ferroptosis has recently been implicated in the pathogenesis of multiple forms of heart disease such as myocardial infarction, cardiac hypertrophy, heart failure, and various cardiomyopathies. Important progress has also been made regarding how ferroptosis is regulated in vitro and in vivo as well as its role in cardiac homeostasis and disease pathogenesis. In this review, we discuss molecular mechanisms that regulates ferroptosis in the heart, including pathways leading to iron overload and lipid peroxidation as well as the roles of key organelles in this process. We also discuss recent findings pertaining to the new pathogenic role of ferroptosis in various forms of heart disease as well as genetic and pharmacologic strategies targeting ferroptosis in the heart.

Image-based quantification of mitochondrial iron uptake via Mitoferrin-2

Iron is a trace element that is critical for most living organisms and plays a key role in a wide variety of metabolic processes. In the mitochondrion, iron is involved in producing iron-sulfur clusters and synthesis of heme and kept within physiological ranges by concerted activity of multiple molecules. Mitochondrial iron uptake is mediated by the solute carrier transporters Mitoferrin-1 (SLC25A37) and Mitoferrin-2 (SLC25A28). While Mitoferrin-1 is mainly involved in erythropoiesis, the cellular function of the ubiquitously expressed Mitoferrin-2 remains less well defined. Furthermore, Mitoferrin-2 is associated with several human diseases, including cancer, cardiovascular and metabolic diseases, hence representing a potential therapeutic target. Here, we developed a robust approach to quantify mitochondrial iron uptake mediated by Mitoferrin-2 in living cells. We utilize HEK293 cells with inducible expression of Mitoferrin-2 and measure iron-induced quenching of rhodamine B[(1,10-phenanthroline-5-yl)-aminocarbonyl]benzyl ester (RPA) fluorescence and validate this assay for medium-throughput screening. This assay may allow identification and characterization of Mitoferrin-2 modulators and could enable drug discovery for this target.

Cancer Therapeutic Potential and Prognostic Value of the SLC25 Mitochondrial Carrier Family: A Review

Transporters of the solute carrier family 25 (SLC25) regulate the intracellular distribution and concentration of nucleotides, amino acids, dicarboxylates, and vitamins within the mitochondrial and cytoplasmic matrices. This mechanism involves changes in mitochondrial function, regulation of cellular metabolism, and the ability to provide energy. In this review, important members of the SLC25 family and their pathways affecting tumorigenesis and progression are elucidated, highlighting the diversity and complexity of these pathways. Furthermore, the significant potential of the members of SLC25 as both cancer therapeutic targets and biomarkers will be emphasized.

Oxidative stress induces mitochondrial iron overload and ferroptotic cell death

Oxidative stress has been shown to induce cell death in a wide range of human diseases including cardiac ischemia/reperfusion injury, drug induced cardiotoxicity, and heart failure. However, the mechanism of cell death induced by oxidative stress remains incompletely understood. Here we provide new evidence that oxidative stress primarily induces ferroptosis, but not apoptosis, necroptosis, or mitochondria-mediated necrosis, in cardiomyocytes. Intriguingly, oxidative stress induced by organic oxidants such as tert-butyl hydroperoxide (tBHP) and cumene hydroperoxide (CHP), but not hydrogen peroxide (H2O2), promoted glutathione depletion and glutathione peroxidase 4 (GPX4) degradation in cardiomyocytes, leading to increased lipid peroxidation. Moreover, elevated oxidative stress is also linked to labile iron overload through downregulation of the transcription suppressor BTB and CNC homology 1 (Bach1), upregulation of heme oxygenase 1 (HO-1) expression, and enhanced iron release via heme degradation. Strikingly, oxidative stress also promoted HO-1 translocation to mitochondria, leading to mitochondrial iron overload and lipid reactive oxygen species (ROS) accumulation. Targeted inhibition of mitochondrial iron overload or ROS accumulation, by overexpressing mitochondrial ferritin (FTMT) or mitochondrial catalase (mCAT), respectively, markedly inhibited oxidative stress-induced ferroptosis. The levels of mitochondrial iron and lipid peroxides were also markedly increased in cardiomyocytes subjected to simulated ischemia and reperfusion (sI/R) or the chemotherapeutic agent doxorubicin (DOX). Overexpressing FTMT or mCAT effectively prevented cardiomyocyte death induced by sI/R or DOX. Taken together, oxidative stress induced by organic oxidants but not H2O2 primarily triggers ferroptotic cell death in cardiomyocyte through GPX4 and Bach1/HO-1 dependent mechanisms. Our results also reveal mitochondrial iron overload via HO-1 mitochondrial translocation as a key mechanism as well as a potential molecular target for oxidative stress-induced ferroptosis in cardiomyocytes.

Membrane Transporters Involved in Iron Trafficking: Physiological and Pathological Aspects

Iron is an essential transition metal for its involvement in several crucial biological functions, the most notable being oxygen storage and transport. Due to its high reactivity and potential toxicity, intracellular and extracellular iron levels must be tightly regulated. This is achieved through transport systems that mediate cellular uptake and efflux both at the level of the plasma membrane and on the membranes of lysosomes, endosomes and mitochondria. Among these transport systems, the key players are ferroportin, the only known transporter mediating iron efflux from cells; DMT1, ZIP8 and ZIP14, which on the contrary, mediate iron influx into the cytoplasm, acting on the plasma membrane and on the membranes of lysosomes and endosomes; and mitoferrin, involved in iron transport into the mitochondria for heme synthesis and Fe-S cluster assembly. The focus of this review is to provide an updated view of the physiological role of these membrane proteins and of the pathologies that arise from defects of these transport systems.

Critical PDT Theory V: What it Takes

First evidence for an apoptotic response to photodamage provided by Oleinick's group in 1991.

Deficiency of mitochondrial calcium uniporter abrogates iron overload-induced cardiac dysfunction by reducing ferroptosis

Iron overload associated cardiac dysfunction remains a significant clinical challenge whose underlying mechanism(s) have yet to be defined. We aim to evaluate the involvement of the mitochondrial Ca2+ uniporter (MCU) in cardiac dysfunction and determine its role in the occurrence of ferroptosis. Iron overload was established in control (MCUfl/fl) and conditional MCU knockout (MCUfl/fl-MCM) mice. LV function was reduced by chronic iron loading in MCUfl/fl mice, but not in MCUfl/fl-MCM mice. The level of mitochondrial iron and reactive oxygen species were increased and mitochondrial membrane potential and spare respiratory capacity (SRC) were reduced in MCUfl/fl cardiomyocytes, but not in MCUfl/fl-MCM cardiomyocytes. After iron loading, lipid oxidation levels were increased in MCUfl/fl, but not in MCUfl/fl-MCM hearts. Ferrostatin-1, a selective ferroptosis inhibitor, reduced lipid peroxidation and maintained LV function in vivo after chronic iron treatment in MCUfl/fl hearts. Isolated cardiomyocytes from MCUfl/fl mice demonstrated ferroptosis after acute iron treatment. Moreover, Ca2+ transient amplitude and cell contractility were both significantly reduced in isolated cardiomyocytes from chronically Fe treated MCUfl/fl hearts. However, ferroptosis was not induced in cardiomyocytes from MCUfl/fl-MCM hearts nor was there a reduction in Ca2+ transient amplitude or cardiomyocyte contractility. We conclude that mitochondrial iron uptake is dependent on MCU, which plays an essential role in causing mitochondrial dysfunction and ferroptosis under iron overload conditions in the heart. Cardiac-specific deficiency of MCU prevents the development of ferroptosis and iron overload-induced cardiac dysfunction.

