Mitochondrial Iron Metabolism Research Articles

Abstract 5937: KS20226, a first-in-class IRP2 inhibitor, preferentially induces reprogramming of iron metabolism and suppresses tumor growth in colorectal cancer

Abstract Background: Iron homeostasis plays a role in redox activity, mitochondrial function, and cell cycle regulation. Specifically, the dysregulation of iron metabolism is implicated in malignant transformation and cancer progression. Iron regulatory protein 2 (IRP2) is crucial for maintaining balance within iron levels and primarily controls genes associated with iron utilization. Our discovery of KS20226, a first-in-class IRP2 inhibitor, reveals remarkable potential as an effective anti-cancer drug candidate. We have elucidated the mechanisms underlying the therapeutic effects of KS20226, thereby highlighting the feasibility of targeting IRP2 in colorectal cancer (CRC). Methods: The proliferation inhibitory activity of KS20226 was assessed the CCK-8 and 3D Spheroid models. To investigate mitochondrial function, we conducted measurement of oxidative phosphorylation (OCR) and glycoPER. Furthermore, gene expression analysis was performed through RNA sequencing (RNA-seq) using the Illumina HiSeq 2500 system platform (Macrogen Inc.). Differential expressed gene (DEG) analysis was carried out using the R package DESeq2, and pathway enrichment analysis was analyzed using GSEA (v_4.2.3.). For the observation of autophagic morphology, we utilized transmission electron microscopy (TEM), immunofluorescence staining targeting the autophagy marker LC3B, and immunoblotting. Results: KS20226 exhibited a remarkable growth inhibitory effect in 10 different CRC cell lines, with GI50 values ranging from 1 to 10 μM. KS20226 suppressed IRP2 expression and the corresponding occupancy of the iron-responsive elements of ferritin H and transferrin receptor 1, resulting in iron deprivation, and efficiently controlled CRC growth. Pharmacologic inhibition of IRP2 induced reprogramming of mitochondrial respiration, reducing reliance on oxidative phosphorylation. The observed mitochondrial dysfunction induced by KS20226 was correlated with pathway analysis, revealing a decrease in the gene-set for HALLMARK_OXIDATIVE_PHOSPHORYLATION (NES = -1.38, p-value = 0.008) and an increase in the gene-set for POSITIVE_REGULATION_OF_AUTOPHAGY (NES=1.43, p-value=0.016). Immunoblotting demonstrated that KS20226 activated the phosphorylation of AMPK, ULK, and Beclin1 cascade, leading to the autophagic cell death of the CRC cells. Conclusions: The reliance of cancer cells on iron metabolism to drive proliferation has led to the development of therapeutic strategies targeting the inhibition of IRP2. KS20226, a first-in-class IPR2 inhibitor, exhibits distinct therapeutic properties, such as perturbation of mitochondrial iron metabolism and induction of autophagic cell death. Our findings represent important progress toward exploiting IRP2-mediated abnormal iron metabolism in cancer, an unexplored area of research with substantial unmet needs. Citation Format: Jieon Hwang, Areum Park, Chinwoo Kim, Chang Gon Kim, Jaesung Kwak, Byungil Kim, Hyunjin Shin, Minhee Ku, Jaemoon Yang, Ayoung Baek, Jiwon Choi, Hocheol Lim, Kyoung Tai No, Xianghua Zhao, Uyeong Choi, Tae Il Kim, Kyu-Sung Jeong, Hyuk Lee, Sang Joon Shin. KS20226, a first-in-class IRP2 inhibitor, preferentially induces reprogramming of iron metabolism and suppresses tumor growth in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5937.

Abnormal mitochondrial iron metabolism damages alveolar type II epithelial cells involved in bleomycin-induced pulmonary fibrosis.