Mitoferrin-1 Promotes Proliferation and Abrogates Protein Oxidation via the Glutathione Pathway in Glioblastoma

Median overall survival is very low in patients with glioblastoma (GBM), largely because these tumors become resistant to therapy. Recently, we found that a decrease in the cytosolic labile iron pool underlies the acquisition of radioresistance. Both cytosolic and mitochondrial iron are important for regulating ROS production, which largely facilitates tumor progression and response to therapy. Here, we investigated the role of the mitochondrial iron transporters mitoferrin-1 (MFRN1) and mitoferrin-2 (MFRN2) in GBM progression. Analysis of The Cancer Genome Atlas database revealed upregulation of MFRN1 mRNA and downregulation of MFRN2 mRNA in GBM tumor tissue compared with non-GBM tissue, yet only the tumor expression level of MFRN1 mRNA negatively correlated with overall survival in patients. Overexpression of MFRN1 in glioma cells significantly increased the level of mitochondrial iron, enhanced the proliferation rate and anchorage-independent growth of these cells, and significantly decreased mouse survival in an orthotopic model of glioma. Finally, MFRN1 overexpression stimulated the upregulation of glutathione, which protected glioma cells from 4-hydroxynonenal-induced protein damage. Overall, these results demonstrate a mechanistic link between MFRN1-mediated mitochondrial iron metabolism and GBM progression. Manipulation of MFRN1 may provide a new therapeutic strategy for improving clinical outcomes in patients with GBM.

Mitochondrial Iron Metabolism: The Crucial Actors in Diseases

Iron is a trace element necessary for cell growth, development, and cellular homeostasis, but insufficient or excessive level of iron is toxic. Intracellularly, sufficient amounts of iron are required for mitochondria (the center of iron utilization) to maintain their normal physiologic function. Iron deficiency impairs mitochondrial metabolism and respiratory activity, while mitochondrial iron overload promotes ROS production during mitochondrial electron transport, thus promoting potential disease development. This review provides an overview of iron homeostasis, mitochondrial iron metabolism, and how mitochondrial iron imbalances-induced mitochondrial dysfunction contribute to diseases.

Mitoferrin, Cellular and Mitochondrial Iron Homeostasis

Iron is essential for many cellular processes, but cellular iron homeostasis must be maintained to ensure the balance of cellular signaling processes and prevent disease. Iron transport in and out of the cell and cellular organelles is crucial in this regard. The transport of iron into the mitochondria is particularly important, as heme and the majority of iron-sulfur clusters are synthesized in this organelle. Iron is also required for the production of mitochondrial complexes that contain these iron-sulfur clusters and heme. As the principal iron importers in the mitochondria of human cells, the mitoferrins have emerged as critical regulators of cytosolic and mitochondrial iron homeostasis. Here, we review the discovery and structure of the mitoferrins, as well as the significance of these proteins in maintaining cytosolic and mitochondrial iron homeostasis for the prevention of cancer and many other diseases.

A new fluorescent sensor mitoferrofluor indicates the presence of chelatable iron in polarized and depolarized mitochondria

Mitochondrial chelatable iron contributes to the severity of several injury processes, including ischemia/reperfusion, oxidative stress, and drug toxicity. However, methods to measure this species in living cells are lacking. To measure mitochondrial chelatable iron in living cells, here we synthesized a new fluorescent indicator, mitoferrofluor (MFF). We designed cationic MFF to accumulate electrophoretically in polarized mitochondria, where a reactive group then forms covalent adducts with mitochondrial proteins to retain MFF even after subsequent depolarization. We also show in cell-free medium that Fe²⁺ (and Cu²⁺), but not Fe³⁺, Ca²⁺, or other biologically relevant divalent cations, strongly quenched MFF fluorescence. Using confocal microscopy, we demonstrate in hepatocytes that red MFF fluorescence colocalized with the green fluorescence of the mitochondrial membrane potential (ΔΨ_m) indicator, rhodamine 123 (Rh123), indicating selective accumulation into the mitochondria. Unlike Rh123, mitochondria retained MFF after ΔΨ_m collapse. Furthermore, intracellular delivery of iron with membrane-permeant Fe³⁺/8-hydroxyquinoline (FeHQ) quenched MFF fluorescence by ∼80% in hepatocytes and other cell lines, which was substantially restored by the membrane-permeant transition metal chelator pyridoxal isonicotinoyl hydrazone. We also show FeHQ quenched the fluorescence of cytosolically coloaded calcein, another Fe²⁺ indicator, confirming that Fe³⁺ in FeHQ undergoes intracellular reduction to Fe²⁺. Finally, MFF fluorescence did not change after addition of the calcium mobilizer thapsigargin, which shows MFF is insensitive to physiologically relevant increases of mitochondrial Ca²⁺. In conclusion, the new sensor reagent MFF fluorescence is an indicator of mitochondrial chelatable Fe²⁺ in normal hepatocytes with polarized mitochondria as well as in cells undergoing loss of ΔΨ_m.

The Emerging Role of RNA N6-Methyladenosine Modification in Pancreatic Cancer

Pancreatic cancer (PC) is one of the most common malignant cancers, ranking the seventh highest causes of cancer-related deaths globally. Recently, RNA N6-methyladenosine (m6A) is emerging as one of the most abundant RNA modifications in eukaryote cells, involved in multiple RNA processes including RNA translocation, alternative splicing, maturation, stability, and degradation. As reported, m6A was dynamically and reversibly regulated by its “writers”, “erasers”, and “readers”, Increasing evidence has revealed the vital role of m6A modification in the development of multiple types of cancers including PC. Currently, aberrant m6A modification level has been found in both PC tissues and cell lines. Moreover, abnormal expressions of m6A regulators and m6A-modified genes have been reported to contribute to the malignant development of PC. Here in this review, we will focus on the function and molecular mechanism of m6A-modulated RNAs including coding RNAs as well as non-coding RNAs. Then the m6A regulators will be summarized to reveal their potential applications in the clinical diagnosis, prognosis, and therapeutics of PC.

FDX2 and ISCU Gene Variations Lead to Rhabdomyolysis With Distinct Severity and Iron Regulation

Background and Objectives

To determine common clinical and biological traits in 2 individuals with variants in ISCU and FDX2, displaying severe and recurrent rhabdomyolyses and lactic acidosis.

Methods

We performed a clinical characterization of 2 distinct individuals with biallelic ISCU or FDX2 variants from 2 separate families and a biological characterization with muscle and cells from those patients.