Rationale: Pulmonary fibrosis is a chronic progressive lung disease with limited therapeutic options. We previously revealed that there is iron deposition in alveolar epithelial type II cell (AECII) in pulmonary fibrosis, which can be prevented by the iron chelator deferoxamine. However, iron in the cytoplasm and the mitochondria has two relatively independent roles and regulatory systems. In this study, we aimed to investigate the role of mitochondrial iron deposition in AECII injury and pulmonary fibrosis, and to find potential therapeutic strategies. Methods: BLM-treated mice, MLE-12 cells, and primary AECII were employed to establish the mouse pulmonary fibrosis model and epithelial cells injury model, respectively. Mitochondrial transplantation, siRNA and plasmid transfection, western blotting (WB), quantitative real-time polymerase chain reaction (RT-qPCR), polymerase chain reaction (PCR), immunofluorescence, immunoprecipitation (IP), MitoSOX staining, JC-1 staining, oxygen consumption rate (OCR) measurement, and Cell Counting Kit-8 (CCK8) assay were utilized to elucidate the role of mitochondrial iron deposition in cell and lung fibrosis and determine its mechanism. Results: This study showed that prominent mitochondrial iron deposition occurs within AECII in bleomycin (BLM)-induced pulmonary fibrosis mouse model and in BLM-treated MLE-12 epithelial cells. Further, the study revealed that healthy mitochondria rescue BLM-damaged AECII mitochondrial iron deposition and cell damage loss. Mitoferrin-2 (MFRN2) is the main transporter that regulates mitochondrial iron metabolism by transferring cytosolic iron into mitochondria, which is upregulated in BLM-treated MLE-12 epithelial cells. Direct overexpression of MFRN2 causes mitochondrial iron deposition and cell damage. In this study, decreased ubiquitination of the ubiquitin ligase F-box/LRR-repeat protein 5 (FBXL5) degraded iron-reactive element-binding protein 2 (IREB2) and promoted MFRN2 expression as well as mitochondrial iron deposition in damaged AECII. Activation of the prostaglandin E2 receptor EP4 subtype (EP4) receptor signaling pathway counteracted mitochondrial iron deposition by downregulating IREB2-MFRN2 signaling through upregulation of FBXL5. This intervention not only reduced mitochondrial iron content but also preserved mitochondrial function and protected against AECII damage after BLM treatment. Conclusion: Our findings highlight the unexplored roles, mechanisms, and regulatory approaches of abnormal mitochondrial iron metabolism of AECII in pulmonary fibrosis. Therefore, this study deepens the understanding of the mechanisms underlying pulmonary fibrosis and offers a promising strategy for developing effective therapeutic interventions using the EP4 receptor activator.

Omaveloxolone (SkyclarysTM) for patients with Friedreich’s ataxia

STRUCTURE: Omaveloxolone {N-[(4aS,6aR,6bS,8aR,12aS,14aR,14bS)-11-cyano-2,2,6a,6b, 9,9,12a-heptamethyl-10,14-dioxo-1,2,3,4,4a,5,6,6a,6b,7,8,8a,9,10,12a,14,14a,14b-octa-decahydropicen-4a-yl]-2,2-difluoropropanamide} is a semisynthetic triterpenoid based on the natural product oleanolic acid. It contains a highly reactive cyanoenone functionality, which binds covalently and reversibly by Michael addition to cysteines in proteins, such as the cysteine-based sensor protein Kelch-like ECH-associated protein 1 (KEAP1). The molecular formula of omaveloxolone is C33H44F2N2O3 and its molecular weight is 554.7 g/mol. MECHANISM OF ACTION: The pathophysiology of Friedreich’s ataxia is associated with an expansion of GAA repeats in the first intron of FXN encoding the small mitochondrial protein frataxin (FXN), which has a role in mitochondrial homeostasis and iron metabolism. FXN deficiency leads to impaired mitochondrial function and increased production of reactive oxygen species (ROS) (1), in turn causing inflammation with further ROS production, creating a vicious cycle that ultimately results in cellular dysfunction (2). Omaveloxolone is an inducer of a network of endogenous cytoprotective proteins regulated by transcription factor nuclear factor-erythroid 2 p45-related factor 2 (NRF2), the master regulator of cellular redox homeostasis. At basal state, the levels of NRF2 are low due to its continuous proteasomal degradation (3) mediated by KEAP1, a substrate adapter of a Cullin-RING E3 ubiquitin ligase. Omaveloxolone (pink circle), via its cyanoenone functionality, binds to sensor cysteines (primarily C151) in KEAP1 and inactivates it (4). As a result, the newly synthesized NRF2 accumulates, forms a heterodimer with a small musculoaponeurotic fibrosarcoma (sMAF) protein, and induces transcription of its target genes by binding to the antioxidant response element (ARE) sequences in their regulatory regions. The NRF2-transcriptional network encompasses an array of broadly cytoprotective proteins (5), including those responsible for the biosynthesis of glutathione (GSH). Collectively, the NRF2 transcriptional targets counteract oxidative and inflammatory stress, and support proteostasis, mitochondrial function, and bioenergetics. Omaveloxolone also inhibits inflammation, in part via NRF2 (which inhibits transcription of proinflammatory genes) and, in part, due to its potential to bind to cysteines in proteins involved in inflammatory cascades (e.g., IKKβ, shown in beige). Omaveloxolone (also known as RTA-408); brand name is SKYCLARYS. Omaveloxolone is the first and only FDA-approved drug for patients with Friedreich’s ataxia. Omaveloxolone has received Orphan Drug, Fast Track, and Rare Pediatric Disease Designations from the FDA, and Orphan Drug Designation for the treatment of Friedreich’s ataxia from the European Commission. It is currently not approved outside of the USA. SKYCLARYS™ is approved in the USA by the FDA and is indicated for the treatment of Friedreich’s ataxia in adults and adolescents aged 16 years and older. The recommended dosage is 150 mg to be taken orally daily. Reata Pharmaceuticals. Headache (37%), nausea (33%), diarrhea (20%), abdominal pain (29%), fatigue (24%), musculoskeletal pain (20%), vomiting (16%), oropharyngeal pain (18%), influenza (16%), muscle spasms (14%), back pain (13%), decreased appetite (12%), rash (10%). The most common laboratory abnormalities (occurring in 37% of patients) are elevated aspartate/alanine transaminases. In clinical trials, most effects diminished or stopped after 12 weeks of treatment. 2014–2020: Phase 1 trials (NCT02029716, NCT03664453, NCT04008186) 2014–present: Phase 2 trials (NCT02255435, NCT02029729, NCT02142959, NCT02128113, NCT03902002, NCT02255422) February 2023: FDA approval for SKYCLARYS™ (omaveloxolone). We thank the Medical Research Council (MR/W023806/1 and MR/T014644/1), the Biotechnology and Biological Sciences Research Council and GlaxoSmithKline (BB/T508111/1, BB/X00029X/1, and BB/T017546/1), Tenovus Scotland (T19/30 and T22/08), Reata Pharmaceuticals, and Medical Research Scotland (PHD-50477-2021) for supporting our research and the COST Action CA20121, supported by the European Cooperation in Science and Technology (www.cost.eu) (https://benbedphar.org/about-benbedphar/). A.T. D.-K. is a member of the Scientific Advisory Board of Evgen Pharma and collaborates with GlaxoSmithKline and Reata Pharmaceuticals. S.D.N. has no interests to declare.