Results

The individual with FDX2 variants was clinically more affected than the individual with ISCU variants. Affected FDX2 individual fibroblasts and myoblasts showed reduced oxygen consumption rates and mitochondrial complex I and PDHc activities, associated with high levels of blood FGF21. ISCU individual fibroblasts showed no oxidative phosphorylation deficiency and moderate increase of blood FGF21 levels relative to controls. The severity of the FDX2 individual was not due to dysfunctional autophagy. Iron was excessively accumulated in ISCU-deficient skeletal muscle, which was accompanied by a downregulation of IRP1 and mitoferrin2 genes and an upregulation of frataxin (FXN) gene expression. This excessive iron accumulation was absent from FDX2 affected muscle and could not be correlated with variable gene expression in muscle cells.

Discussion

We conclude that FDX2 and ISCU variants result in a similar muscle phenotype, that differ in severity and skeletal muscle iron accumulation. ISCU and FDX2 are not involved in mitochondrial iron influx contrary to frataxin.

Low Blue Dose Photodynamic Therapy with Porphyrin-Iron Oxide Nanoparticles Complexes: In Vitro Study on Human Melanoma Cells

The purpose of this study was to investigate the effectiveness in photodynamic therapy of iron oxide nanoparticles (γ-Fe2O3 NPs), synthesized by laser pyrolysis technique, functionalized with 5,10,15,20-(Tetra-4-sulfonatophenyl) porphyrin tetraammonium (TPPS) on human cutaneous melanoma cells, after only 1 min blue light exposure. The efficiency of porphyrin loading on the iron oxide nanocarriers was estimated by using absorption and FTIR spectroscopy. The singlet oxygen yield was determined via transient characteristics of singlet oxygen phosphorescence at 1270 nm both for porphyrin functionalized nanoparticles and rose bengal used as standard. The irradiation was performed with a LED (405 nm, 1 mW/cm2) for 1 min after melanoma cells were treated with TPPS functionalized iron oxide nanoparticles (γ-Fe2O3 NPs_TPPS) and incubated for 24 h. Biological tests revealed a high anticancer effect of γ-Fe2O3 NPs_TPPS complexes indi-cated by the inhibition of tumor cell proliferation, reduction of cell adhesion, and induction of cell death through ROS generated by TPPS under light exposure. The biological assays were combined with the pharmacokinetic prediction of the porphyrin.

Iron overload and mitochondrial dysfunction orchestrate pulmonary fibrosis

Pulmonary fibrosis (PF) is a chronic, progressive heterogeneous disease of lung tissues with poor lung function caused by scar tissue. Due to our limited understanding of its mechanism, there is currently no treatment strategy that can prevent the development of PF. In recent years, iron accumulation and mitochondrial damage have been reported to participate in PF, and drugs that reduce iron content and improve mitochondrial function have shown significant efficacy in animal experimental models. Excessive iron leads to mitochondrial impairment, which may be the key cause that results in the dysfunction of various kinds of pulmonary cells and further promotes PF. As an emerging research hotspot, there are few targeted effective therapeutic strategies at present due to limited mechanistic understanding. In this review, the roles of iron homeostasis imbalance and mitochondrial damage in PF are summarized and discussed, highlighting a promising direction for finding truly effective therapeutics for PF.

Molecular characteristics of proteins within the mitochondrial Fe-S cluster assembly complex

Iron-Sulfur (Fe-S) clusters are essential for life, as they are widely utilized in nearly every biochemical pathway. When bound to proteins, Fe-S clusters assist in catalysis, signal recognition, and energy transfer events, as well as additional cellular pathways including cellular respiration and DNA repair and replication. In Eukaryotes, Fe-S clusters are produced through coordinated activity by mitochondrial Iron-Sulfur Cluster (ISC) assembly pathway proteins through direct assembly, or through the production of the activated sulfur substrate used by the Cytosolic Iron-Sulfur Cluster Assembly (CIA) pathway. In the mitochondria, Fe-S cluster assembly is accomplished through the coordinated activity of the ISC pathway protein complex composed of a cysteine desulfurase, a scaffold protein, the accessory ISD11 protein, the acyl carrier protein, frataxin, and a ferredoxin; downstream events that accomplish Fe-S cluster transfer and delivery are driven by additional chaperone/delivery proteins that interact with the ISC assembly complex. Deficiency in human production or activity of Fe-S cluster containing proteins is often detrimental to cell and organism viability. Here we summarize what is known about the structure and functional activities of the proteins involved in the early steps of assembling [2Fe-2S] clusters before they are transferred to proteins devoted to their delivery. Our goal is to provide a comprehensive overview of how the ISC assembly apparatus proteins interact to make the Fe-S cluster which can be delivered to proteins downstream to the assembly event.

Down the Iron Path: Mitochondrial Iron Homeostasis and Beyond

Cellular iron homeostasis and mitochondrial iron homeostasis are interdependent. Mitochondria must import iron to form iron–sulfur clusters and heme, and to incorporate these cofactors along with iron ions into mitochondrial proteins that support essential functions, including cellular respiration. In turn, mitochondria supply the cell with heme and enable the biogenesis of cytosolic and nuclear proteins containing iron–sulfur clusters. Impairment in cellular or mitochondrial iron homeostasis is deleterious and can result in numerous human diseases. Due to its reactivity, iron is stored and trafficked through the body, intracellularly, and within mitochondria via carefully orchestrated processes. Here, we focus on describing the processes of and components involved in mitochondrial iron trafficking and storage, as well as mitochondrial iron–sulfur cluster biogenesis and heme biosynthesis. Recent findings and the most pressing topics for future research are highlighted.

Non-enzymatic lipid peroxidation initiated by photodynamic therapy drives a distinct ferroptosis-like cell death pathway

Ferroptosis is primarily triggered by a failure of the glutathione (GSH)-glutathione peroxidase 4 (GPX4) reductive system and associated overwhelming lipid peroxidation, in which enzymes regulating polyunsaturated fatty acid (PUFA) metabolism, and in particular acyl-CoA synthetase long chain family member 4 (ACSL4), are central. Here, we found that exogenous oxygen radicals generated by photodynamic therapy (PDT) can directly peroxidize PUFAs and initiate lipid autoxidation, coinciding with cellular GSH depletion. Different from canonical ferroptosis induced by RSL3 or erastin, PDT-initiated lipid peroxidation and ferroptotis-like cell death is independent of lipoxygenase (ALOXs) and ACSL4. Especially, this form of cell death modality can be triggered in malignant cells insensitive to or acquired resistance to canonical ferroptosis inducers. We also observed a distinct iron metabolism pathway in this PDT-triggered cell death modality, in which cytosolic labile iron is decreased probably due to its relocation to mitochondria. Inhibition of the mitochondrial Ca²⁺ and Fe²⁺ uniporter (MCU) effectively prevented PDT-triggered lipid peroxidation and subsequent cell death. Therefore, we tentatively term this distinct ferroptosis-like cell death as liperoptosis. Moreover, using the clinically approved photosensitizer Verteporfin, PDT inhibited tumor growth through inducing prevailing ferroptosis-like cell death in a mouse xenograft model. With its site-specific advantages, these findings highlight the value of using PDT to trigger lipid peroxidation and ferroptosis-like cell death in vivo, and will benefit exploring the exact molecular mechanism of immunological effects of PDT in cancer treatment.

The localization of the photosensitizer determines the dynamics of the secondary production of hydrogen peroxide in cell cytoplasm and mitochondria

Photodynamic therapy (PDT) is based on the production of the cytotoxic reactive oxygen species (ROS) by light irradiation of a photosensitizer dye in the presence of molecular oxygen. Along with photochemical ROS production, it becomes evident that PDT induces massive secondary production of ROS which is registered long after the irradiation is completed. We created cell lines of human epidermoid carcinoma with the cytoplasmic and mitochondrial localization of protein sensor HyPer sensitive to hydrogen peroxide to compare its concentration in two cellular compartments. The lag-period between irradiation and accumulation of hydrogen peroxide in cells was registered; its duration was dose-dependent and increased up to 80 min when lowering the exposition dose from 50 to 15 J/cm². We have shown that localization of the photosensitizer determines the spatiotemporal pattern of the cell response to PDT: secondary hydrogen peroxide accumulation in cell cytoplasm induced by photodynamic treatment with lysosome-localized phtalocyianine Photosens occurs several minutes prior to that in mitochondria; on the contrary, membranotropic arylcyanoporphyrazine dye leads to massive mitochondrial hydrogen peroxide production followed by its cytoplasmic accumulation. We hypothesize that photosensitizers with various physicochemical properties and intracellular localization can trigger different patterns not only of primary but also secondary ROS production leading to different cell fate outcomes.

Mitoferrin 2 deficiency prevents mitochondrial iron overload-induced endothelial injury and alleviates atherosclerosis

Endothelial dysfunction is an early step in the development of atherosclerotic cardiovascular disease. Iron overload can lead to excessive mitochondrial reactive oxygen species (mtROS) production, resulting in mitochondrial dysfunction and vascular endothelial cell (EC) damage. Mitoferrin 2 (Mfrn2) is an iron transporter in the inner mitochondrial membrane. This study aimed to assess whether Mfrn2 and mitochondrial iron overload were involved in atherosclerosis progression and to explore the potential mechanism. We observed significant upregulation of Mfrn2 in the arteries of high-fat diet (HFD)-fed Apolipoprotein E-/- (ApoE-/-) mice and in TNF-α-induced mouse aortic endothelial cells (MAECs). Mfrn2 gene silencing inhibited mitochondrial iron overload, stabilized mitochondrial membrane potential and improved mitochondrial function in TNF-α-induced MAECs. Vascular EC-specific knockdown of Mfrn2 in ApoE-/- mice markedly decreased atherosclerotic lesion formation and the levels of ICAM-1 in aortas and reduced monocyte infiltration into the vascular wall. Furthermore, TNF-α increased the binding of 14-3-3 epsilon (ε) and Mfrn2, preventing Mfrn2 degradation and leading to mitochondrial iron overload in ECs, while 14-3-3ε overexpression increased Mfrn2 stability by inhibiting its ubiquitination. Together, our results reveal that Mfrn2 deficiency attenuates endothelial dysfunction by decreasing iron levels within the mitochondria and mitochondrial dysfunction. These findings may provide new insights into preventive and therapeutic strategies against vascular endothelial dysfunction in atherosclerotic disease.

Photodynamic therapy: autophagy and mitophagy, apoptosis and paraptosis

Macroautophagy/autophagy can play a cytoprotective role after photodynamic damage to malignant cells, depending on the site of subcellular damage initiated by reactive oxygen species. There is evidence for such protection when mitochondria are among the targets. Targeting lysosomes has been reported to be more effective for photokilling, perhaps because autophagy offers no cytoprotection. Photodynamic damage to both lysosomes and mitochondria can, however, markedly enhance the overall level of photokilling. Two mechanisms have been proposed to account for this result. Lysosomal photodamage leads to the release of calcium ions, resulting in the activation of the protease CAPN (calpain). CAPN then cleaves ATG5 to a fragment (tATG5) capable of interacting with mitochondria to enhance pro-apoptotic signals. It has also been proposed that targeting lysosomes for photodynamic damage can impair mitophagy, a process that could mitigate the pro-apoptotic effects of mitochondrial targeting. The level of lysosomal photodamage required for suppression of mitophagy is unclear. The "tATG5 route" involves the catalytic action of CAPN, activated by a degree of lysosomal photodamage barely detectible by a viability assay. ER photodamage can also initiate paraptosis, a death pathway functional even in cell types with impaired apoptosis and apparently unaffected by autophagy. Abbreviations: ALLN: N-acetyl-Leu-Leu-norleucinal (cell-permeable inhibitor of calpain); ATG: autophagy related; BPD: benzoporphyrin derivative (Visudyne); ER: endoplasmic reticulum; EtNBS: 5-ethylamino-9-diethyl-aminobenzo[a]phenothiazinium chloride; MTT: a tetrazolium dye; NPe6: mono N-aspartyl chlorin e6; PDT: photodynamic therapy; ROS: reactive oxygen species.

The BRD7-P53-SLC25A28 axis regulates ferroptosis in hepatic stellate cells

Ferroptosis is a recently discovered form of programmed cell death, but its regulatory mechanisms are not fully understood. In the current study, we reported that the BRD7-P53-SLC25A28 axis played a crucial role in regulating ferroptosis in hepatic stellate cells (HSCs). Upon exposure to ferroptosis inducers, bromodomain-containing protein 7 (BRD7) protein expression was remarkably increased through the inhibition of the ubiquitin-proteasome pathway. CRISPR/Cas9-mediated BRD7 knockout conferred resistance to HSC ferroptosis, whereas specific BRD7 plasmid-mediated BRD7 overexpression facilitated HSC ferroptosis. Interestingly, the elevated BRD7 expression exhibited to promote p53 mitochondrial translocation via direct binding with p53 N-terminal transactivation domain (TAD), which may be the underlying mechanisms for BRD7-enhanced HSC ferroptosis. Site-directed mutations of serine 392 completely blocked the binding of BRD7 to p53, and, in turn, prevented p53 mitochondrial translocation and HSC ferroptosis. Importantly, mitochondrial p53 interacted with solute carrier family 25 member 28 (SLC25A28) to form complex and enhanced the activity of SLC25A28, which could lead to the abnormal accumulation of redox-active iron and hyperfunction of electron transfer chain (ETC). SLC25A28 knockdown impaired BRD7-or p53-mediated ferroptotic events. In mice, erastin treatment ameliorated pathological damage of liver fibrosis through inducing HSC ferroptosis. HSC-specific blockade of BRD7-P53-SLC25A28 axis could abrogate erastin-induced HSC ferroptosis. Of note, we analyzed the effect of sorafenib on HSC ferroptosis in advanced fibrotic patients with hepatocellular carcinoma receiving sorafenib monotherapy. Attractively, BRD7 upregulation, p53 mitochondrial translocation, combination of SLC25A28 and p53, and ferroptosis induction occurred in primary human HSCs. Overall, these findings reveal novel signal transduction and regulatory mechanism of ferroptosis, and also suggest BRD7-P53-SLC25A28 axis as potential targets for liver fibrosis.