Salidroside alleviates cognitive impairment by inhibiting ferroptosis via activation of the Nrf2/GPX4 axis in SAMP8 mice

BackgroundAlzheimer's disease (AD) is a neurogenerative disease and remains no effective method for stopping its progress. Ferroptosis and adaptive immunity have been proven to contribute to AD pathogenesis. Salidroside exhibits neuroprotective and immunomodulatory effects. However, the underlying mechanisms linking salidroside, ferroptosis, and adaptive immunity in AD remain uncertain. PurposeThe objective of this study is to explore the neuroprotective effects and the potential molecular mechanisms of salidroside against neuronal ferroptosis and CD8+ T cell infiltration in senescence-accelerated mouse prone 8 (SAMP8) mice. Study design and methodsSAMP8 mice were employed as an AD model and were treated with salidroside for 12 weeks. Behavioral tests, immunohistochemistry, HE and Nissl staining, immunofluorescence, transmission electron microscopy, quantitative proteomics, bioinformatic analysis, flow cytometry, iron staining, western blotting, and molecular docking were performed. ResultsTreatment with salidroside dose-dependently attenuated cognitive impairment, reduced the accumulation of Aβ plaques and restored neuronal damage. Salidroside also suppressed the infiltration of CD8+T cells, oxidative stress, and inflammatory cytokines, and improved mitochondrial metabolism, iron metabolism, lipid metabolism, and redox in the SAMP8 mice brain. The administration of salidroside decreased iron deposition, reduced TFR1, and ACSL4 protein expression, upregulated SLC7A11, and GPX4 protein expression, and promoted the Nrf2/GPX4 axis activation. ConclusionIn conclusion, neuronal ferroptosis and CD8+T cells are involved in the process of cognitive impairment in SAMP8 mice. Salidroside alleviates cognitive impairment and inhibits neuronal ferroptosis. The underlying mechanisms may involve the Nrf2/GPX4 axis activation and reduction in CD8+T cells infiltration. This study provides some evidence for the roles of salidroside in adaptive immunity and neuronal ferroptosis in SAMP8 mice.

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.

Elevated SFXN2 limits mitochondrial autophagy and increases iron-mediated energy production to promote multiple myeloma cell proliferation.

Human sideroflexin 2 (SFXN2) belongs to the SFXN protein family, which is a mitochondrial outer membrane protein involved in mitochondrial iron metabolism. Mitochondria are indispensable for cellular energy production and iron metabolism. However, it remains elusive how SFXN2 modulates mitochondrial homeostasis and cellular iron metabolism in multiple myeloma (MM). In this study, we first found that SFXN2 was significantly elevated and correlated to poor outcomes in MM patients from clinical datasets. SFXN2 overexpression promoted MM cell proliferation and suppressed starvation-induced autophagy/mitophagy, while SFXN2 knockdown aggravated mitochondria damage and autophagic processes in ARP1 and H929 MM cell lines. Furthermore, inhibition of SFXN2 exerted effectively anti-myeloma activity in vivo by using myeloma xenograft model. Mechanism studies indicated that heme oxygenase 1 (HO1) with anti-oxidant function contributed to the process of autophagy suppression and cellular proliferation mediated by SFXN2. Our study revealed the critical role of SFXN2 in regulating mitochondrial bioenergetics, mitophagy, cellular iron metabolism, and redox homeostasis in interconnected and intricate way. Collectively, these findings not only provide insights into the metabolic reprogramming of tumor cells, but also highlight the therapeutic potential of SFXN2 in combination with iron metabolism as target for prognosis and treatment in MM patients.

Physiological and Pathological Functions of Neuronal Hemoglobin: A Key Underappreciated Protein in Parkinson's Disease.