The mitochondrial metal transporters mitoferrin1 and mitoferrin2 are required for liver regeneration and cell proliferation in mice

Mitochondrial iron import is essential for iron-sulfur cluster formation and heme biosynthesis. Two nuclear-encoded vertebrate mitochondrial high-affinity iron importers, mitoferrin1 (Mfrn1) and Mfrn2, have been identified in mammals. In mice, the gene encoding Mfrn1, solute carrier family 25 member 37 (Slc25a37), is highly expressed in sites of erythropoiesis, and whole-body Slc25a37 deletion leads to lethality. Here, we report that mice with a deletion of Slc25a28 (encoding Mfrn2) are born at expected Mendelian ratios, but show decreased male fertility due to reduced sperm numbers and sperm motility. Mfrn2 ^-/- mice placed on a low-iron diet exhibited reduced mitochondrial manganese, cobalt, and zinc levels, but not reduced iron. Hepatocyte-specific loss of Slc25a37 (encoding Mfrn1) in Mfrn2 ^-/- mice did not affect animal viability, but resulted in a 40% reduction in mitochondrial iron and reduced levels of oxidative phosphorylation proteins. Placing animals on a low-iron diet exaggerated the reduction in mitochondrial iron observed in liver-specific Mfrn1/2-knockout animals. Mfrn1 ^-/-/Mfrn2 ^-/- bone marrow-derived macrophages or skin fibroblasts in vitro were unable to proliferate, and overexpression of Mfrn1-GFP or Mfrn2-GFP prevented this proliferation defect. Loss of both mitoferrins in hepatocytes dramatically reduced regeneration in the adult mouse liver, further supporting the notion that both mitoferrins transport iron and that their absence limits proliferative capacity of mammalian cells. We conclude that Mfrn1 and Mfrn2 contribute to mitochondrial iron homeostasis and are required for high-affinity iron import during active proliferation of mammalian cells.

Suppression of iron mobilization from lysosomes to mitochondria attenuates liver injury after acetaminophen overdose in vivo in mice: Protection by minocycline

Acetaminophen (APAP) overdose causes hepatotoxicity involving mitochondrial dysfunction. Previous studies showed that translocation of Fe²⁺ from lysosomes into mitochondria by the mitochondrial Ca²⁺ uniporter (MCU) promotes the mitochondrial permeability transition (MPT) after APAP. Here, our Aim was to assess protection by iron chelation and MCU inhibition against APAP hepatotoxicity in mice. C57BL/6 mice and hepatocytes were administered toxic doses of APAP with and without starch-desferal (an iron chelator), minocycline (MCU inhibitor), or N-acetylcysteine (NAC). In mice, starch-desferal and minocycline pretreatment decreased ALT and liver necrosis after APAP by >60%. At 24 h after APAP, loss of fluorescence of mitochondrial rhodamine 123 occurred in pericentral hepatocytes often accompanied by propidium iodide labeling, indicating mitochondrial depolarization and cell death. Starch-desferal and minocycline pretreatment decreased mitochondrial depolarization and cell death by more than half. In cultured hepatocytes, cell killing at 10 h after APAP decreased from 83% to 49%, 35% and 27%, respectively, by 1 h posttreatment with minocycline, NAC, and minocycline plus NAC. With 4 h posttreatment in vivo, minocycline and minocycline plus NAC decreased ALT and necrosis by ~20% and ~50%, respectively, but NAC alone was not effective. In conclusion, minocycline and starch-desferal decrease mitochondrial dysfunction and severe liver injury after APAP overdose, suggesting that the MPT is likely triggered by iron uptake into mitochondria through MCU. In vivo, minocycline and minocycline plus NAC posttreatment after APAP protect at later time points than NAC alone, indicating that minocycline has a longer window of efficacy than NAC.

Systemic MEK inhibition enhances the efficacy of 5-aminolevulinic acid-photodynamic therapy

BACKGROUND

Protoporphyrin IX (PpIX) gets accumulated preferentially in 5-aminolevulinic acid (5-ALA)-treated cancer cells. Photodynamic therapy (PDT) utilises the accumulated PpIX to trigger cell death by light-induced generation of reactive oxygen species (ROS). We previously demonstrated that oncogenic Ras/MEK decreases PpIX accumulation in cancer cells. Here, we investigated whether combined therapy with a MEK inhibitor would improve 5-ALA-PDT efficacy.

METHODS

Cancer cells and mice models of cancer were treated with 5-ALA-PDT, MEK inhibitor or both MEK inhibitor and 5-ALA-PDT, and treatment efficacies were evaluated.

RESULTS

Ras/MEK negatively regulates the cellular sensitivity to 5-ALA-PDT as cancer cells pre-treated with a MEK inhibitor were killed more efficiently by 5-ALA-PDT. MEK inhibition promoted 5-ALA-PDT-induced ROS generation and programmed cell death. Furthermore, the combination of 5-ALA-PDT and a systemic MEK inhibitor significantly suppressed tumour growth compared with either monotherapy in mouse models of cancer. Remarkably, 44% of mice bearing human colon tumours showed a complete response with the combined treatment.

CONCLUSION

We demonstrate a novel strategy to promote 5-ALA-PDT efficacy by targeting a cell signalling pathway regulating its sensitivity. This preclinical study provides a strong basis for utilising MEK inhibitors, which are approved for treating cancers, to enhance 5-ALA-PDT efficacy in the clinic.

The significance, trafficking and determination of labile iron in cytosol, mitochondria and lysosomes

The labile iron pool (LIP) is a pool of chelatable and redox-active iron, not only essential for a wide variety of metabolic process, but also as a catalyst in the Fenton reaction, causing the release of hazardous reactive oxygen species (ROS) with potential for inducing oxidative stress and cell damage. The cellular LIP represents the entirety of every heterogenous sub-pool of iron, not only present in the cytosol, but also in mitochondria, lysosomes and the nucleus, which have all been detected and characterized by various fluorescent methods. Accumulated evidence indicates that alterations in the intracellular LIP can substantially contribute to a variety of injurious processes and initiate pathological development. Herein, we present our understanding of the role of the cellular LIP. To fully review the LIP, firstly, the significance of cellular labile iron in different subcellular compartments is presented. And then, the trafficking processes of cellular labile iron between/in cytosol, mitochondria and lysosomes are discussed in detail. Then, the recent progress in uncovering and assessing the cellular LIP by fluorescent methods have been noted. Overall, this summary may help to comprehensively envision the important physiological and pathological roles of the LIP and shed light on profiling the LIP in a real-time and nondestructive manner with fluorescent methods. Undoubtedly, with the advent and development of iron biology, a better understanding of iron, especially the LIP, may also enhance treatments for iron-related diseases.

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.

Mitochondrial Iron in Human Health and Disease

Mitochondria are an iconic distinguishing feature of eukaryotic cells. Mitochondria encompass an active organellar network that fuses, divides, and directs a myriad of vital biological functions, including energy metabolism, cell death regulation, and innate immune signaling in different tissues. Another crucial and often underappreciated function of these dynamic organelles is their central role in the metabolism of the most abundant and biologically versatile transition metals in mammalian cells, iron. In recent years, cellular and animal models of mitochondrial iron dysfunction have provided vital information in identifying new proteins that have elucidated the pathways involved in mitochondrial homeostasis and iron metabolism. Specific signatures of mitochondrial iron dysregulation that are associated with disease pathogenesis and/or progression are becoming increasingly important. Understanding the molecular mechanisms regulating mitochondrial iron pathways will help better define the role of this important metal in mitochondrial function and in human health and disease.

Mitoferrin-1 is Involved in the Progression of Alzheimer's Disease Through Targeting Mitochondrial Iron Metabolism in a Caenorhabditis elegans Model of Alzheimer's Disease

In mammals, mitoferrin-1 and mitoferrin-2, two homologous proteins of the mitochondrial solute carrier family are required for iron delivery into mitochondria. However, there is only one kind, called W02B12 (mitoferrin-1 or mfn-1), in Caenorhabditis elegans and its regulatory mechanism is unknown. In this study, we used C. elegans strains CL2006 and GMC101 as models to investigate what role mitoferrin-1 played in Alzheimer's disease (AD). We found that knockdown of mitoferrin-1 by feeding-RNAi treatment extended lifespans of both strains of C. elegans. In addition, it reduced the paralysis rate in the GMC101 strain. These results suggest that mitoferrin-1 may be involved in the progression of Alzheimer's disease. Knockdown of mitoferrin-1 was seen to disturb mitochondrial morphology in the CB5600 strain. We tested whether knockdown of mitoferrin-1 could influence mitochondrial metabolism. Analysis of mitochondrial iron metabolism and mitochondrial ROS showed that knockdown of mitoferrin-1 could reduce mitochondrial iron content and reduce the level of mitochondrial ROS in the CL2006 and GMC101 strains. These results confirm that knockdown of mitoferrin-1 can slow the progress of disease in Alzheimer model of C. elegans and suggest that mitoferrin-1 plays a major role in mediating mitochondrial iron metabolism in this process.

Expression and putative role of mitochondrial transport proteins in cancer

Cancer cells undergo major changes in energy and biosynthetic metabolism. One of them is the Warburg effect, in which pyruvate is used for fermentation rather for oxidative phosphorylation. Another major one is their increased reliance on glutamine, which helps to replenish the pool of Krebs cycle metabolites used for other purposes, such as amino acid or lipid biosynthesis. Mitochondria are central to these alterations, as the biochemical pathways linking these processes run through these organelles. Two membranes, an outer and inner membrane, surround mitochondria, the latter being impermeable to most organic compounds. Therefore, a large number of transport proteins are needed to link the biochemical pathways of the cytosol and mitochondrial matrix. Since the transport steps are relatively slow, it is expected that many of these transport steps are altered when cells become cancerous. In this review, changes in expression and regulation of these transport proteins are discussed as well as the role of the transported substrates. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

Iron Chelators and Exogenic Photosensitizers. Synergy through Oxidative Stress Gene Expression

In non-invasive anticancer photodynamic therapy (PDT), a nontoxic photosensitizer (PS), which is activated by visible light, is used as a magic bullet that selectively destroys cancer cells. Recently, we described the combined therapy of 5-aminolevulinic acid (ALA-PDT) with thiosemicarbazone (TSC), i.e. an iron-chelating agent. This resulted in a strong synergistic effect. Herein, we investigated a novel strategy using a combination of PDT consist of the xenobiotic-porphyrin type PS with TSC. We observed a synergistic effect for all of the pairs of TSC-PS. This approach can be rationalized by the fact that both chlorin and TSC can affect the generation of reactive oxygen species (ROS). In order to elucidate the plausible mechanism of action, we also combined the investigated PSs with DFO, which forms complexes that are redox inactive. We detected a slight antagonism or additivity for this combination. This may suggest that the ability of an iron chelator (IC) to participate in the production of ROS and the generation of oxidative stress is important.

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.

Mitochondria and Iron: current questions

INTRODUCTION

Mitochondria are cellular organelles that perform numerous bioenergetic, biosynthetic, and regulatory functions and play a central role in iron metabolism. Extracellular iron is taken up by cells and transported to the mitochondria, where it is utilized for synthesis of cofactors essential to the function of enzymes involved in oxidation-reduction reactions, DNA synthesis and repair, and a variety of other cellular processes. Areas covered: This article reviews the trafficking of iron to the mitochondria and normal mitochondrial iron metabolism, including heme synthesis and iron-sulfur cluster biogenesis. Much of our understanding of mitochondrial iron metabolism has been revealed by pathologies that disrupt normal iron metabolism. These conditions affect not only iron metabolism but mitochondrial function and systemic health. Therefore, this article also discusses these pathologies, including conditions of systemic and mitochondrial iron dysregulation as well as cancer. Literature covering these areas was identified via PubMed searches using keywords: Iron, mitochondria, Heme Synthesis, Iron-sulfur Cluster, and Cancer. References cited by publications retrieved using this search strategy were also consulted. Expert commentary: While much has been learned about mitochondrial and its iron, key questions remain. Developing a better understanding of mitochondrial iron and its regulation will be paramount in developing therapies for syndromes that affect mitochondrial iron.

PLGA nanoparticle encapsulation reduces toxicity while retaining the therapeutic efficacy of EtNBS-PDT in vitro

Photodynamic therapy regimens, which use light-activated molecules known as photosensitizers, are highly selective against many malignancies and can bypass certain challenging therapeutic resistance mechanisms. Photosensitizers such as the small cationic molecule EtNBS (5-ethylamino-9-diethyl-aminobenzo[a]phenothiazinium chloride) have proven potent against cancer cells that reside within acidic and hypoxic tumour microenvironments. At higher doses, however, these photosensitizers induce "dark toxicity" through light-independent mechanisms. In this study, we evaluated the use of nanoparticle encapsulation to overcome this limitation. Interestingly, encapsulation of the compound within poly(lactic-co-glycolic acid) (PLGA) nanoparticles (PLGA-EtNBS) was found to significantly reduce EtNBS dark toxicity while completely retaining the molecule's cytotoxicity in both normoxic and hypoxic conditions. This dual effect can be attributed to the mechanism of release: EtNBS remains encapsulated until external light irradiation, which stimulates an oxygen-independent, radical-mediated process that degrades the PLGA nanoparticles and releases the molecule. As these PLGA-encapsulated EtNBS nanoparticles are capable of penetrating deeply into the hypoxic and acidic cores of 3D spheroid cultures, they may enable the safe and efficacious treatment of otherwise unresponsive tumour regions.