The expression of Hemoglobin (Hb) is not restricted to erythrocytes but is also present in neurons. Hb is selectively enriched in vulnerable mesencephalic dopaminergic neurons of Parkinson’s disease (PD) instead of resistant neurons. Controversial results of neuronal Hb levels have been reported in postmortem brains of PD patients: although neuronal Hb levels may decline in PD patients, elderly men with higher Hb levels have an increased risk of developing PD. α-synuclein, a key protein involved in PD pathology, interacts directly with Hb protein and forms complexes in erythrocytes and brains of monkeys and humans. These complexes increase in erythrocytes and striatal cytoplasm, while they decrease in striatal mitochondria with aging. Besides, the colocalization of serine 129-phosphorylated (Pser129) α-synuclein and Hb β chains have been found in the brains of PD patients. Several underlying molecular mechanisms involving mitochondrial homeostasis, α-synuclein accumulation, iron metabolism, and hormone-regulated signaling pathways have been investigated to assess the relationship between neuronal Hb and PD development. The formation of fibrils with neuronal Hb in various neurodegenerative diseases may indicate a common fibrillization pathway and a widespread target that could be applied in neurodegeneration therapy.

The multi-faced role of FUNDC1 in mitochondrial events and human diseases

Mitophagy plays a vital role in the selective elimination of dysfunctional and unwanted mitochondria. As a receptor of mitophagy, FUN14 domain containing 1 (FUNDC1) is attracting considerably critical attention. FUNDC1 is involved in the mitochondria fission, the clearance of unfolded protein, iron metabolism in mitochondria, and the crosstalk between mitochondria and endoplasmic reticulum besides mitophagy. Studies have demonstrated that FUNDC1 is associated with the progression of ischemic disease, cancer, and metabolic disease. In this review, we systematically examine the recent advancements in FUNDC1 and the implications of this protein in health and disease.

Prospective role and immunotherapeutic targets of sideroflexin protein family in lung adenocarcinoma: evidence from bioinformatics validation.

As lung cancer remains the leading cause of cancer deaths globally, characterizing the tumor molecular profiles is crucial to tailoring treatments for individuals at advanced stages. Cancer cells exhibit strong dependence on iron for their proliferation, and several iron-regulatory proteins have been proposed as either oncogenes or tumor suppressive genes. This study aims to evaluate the prospective therapeutic and prognostic values of the sideroflexin (SFXN) gene family, whose functions involve mitochondrial iron metabolism, in lung adenocarcinoma (LUAD). Differential expression analysis using TIMER and UALCAN tools was first employed to compare SFXNs expression levels between normal and LUAD tissues. Next, SFXNs' prognostic values, biological significance, and potential as immunotherapy candidates were examined from GEPIA, cBioPortal, MetaCore, Cytoscape, and TIMER databases. It was found that all members of SFXN family, except SFXN3, were differentially expressed in LUAD compared to normal samples and within different stages of LUAD. Survival analysis then revealed SFXN1 to be related to worse overall survival outcome in patients with LUAD. Furthermore, several correlations between expression of SFXN1 and immune infiltration cells were discovered. To conclude, our study provides evidence of SFXN family gene's relevance to the prognosis and immunotherapeutic targets of LUAD.

The metabolic growth limitations of petite cells lacking the mitochondrial genome.

Eukaryotic cells can survive the loss of their mitochondrial genome, but consequently suffer from severe growth defects. 'Petite yeasts', characterized by mitochondrial genome loss, are instrumental for studying mitochondrial function and physiology. However, the molecular cause of their reduced growth rate remains an open question. Here we show that petite cells suffer from an insufficient capacity to synthesize glutamate, glutamine, leucine and arginine, negatively impacting their growth. Using a combination of molecular genetics and omics approaches, we demonstrate the evolution of fast growth overcomes these amino acid deficiencies, by alleviating a perturbation in mitochondrial iron metabolism and by restoring a defect in the mitochondrial tricarboxylic acid cycle, caused by aconitase inhibition. Our results hence explain the slow growth of mitochondrial genome-deficient cells with a partial auxotrophy in four amino acids that results from distorted iron metabolism and an inhibited tricarboxylic acid cycle.

A systematic review and meta-analysis of studies that have evaluated the role of mitochondrial function and iron metabolism in frailty.

Frailty is a condition of global impairment due to depletion of physiological reserves. However, the underlying biological mechanisms are poorly understood. The aims of the current study were to identify the differences in mitochondrial function and iron metabolism between frail and nonfrail populations, and to investigate the contribution of different methodological approaches to the results. Searches were performed, using five online databases up to November 2019. Studies reporting measurements of mitochondrial function or iron metabolism in frail and nonfrail subjects or subjects with and without sarcopenia, were included. Pooled effect estimates were expressed as Standardized Mean Differences. Heterogeneity, expressed as I 2, was explored using regression analyses. In total, 107 studies, reporting 75 measures of mitochondrial function or iron metabolism, using six different experimental approaches, in three species were identified. Significant decreases in measures of oxygen consumption were observed for frail humans but not in animal models. Conversely, no differences between frail and nonfrail humans were observed for apoptosis and autophagy, in contrast to animal models. The most significant effect of the type of frailty assessment was observed for respiratory chain complexes where only subjects categorized as frail by the Fried Frailty Index showed a significant decrease in activity. We identified iron metabolism in frailty as an important knowledge gap, highlighted the need of consistent frailty diagnostic tools, and pointed out the limited translational potential of animal models. Inconsistency between studies evaluating the molecular mechanisms underlying frailty may present a barrier to the development of effective therapies.

Mechanisms of impaired mitochondrial homeostasis and NAD+ metabolism in a model of mitochondrial heart disease exhibiting redox active iron accumulation

Due to the high redox activity of the mitochondrion, this organelle can suffer oxidative stress. To manage energy demands while minimizing redox stress, mitochondrial homeostasis is maintained by the dynamic processes of mitochondrial biogenesis, mitochondrial network dynamics (fusion/fission), and mitochondrial clearance by mitophagy. Friedreich's ataxia (FA) is a mitochondrial disease resulting in a fatal hypertrophic cardiomyopathy due to the deficiency of the mitochondrial protein, frataxin. Our previous studies identified defective mitochondrial iron metabolism and oxidative stress potentiating cardiac pathology in FA. However, how these factors alter mitochondrial homeostasis remains uncharacterized in FA cardiomyopathy. This investigation examined the muscle creatine kinase conditional frataxin knockout mouse, which closely mimics FA cardiomyopathy, to dissect the mechanisms of dysfunctional mitochondrial homeostasis. Dysfunction of key mitochondrial homeostatic mechanisms were elucidated in the knockout hearts relative to wild-type littermates, namely: (1) mitochondrial proliferation with condensed cristae; (2) impaired NAD+ metabolism due to perturbations in Sirt1 activity and NAD+ salvage; (3) increased mitochondrial biogenesis, fusion and fission; and (4) mitochondrial accumulation of Pink1/Parkin with increased autophagic/mitophagic flux. Immunohistochemistry of FA patients' heart confirmed significantly enhanced expression of markers of mitochondrial biogenesis, fusion/fission and autophagy. These novel findings demonstrate cardiac frataxin-deficiency results in significant changes to metabolic mechanisms critical for mitochondrial homeostasis. This mechanistic dissection provides critical insight, offering the potential for maintaining mitochondrial homeostasis in FA and potentially other cardio-degenerative diseases by implementing innovative treatments targeting mitochondrial homeostasis and NAD+ metabolism.

Targeting Mitochondrial Iron Metabolism Suppresses Tumor Growth and Metastasis by Inducing Mitochondrial Dysfunction and Mitophagy.

Deferoxamine (DFO) represents a widely used iron chelator for the treatment of iron overload. Here we describe the use of mitochondrially targeted deferoxamine (mitoDFO) as a novel approach to preferentially target cancer cells. The agent showed marked cytostatic, cytotoxic, and migrastatic properties in vitro, and it significantly suppressed tumor growth and metastasis in vivo. The underlying molecular mechanisms included (i) impairment of iron-sulfur [Fe-S] cluster/heme biogenesis, leading to destabilization and loss of activity of [Fe-S] cluster/heme containing enzymes, (ii) inhibition of mitochondrial respiration leading to mitochondrial reactive oxygen species production, resulting in dysfunctional mitochondria with markedly reduced supercomplexes, and (iii) fragmentation of the mitochondrial network and induction of mitophagy. Mitochondrial targeting of deferoxamine represents a way to deprive cancer cells of biologically active iron, which is incompatible with their proliferation and invasion, without disrupting systemic iron metabolism. Our findings highlight the importance of mitochondrial iron metabolism for cancer cells and demonstrate repurposing deferoxamine into an effective anticancer drug via mitochondrial targeting. SIGNIFICANCE: These findings show that targeting the iron chelator deferoxamine to mitochondria impairs mitochondrial respiration and biogenesis of [Fe-S] clusters/heme in cancer cells, which suppresses proliferation and migration and induces cell death. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/9/2289/F1.large.jpg.