Translocation of iron from lysosomes to mitochondria during acetaminophen-induced hepatocellular injury: Protection by starch-desferal and minocycline

Acetaminophen (APAP) overdose causes hepatotoxicity involving mitochondrial dysfunction and the mitochondrial permeability transition (MPT). Iron is a critical catalyst for ROS formation, and reactive oxygen species (ROS) play an important role in APAP-induced hepatotoxicity. Previous studies show that APAP disrupts lysosomes, which release ferrous iron (Fe(2+)) into the cytosol to trigger the MPT and cell killing. Here, our aim was to investigate whether iron released from lysosomes after APAP is then taken up into mitochondria via the mitochondrial electrogenic Ca(2+), Fe(2+) uniporter (MCFU) to cause mitochondrial dysfunction and cell death. Hepatocytes were isolated from fasted male C57BL/6 mice. Necrotic cell killing was assessed by propidium iodide fluorimetry. Mitochondrial membrane potential (ΔΨ) was visualized by confocal microscopy of rhodamine 123 (Rh123) and tetramethylrhodamine methylester (TMRM). Chelatable Fe(2+) was monitored by quenching of calcein (cytosol) and mitoferrofluor (MFF, mitochondria). ROS generation was monitored by confocal microscopy of MitoSox Red and plate reader fluorimetry of chloromethyldihydrodichlorofluorescein diacetate (cmH2DCF-DA). Administered 1h before APAP (10mM), the lysosomally targeted iron chelator, starch-desferal (1mM), and the MCFU inhibitors, Ru360 (100nM) and minocycline (4µM), decreased cell killing from 83% to 41%, 57% and 53%, respectively, after 10h. Progressive quenching of calcein and MFF began after ~4h, signifying increased cytosolic and mitochondrial chelatable Fe(2+). Mitochondria then depolarized after ~10h. Dipyridyl, a membrane-permeable iron chelator, dequenched calcein and MFF fluorescence after APAP. Starch-desferal, but not Ru360 and minocycline, suppressed cytosolic calcein quenching, whereas starch-desferal, Ru360 and minocycline all suppressed mitochondrial MFF quenching and mitochondrial depolarization. Starch-desferal, Ru360 and minocycline also each decreased ROS formation. Moreover, minocycline 1h after APAP decreased cell killing by half. In conclusion, release of Fe(2+) from lysosomes followed by uptake into mitochondria via MCFU occurs during APAP hepatotoxicity. Mitochondrial iron then catalyzes toxic hydroxyl radical formation, which triggers the MPT and cell killing. The efficacy of minocycline post-treatment shows minocycline as a possible therapeutic agent against APAP hepatotoxicity.

Biomedical applications of nano-titania in theranostics and photodynamic therapy

Titanium dioxide (TiO₂) is one of the most abundantly used nanomaterials for human life. It is used in sunscreen, photovoltaic devices, biomedical applications and as a food additive and environmental scavenger.

Upconversion nanoparticle-mediated photodynamic therapy induces THP-1 macrophage apoptosis via ROS bursts and activation of the mitochondrial caspase pathway

Atherosclerosis (AS) is the most vital cardiovascular disease, which poses a great threat to human health. Macrophages play an important role in the progression of AS. Photodynamic therapy (PDT) has emerged as a useful therapeutic modality not only in the treatment of cancer but also in the treatment of AS. The purpose of this study was to determine the molecular mechanisms underlying the activity of PDT, using mesoporous-silica-coated upconversion fluorescent nanoparticles encapsulating chlorin e6 (UCNPs-Ce6) in the induction of apoptosis in THP-1 macrophages. Here, we investigated the ability of UCNPs-Ce6-mediated PDT to induce THP-1 macrophage apoptosis by facilitating the induction of reactive oxygen species (ROS) and regulation of mitochondrial permeability transition pore (MPTP) to depolarize mitochondrial membrane potential (MMP). Both Bax translocation and the release of cytochrome C were examined using immunofluorescence and Western blotting. Our results indicated that the levels of ROS were significantly increased in the PDT group, resulting in both MPTP opening and MMP depolarization, which led to apoptosis. In addition, immunofluorescence and Western blotting revealed that PDT induced both Bax translocation and the release of cytochrome C, as well as upregulation of cleaved caspase-9, cleaved caspase-3, and cleaved poly(ADP-ribose) polymerase. Therefore, we demonstrated that UCNPs-Ce6-mediated PDT induces apoptosis in THP-1 macrophages via ROS bursts. The proapoptotic factor Bax subsequently translocates from the cytosol to the mitochondria, resulting in the MPTP opening and cytochrome C release. This study demonstrated the great potential of UCNPs-Ce6-mediated PDT in the treatment of AS.

Promotion of Proapoptotic Signals by Lysosomal Photodamage

We previously reported that low-level lysosomal photodamage enhanced the efficacy of subsequent mitochondrial photodamage, resulting in a substantial promotion of apoptotic cell death. We now extend our analysis of the sequential PDT protocol to include two additional lysosomal-targeting photosensitizers. These agents, because of enhanced permeability, are more potent than the agent (N-aspartyl chlorin E6, NPe6) used in the initial study. Addition of the cell-permeable cysteine protease inhibitor E-64d and calcium chelator BAPTA-AM almost completely suppressed sequential PDT-induced loss of mitochondrial membrane potential and activation of procaspases-3 and -7. These inhibitors did not, however, suppress the proapoptotic effect of a BH3 mimetic or mitochondrial photodamage. Knockdowns of ATG7 or ATG5, proteins normally associated with autophagy, suppressed photodamage induced by the sequential PDT protocol. These effects appear to be independent of the autophagic process as pharmacological inhibition of autophagy offered no such protection. Effects of ATG7 and ATG5 knockdowns may reflect the role that ATG7 plays in regulating lysosome permeability, and the likelihood that a proteolytic fragment of ATG5 amplifies mitochondrial proapoptotic processes. Our results suggest that low-dose photodamage that sequentially targets lysosomes and mitochondria may offer significant advantages over the use of single photosensitizers.

Iron alters cell survival in a mitochondria-dependent pathway in ovarian cancer cells

The role of iron in the development of cancer remains unclear. We previously reported that iron reduces cell survival in a Ras/mitogen-activated protein kinase (MAPK)-dependent manner in ovarian cells; however, the underlying downstream pathway leading to reduced survival was unclear. Although levels of intracellular iron, ferritin/CD71 protein and reactive oxygen species did not correlate with iron-induced cell survival changes, we identified mitochondrial damage (via TEM) and reduced expression of outer mitochondrial membrane proteins (translocase of outer membrane: TOM20 and TOM70) in cell lines sensitive to iron. Interestingly, Ru360 (an inhibitor of the mitochondrial calcium uniporter) reversed mitochondrial changes and restored cell survival in HEY ovarian carcinoma cells treated with iron. Further, cells treated with Ru360 and iron also had reduced autophagic punctae with increased lysosomal numbers, implying cross-talk between these compartments. Mitochondrial changes were dependent on activation of the Ras/MAPK pathway since treatment with a MAPK inhibitor restored expression of TOM20/TOM70 proteins. Although glutathione antioxidant levels were reduced in HEY treated with iron, extracellular glutamate levels were unaltered. Strikingly, oxalomalate (inhibitor of aconitase, involved in glutamate production) reversed iron-induced responses in a similar manner to Ru360. Collectively, our results implicate iron in modulating cell survival in a mitochondria-dependent manner in ovarian cancer cells.