Hypomethylating Agents Do Not Alter Novel Splicing Events in Myeloid Neoplasms

Background: Aberrant mRNA splicing occurs in myeloid malignancies and affects genes involved in tumor suppression, heme biosynthesis and mitochondrial iron metabolism. Functional studies demonstrated impaired cellular differentiation upon targeting of aberrant splice variants. Hypomethylating agents (HMA) constitute the backbone of therapy of myeloid malignancies. Whether HMA treatment in myeloid malignancies alters the novel splicing transcriptional landscape and whether it correlates with responses remain largely unexplored. Methods: Total RNA sequencing was done on CD34+ cells from 79 patients bone marrow samples involved by acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), chronic myelomonocytic leukemia (CMML), TF1 cell lines and CD34+ murine bone marrow cells. Novel alternatively spliced transcripts were detected using SplAdder and included the following splicing events: alterative 3' splice junction, alternative 5' splice junction, exon skipping, intron retention, multiple exon skipping and mutually exclusive exons. All alternatively splicing events were normalized to total transcript count in order to correct for total transcript levels. A false discovery rate of <0.1 was used to identify significant events. Results: A total of 79 myeloid disease patients (27.8% females, 72.1% males) with a median age of 70 years (range, 31-87 years) were included in this study. In aggregate analysis of all 79 myeloid malignancies (39.2% (n=31) pre-treatment and 60.8% (n=48) post-treatment), there were 160 versus 37 (4.3 folds), 112 versus 40 (2.8 folds), 292 versus 51 (5.7 folds), 172 versus 80 (2.1 folds) and 29 versus 9 (3.2 folds) and 2 versus 0 novel splicing events occurring in pre- versus post- HMA treatment, respectively, in alterative 3' splice junction, alternative 5' splice junction, exon skipping, intron retention, multiple exon skipping and mutually exclusive exons, respectively. This suggested that treatment with HMA led to downregulation of novel alternative splicing events after normalization to total transcripts. However, upon excluding AML patients from the analysis, there were no significant events associated with treatment suggesting that the findings could be due to random events. To further explore whether HMA therapy influenced novel splicing events, we examined the novel splicing pattern in 7 MDS patients with paired BM samples at pre- and post-HMA and found no significant differences in alternative splicing events before and after the treatment. We then examined TF1 (human erythroleukemia) cell lines at pre- and post- HMA time points, but did not identify notable differences in the novel alternative splicing events with respect to HMA treatment. To assess whether CD34+ bone marrow cells from mice treated with hypomethylating agents have differential novel alternatively spliced events, we conducted similar analysis and did not find any discernible differences pre- and post- HMA treatment. These findings suggest that HMA does not influence novel alternative splicing events. Conclusions: Aberrant splicing has been linked to myeloid neoplasms especially myelodysplastic syndrome with mutations in splice variant genes. Our findings suggest that HMA does not influence novel alternative splicing events in myeloid malignancies. Therefore, the alternative splicing in myeloid disease is inherent to the disease and not affected by treatment. Disclosures Garcia-Manero: H3 Biomedicine: Research Funding; Novartis: Research Funding; Bristol-Myers Squibb: Consultancy, Research Funding; Merck: Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Onconova: Research Funding; Acceleron Pharmaceuticals: Consultancy, Honoraria; Amphivena Therapeutics: Research Funding; Genentech: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Helsinn Therapeutics: Consultancy, Honoraria, Research Funding; Jazz Pharmaceuticals: Consultancy; Astex Pharmaceuticals: Consultancy, Honoraria, Research Funding; AbbVie: Honoraria, Research Funding.

Coordinated Mis-Splicing of Multiple Mitochondrial Iron Metabolism Genes Causes Ring Sideroblast Formation in SF3B1-Mutant MDS

Splicing is a fundamental process by which introns are removed from primary RNA transcripts. Alternative splicing is a major mechanism of gene regulation by which eukaryotic cells expand their transcriptional repertoire. By contrast, aberrant splicing generates novel transcripts not found in normal cells. Heterozygous gain-of-function mutations in a core U2 spliceosome factor SF3B1 are present in ~25% of myelodysplastic syndrome (MDS) patients. Strikingly, SF3B1 mutations are present in ~80% of MDS with ring sideroblasts (MDS-RS) patients suggesting a causal connection between mutant SF3B1 and ring sideroblasts (RS), erythroid precursors with iron-laden mitochondria (Yoshida et al. Nature 2011, Papaemmanuil et al.NEJM 2011). However, the mechanism by which SF3B1 mutations cause RS formation remains poorly understood since existing models of SF3B1-mutant MDS do not recapitulate RS formation in vitro. We have established an induced pluripotent stem cell (iPSC) model of MDS-RS that recapitulates mutant SF3B1-mediated mis-splicing and in vitro ring sideroblast formation. We reprogrammed SF3B1-mutant and SF3B1-wild-type iPSCs from individual MDS-RS patients enabling internal normalization to the isogenic normal clone (Hsu et al. Blood 2019). We established expandable multipotential HPC lines by conditional expression of five transcription factors (5F-HPC), followed by an 18-day erythroid differentiation. We monitored RS formation during the erythroid differentiation of 5F-HPCs using Prussian blue histological staining. RS formation increased during terminal erythropoiesis and peaked at 25-40% on day 18 in SF3B1-mutant lines, whereas SF3B1-wild-type lines showed no detectable RS formation. The frequency of RS was similar in isogenic SF3B1-only and SF3B1/EZH2 co-mutant cells suggesting that the mutant SF3B1 is sufficient to drive RS formation. To identify mis-splicing events that contribute to RS formation, we performed RNA-sequencing and splicing analysis of SF3B1-mutant and SF3B1-wild-type iPSCs at three stages of erythroid differentiation: CD34+ progenitor, CD71+ early erythroblast, and CD71+Glycophorin A+ erythroblast. Global isoform usage was dramatically altered during erythropoiesis, but was more similar in stage-matched SF3B1-mutant and SF3B1-wild-type cells suggesting that mutant SF3B1 selectively mis-splices a subset of transcripts. We identified 2300 transcripts with >10% mis-splicing in SF3B1-mutant lines, and only 120 transcripts with more significant >40% mis-splicing. Of these, TMEM14C and PPOX, inner mitochondrial membrane components of the heme synthesis pathway were strongly mis-spliced throughout erythroid differentiation, consistent with previous studies (Conte et al.BJHaem 2015, Shiozawa et al. Nat Commun 2018). The transcript levels of PPOX but not TMEM14C were reduced in SF3B1-mutant lines as a result of mis-splicing. The expression of ABCB7, a mitochondrial iron sulfur cluster biogenesis component mutated in inherited X-linked sideroblastic anemia (Allikmets et al. Hum Mol Genet 1999), was also reduced in SF3B1-mutant cells as expected (Shiozawa et al. Nat Commun 2018, Dolatshad et al. Leukemia 2016). To investigate the role of these mis-splicing events in ring sideroblast formation, we performed lentiviral overexpression of TMEM14C, PPOX, and ABCB7, in SF3B1-mutant 5F-HPCs and quantified RS formation during late stages of erythroid differentiation. Overexpression of TMEM14C and ABCB7 in SF3B1-mutant cells partially but not completely rescued RS formation compared to luciferase control. These findings confirm the long-standing hypothesis that mis-splicing of mitochondrial iron metabolism genes causes RS formation. Furthermore, these findings suggest that RS formation in MDS is a multigenic event caused by coordinated but incomplete mis-splicing of several critical iron metabolism genes. Disclosures No relevant conflicts of interest to declare.