Cellular iron uptake, trafficking and metabolism: Key molecules and mechanisms and their roles in disease

Iron is a crucial transition metal for virtually all life. Two major destinations of iron within mammalian cells are the cytosolic iron-storage protein, ferritin, and mitochondria. In mitochondria, iron is utilized in critical anabolic pathways, including: iron-storage in mitochondrial ferritin, heme synthesis, and iron-sulfur cluster (ISC) biogenesis. Although the pathways involved in ISC synthesis in the mitochondria and cytosol have begun to be characterized, many crucial details remain unknown. In this review, we discuss major aspects of the journey of iron from its initial cellular uptake, its modes of trafficking within cells, to an overview of its downstream utilization in the cytoplasm and within mitochondria. The understanding of mitochondrial iron processing and its communication with other organelles/subcellular locations, such as the cytosol, has been elucidated by the analysis of certain diseases e.g., Friedreich's ataxia. Increased knowledge of the molecules and their mechanisms of action in iron processing pathways (e.g., ISC biogenesis) will shape the investigation of iron metabolism in human health and disease.

Artesunate induces cell death in human cancer cells via enhancing lysosomal function and lysosomal degradation of ferritin

Artesunate (ART) is an anti-malaria drug that has been shown to exhibit anti-tumor activity, and functional lysosomes are reported to be required for ART-induced cancer cell death, whereas the underlying molecular mechanisms remain largely elusive. In this study, we aimed to elucidate the molecular mechanisms underlying ART-induced cell death. We first confirmed that ART induces apoptotic cell death in cancer cells. Interestingly, we found that ART preferably accumulates in the lysosomes and is able to activate lysosomal function via promotion of lysosomal V-ATPase assembly. Furthermore, we found that lysosomes function upstream of mitochondria in reactive oxygen species production. Importantly, we provided evidence showing that lysosomal iron is required for the lysosomal activation and mitochondrial reactive oxygen species production induced by ART. Finally, we showed that ART-induced cell death is mediated by the release of iron in the lysosomes, which results from the lysosomal degradation of ferritin, an iron storage protein. Meanwhile, overexpression of ferritin heavy chain significantly protected cells from ART-induced cell death. In addition, knockdown of nuclear receptor coactivator 4, the adaptor protein for ferritin degradation, was able to block ART-mediated ferritin degradation and rescue the ART-induced cell death. In summary, our study demonstrates that ART treatment activates lysosomal function and then promotes ferritin degradation, subsequently leading to the increase of lysosomal iron that is utilized by ART for its cytotoxic effect on cancer cells. Thus, our data reveal a new mechanistic action underlying ART-induced cell death in cancer cells.

The roles of mitoferrin-2 in the process of arsenic trioxide-induced cell damage in human gliomas

Background

Among glioma treatment strategies, arsenic trioxide (As₂O₃) has shown efficacy as a therapeutic agent against human gliomas. However, the exact antitumor mechanism of action of As₂O₃ is still unclear. Mitochondria are considered to be the major source of intracellular reactive oxygen species (ROS), which are known to be associated with As₂O₃-induced cell damage. Therefore, we investigated whether mitoferrin-2, a mitochondrial iron uptake transporter, participates in As₂O₃-induced cell killing in human gliomas.

Methods

Human glioma cell lines were used to explore the mechanism of As₂O₃’s antitumor effects. First, expression of mitoferrin-2 was analyzed in glioma cells that were pretreated with As₂O_3. Changes in ROS production and apoptosis were assessed. Furthermore, cell viability was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT).

Results

In the present study we found that As₂O₃ induced ROS production and apoptosis in glioma cells. In addition, gene expression of mitoferrin-2, a mitochondrial iron uptake transporter, was increased 4 to 5 fold after exposure to As₂O₃ (5 μM) for 48 hours. Furthermore, apoptosis and cytotoxicity induced by As₂O₃ in glioma cells were decreased after silencing the mitoferrin-2 gene.

Conclusions

Our findings indicated that mitoferrin-2 participates in mitochondrial ROS-dependent mechanisms underlying As₂O₃-mediated damage in glioma cells.

Enhanced efficacy of photodynamic therapy via a sequential targeting protocol

This study was designed to examine determinants of the discovery that low-dose lysosomal photodamage (lyso-PDT) could potentiate the efficacy of subsequent low-dose mitochondrial photodamage (mito-PDT). The chlorin NPe6 and the benzoporphyrin derivative (BPD) were used to separately target lysosomes and mitochondria, respectively, in murine hepatoma cells. Lyso-PDT (LD(5) conditions) followed by mito-PDT (LD(15) conditions) enhanced the loss of the mitochondrial membrane potential, activation of procaspases-3/7 and photokilling. Reversing the sequence was less effective. The optimal sequence did not enhance reactive oxygen species formation above that obtained with low-dose mito-PDT. In contrast, alkalinization of lysosomes with bafilomycin also enhanced low-dose mito-PDT photokilling, but via a different pathway. This involves redistribution of iron from lysosomes to mitochondria leading to enhanced hydroxyl radical formation, effects not observed after the sequential procedure. Moreover, Ru360, an inhibitor of mitochondrial calcium and iron uptake, partially suppressed the ability of bafilomycin to enhance mito-PDT photokilling without affecting the enhanced efficacy of the sequential protocol. We conclude that sequential PDT protocol promotes PDT efficacy by a process not involving iron translocation, but via promotion of the pro-apoptotic signal that derives from mitochondrial photodamage.

Blockade of mitochondrial calcium uniporter prevents cardiac mitochondrial dysfunction caused by iron overload

AIM

Iron overload in the heart can lead to iron-overload cardiomyopathy and cardiac arrhythmia. In the past decades, growing evidence has suggested that cardiac mitochondrial dysfunction is associated with the development of cardiac dysfunction and lethal arrhythmias. Despite these facts, the effect of iron overload on cardiac mitochondrial function is still unclear. In this study, we determined the effects of iron overload on the cardiac mitochondrial function and the routes of cardiac mitochondrial iron uptake. We tested the hypothesis that iron overload can lead to cardiac mitochondrial dysfunction and that mitochondrial calcium uniporter (MCU) plays a major role for cardiac mitochondrial iron uptake under iron-overload condition. Cardiac mitochondrial function was assessed via the determination of mitochondrial swelling, mitochondrial reactive oxygen species (ROS) production and mitochondrial membrane potential changes.

METHODS

Isolated cardiac mitochondria from male Wistar rats were used in this study. To determine the routes for cardiac mitochondrial iron uptake, isolated mitochondria were exposed to MCU blocker (Ru360), mitochondrial permeability transition pore (mPTP) blocker (cyclosporin A) and an iron chelator (deferoxamine).

RESULTS

We found that (i) iron overload caused cardiac mitochondrial dysfunction, indicated by increased ROS production, mitochondrial membrane depolarization and mitochondrial swelling; and (ii) only MCU blocker completely protected cardiac mitochondrial dysfunction caused by iron overload.

CONCLUSIONS

These findings strongly suggest that MCU could be the major route for iron uptake into cardiac mitochondria. The inhibition of MCU could be the novel pharmacological intervention for preventing iron-overload cardiomyopathy.