Involvement of Mrs3/4 in Mitochondrial Iron Transport and Metabolism in Cryptococcus neoformans.

Mitochondria play a vital role in iron uptake and metabolism in pathogenic fungi, and also influence virulence and drug tolerance. However, the regulation of iron transport within the mitochondria of Cryptococcus neoformans, a causative agent of fungal meningoencephalitis in immunocompromised individuals, remains largely uncharacterized. In this study, we identified and functionally characterized Mrs3/4, a homolog of the Saccharomyces cerevisiae mitochondrial iron transporter, in C. neoformans var. grubii. A strain expressing an Mrs3/4-GFP fusion protein was generated, and the mitochondrial localization of the fusion protein was confirmed. Moreover, a mutant lacking the MRS3/4 gene was constructed; this mutant displayed significantly reduced mitochondrial iron and cellular heme accumulation. In addition, impaired mitochondrial iron-sulfur cluster metabolism and altered expression of genes required for iron uptake at the plasma membrane were observed in the mrs3/4 mutant, suggesting that Mrs3/4 is involved in iron import and metabolism in the mitochondria of C. neoformans. Using a murine model of cryptococcosis, we demonstrated that an mrs3/4 mutant is defective in survival and virulence. Taken together, our study suggests that Mrs3/4 is responsible for iron import in mitochondria and reveals a link between mitochondrial iron metabolism and the virulence of C. neoformans.

Genomic Multiplication and Drug Efflux Influence Ketoconazole Resistance in Malassezia restricta.

Malassezia restricta is an opportunistic fungal pathogen on human skin; it is associated with various skin diseases, including seborrheic dermatitis and dandruff, which are usually treated using ketoconazole. In this study, we clinically isolated ketoconazole-resistant M. restricta strains (KCTC 27529 and KCTC 27550) from patients with dandruff. To understand the mechanisms of ketoconazole resistance in the isolates, their genomes were sequenced and compared with the susceptible reference strain M. restricta KCTC 27527. Using comparative genome analysis, we identified tandem multiplications of the genomic loci containing ATM1 and ERG11 homologs in M. restricta KCTC 27529 and KCTC 27550, respectively. Additionally, we found that the copy number increase of ATM1 and ERG11 is reflected in the increased expression of these genes; moreover, we observed that overexpression of these homologs caused ketoconazole resistance in a genetically tractable fungal pathogen, Cryptococcus neoformans. In addition to tandem multiplications of the genomic region containing the ATM1 homolog, the PDR5 homolog, which encodes the drug efflux pump protein was upregulated in M. restricta KCTC 27529 compared to the reference strain. Biochemical analysis confirmed that drug efflux was highly activated in M. restricta KCTC 27529, implying that upregulation of the PDR5 homolog may also contribute to ketoconazole resistance in the strain. Overall, our results suggest that multiplication of the genomic loci encoding genes involved in ergosterol synthesis, mitochondrial iron metabolism, and oxidative stress response and overexpression of the drug efflux pumps are the mechanisms underlying ketoconazole resistance in M. restricta.

Exercise Preconditioning Prevents Chemotherapy‐Induced Alterations to Transport Proteins Required for Mitochondrial Iron Uptake and Metabolism in the Diaphragm

Doxorubicin (DOX) is an anthracycline antibiotic used in cancer treatment. Unfortunately, the clinical use of this highly efficacious anticancer drug is limited due to the development of respiratory and diaphragm muscle dysfunction in patients. DOX‐induced ventilatory impairment is a debilitating condition that promotes the onset of dyspnea, fatigue and exercise intolerance. While the mechanisms responsible for DOX‐induced respiratory insufficiency are unclear, it has been proposed that DOX myotoxicity develops as a result of increased cellular reactive oxygen species production and mitochondrial iron accumulation. Conversely, endurance exercise training performed prior to DOX administration has been shown to prevent the chemotherapy‐induced diaphragm weakness and mitochondrial dysfunction. While the mechanisms for this protection remain elusive, evidence suggests that exercise may provide protection by attenuating DOX‐induced alterations to proteins required to maintain mitochondrial iron homeostasis. Cause and effect were determined by exposing Sprague‐Dawley rats to a two‐week treadmill preconditioning protocol (5 days/week; 60 min/day; 30 m/min). Twenty‐four hours following the last exercise training period, animals received either saline or DOX (20mg/kg ip) treatment. Our results demonstrate that DOX administration in sedentary animals results in increased expression of the mitochondrial iron importer mitoferrin‐1 and a significant reduction in the cellular iron importer transferrin receptor 1 in the diaphragm. In addition, DOX also stimulated a significant decrease in the expression of sideroflexin 2 and SLC25A28, proteins required for mitochondrial heme biosynthesis. Importantly, exercise training prior to DOX administration prevented the DOX‐induced increase in mitoferrin‐1, and also reduced mitoferrin‐2 below that of sedentary animals treated with DOX. Moreover, diaphragm expression of SLC25A38 was maintained in exercise trained animals receiving DOX. These findings confirm a role for mitochondrial iron accumulation and dysregulation of heme synthesis in the development of DOX‐induced respiratory dysfunction, and demonstrate that exercise preconditioning may ameliorate DOX‐mediated diaphragm muscle pathology through the prevention of mitochondrial iron dysregulation.Support or Funding InformationThis work was supported by NIH R01 HL146443 awarded to AJS

Enzymatic Oxidation of Biological Thiols by MitoNEET

MitoNEET is a mitochondrial [2Fe‐2S] protein known for its involvement in mitochondrial bioenergetics, iron metabolism, and oxidative stress. The protein has been extensively characterized at the biochemical level since its discovery in 2004, but the mechanisms of physiological function(s) remain unresolved. Two observations seem especially relevant for understanding the physiological function of mitoNEET: (1) under anaerobic conditions in vitro thiol compounds such as glutathione (GSH) and cysteine (Cys) reduce the 2Fe‐2S cluster from the Fe+3Fe+3 to the Fe3+Fe2+ state, and (2) mitoNEET expression levels increase in cells when the glutathione redox balance is shifted towards the oxidized state. We hypothesize that mitoNEET’s in vivo function can be elucidated by bridging these two observations and postulate that mitoNEET is an enzyme responsible for the oxidation of thiol groups on either low molecular‐weight compounds or cysteine residues on target proteins involved in redox signaling. To test this hypothesis, we monitored thiol oxidation by following oxygen consumption rates and demonstrate that mitoNEET accelerates thiol oxidation of cysteine and glutathione (GSH). Four distinct experiments comparing the reactivity of Fe3+ in the form of FeCl3 to mitoNEET demonstrate that the observed thiol oxidation by mitoNEET was not due to free Fe3+contamination. These observations were: (1) rates of cysteine oxidation (8 mM) catalyzed by mitoNEET (1.5 mM) or Fe3+ (1.5 mM) show different optima in the pH range 6.4 to 9.4 and the mitoNEET‐catalyzed reaction is 137% faster at pH 8.4 than the rate observed for Fe3+ alone. (2) Oxidation rates of the nonphysiological compound, n‐acetylcysteine (NAC) by mitoNEET are 181% higher compared to Fe3+ at a pH of 9.0. (3) MitoNEET‐mediated glutathione oxidation remained unaffected by addition of cysteine while oxidation catalyzed by free Fe3+ decreased after addition of cysteine. (4) Dimedone, a probe used to inhibit thiol oxidation from sulfenic acid to sulfinic and sulfonic acid, was 25% more effective at inhibiting cysteine oxidation when free Fe3+ was the catalyst compared to mitoNEET. These results support our hypothesis that mitoNEET possesses an active site which functions to oxidize biological thiols. While the in vivo substrate is currently unknown, we have narrowed down potential candidates to small thiol containing compounds and/or cysteine residues on specific target proteins. We are currently investigating how the enzymatic activity may be regulated by endogenous compounds and through pharmacological compounds targeting mitoNEET.Support or Funding InformationThis work was supported by NSF CHE‐1806266 to M.A.M and M.E.K and UofL Mentored Undergraduate Research and Creative Activities Grant to R.A.S.