Mitochondrial dysfunction in primary human fibroblasts triggers an adaptive cell survival program that requires AMPK-α

Small Molecule Inhibitors of Arylamine N-Acetyltransferase 1 Attenuate Cellular Respiration

Arylamine N-acetyltransferase 1 (NAT1) expression has been shown to attenuate mitochondrial function, suggesting it is a promising drug target in diseases of mitochondrial dysfunction. Here, several second-generation naphthoquinones have been investigated as small molecule inhibitors of NAT1. The results show that the compounds inhibit both in vitro and in whole cells. A lead compound (Cmp350) was further investigated for its ability to alter mitochondrial metabolism in MDA-MB-231 cells. At concentrations that inhibited NAT1 by over 85%, no overt toxicity was observed. Moreover, the inhibitor decreased basal respiration and reserve respiratory capacity without affecting ATP production. Cells treated with Cmp350 were almost exclusively dependent on glucose as a fuel source. We postulate that Cmp350 is an excellent lead compound for the development of NAT1-targeted inhibitors as both experimental tools and therapeutics in the treatment of hypermetabolic diseases such as amyotrophic lateral sclerosis, cancer cachexia, and sepsis.

Mitochondria dysfunction induced by decyl-TPP mitochondriotropic antioxidant based on caffeic acid AntiOxCIN6 sensitizes cisplatin lung anticancer therapy due to a remodeling of energy metabolism

The pharmacological interest in mitochondria is very relevant since these crucial organelles are involved in the pathogenesis of multiple diseases, such as cancer. In order to modulate cellular redox/oxidative balance and enhance mitochondrial function, numerous polyphenolic derivatives targeting mitochondria have been developed. Still, due to the drug resistance emergence in several cancer therapies, significant efforts are being made to develop drugs that combine the induction of mitochondrial metabolic reprogramming with the ability to generate reactive oxygen species, taking into consideration the varying metabolic profiles of different cell types. We previously developed a mitochondria-targeted antioxidant (AntiOxCIN6) by linking caffeic acid to lipophilic triphenylphosphonium cation through a 10-carbon aliphatic chain. The antioxidant activity of AntiOxCIN6 has been documented but how the mitochondriotropic compound impact energy metabolism of both normal and cancer cells remains unknown. We demonstrated that AntiOxCIN6 increased antioxidant defense system in HepG2 cells, although ROS clearance was ineffective. Consequently, AntiOxCIN6 significantly decreased mitochondrial function and morphology, culminating in a decreased capacity in complex I-driven ATP production without affecting cell viability. These alterations were accompanied by an increase in glycolytic fluxes. Additionally, we demonstrate that AntiOxCIN6 sensitized A549 adenocarcinoma cells for CIS-induced apoptotic cell death, while AntiOxCIN6 appears to cause metabolic changes or a redox pre-conditioning on lung MRC-5 fibroblasts, conferring protection against cisplatin. We propose that length and hydrophobicity of the C10-TPP+ alkyl linker play a significant role in inducing mitochondrial and cellular toxicity, while the presence of the antioxidant caffeic acid appears to be responsible for activating cytoprotective pathways.

The decylTPP mitochondria-targeting moiety lowers electron transport chain supercomplex levels in primary human skin fibroblasts

Attachment of cargo molecules to lipophilic triphenylphosphonium (TPP⁺) cations is a widely applied strategy for mitochondrial targeting. We previously demonstrated that the vitamin E-derived antioxidant Trolox increases the levels of active mitochondrial complex I (CI), the first complex of the electron transport chain (ETC), in primary human skin fibroblasts (PHSFs) of Leigh Syndrome (LS) patients with isolated CI deficiency. Primed by this finding, we here studied the cellular effects of mitochondria-targeted Trolox (MitoE10), mitochondria-targeted ubiquinone (MitoQ10) and their mitochondria-targeting moiety decylTPP (C₁₀-TPP⁺). Chronic treatment (96 h) with these molecules of PHSFs from a healthy subject and an LS patient with isolated CI deficiency (NDUFS7-V122M mutation) did not greatly affect cell number. Unexpectedly, this treatment reduced CI levels/activity, lowered the amount of ETC supercomplexes, inhibited mitochondrial oxygen consumption, increased extracellular acidification, altered mitochondrial morphology and stimulated hydroethidine oxidation. We conclude that the mitochondria-targeting decylTPP moiety is responsible for the observed effects and advocate that every study employing alkylTPP-mediated mitochondrial targeting should routinely include control experiments with the corresponding alkylTPP moiety.

The beginning of GPX2 and 30 years later

We published the first paper to characterize GPX2 (aka GSHPx-GI) as a selenoenzyme with glutathione peroxidase activity in 1993. Among the four Se-GPX isozymes, GPX1-4, GPX1 and GPX2 are closely related in terms of structure, substrate specificities, and subcellular localization. What sets them apart are distinct patterns of gene regulation, tissue distribution and response to selenium. While we identified the digestive tract epithelium as the main site of GPX2 expression, later work has shown GPX2 is found more widely in epithelial tissues with concentration of expression in stem cell and proliferative compartments. GPX2 expression is regulated over a wide range of levels by many pathways, including NRF2, WNT, p53, RARE and this often results in attaching undue significance to GPX2 as GPX2 is only a part of a system of hydroperoxidase activities, including GPX1, peroxiredoxins and catalase. These other activities may play equal or greater roles, particularly in cell lines cultured without selenium supplementation and often with very low GPX2 levels. This could be assessed by examining levels of mRNA and protein among these various peroxidases at the outset of studies. As an example, it was found that GPX1 responds to the absence of GPX2 in mouse ileum and colon epithelium with higher expression. As such, both Gpx1 and Gpx2 had to be knocked out in mice to produce ileocolitis. However, we note that the actual role of GPX1 and GPX2 in relation to peroxiredoxin function is unclear. There may be an interdependence that requires only low amounts of GPX1 and/or GPX2 in a supporting role to maintain proper peroxiredoxin function. GPX2 levels may be prognostic for cancer progression in colon, breast, prostate and liver, however, there is no consistent trend for higher or lower levels to be favorable.

Pyruvate and uridine rescue the metabolic profile of OXPHOS dysfunction

Introduction

Primary mitochondrial diseases (PMD) are a large, heterogeneous group of genetic disorders affecting mitochondrial function, mostly by disrupting the oxidative phosphorylation (OXPHOS) system. Understanding the cellular metabolic re-wiring occurring in PMD is crucial for the development of novel diagnostic tools and treatments, as PMD are often complex to diagnose and most of them currently have no effective therapy.

Objectives

To characterize the cellular metabolic consequences of OXPHOS dysfunction and based on the metabolic signature, to design new diagnostic and therapeutic strategies.

Methods

In vitro assays were performed in skin-derived fibroblasts obtained from patients with diverse PMD and validated in pharmacological models of OXPHOS dysfunction. Proliferation was assessed using the Incucyte technology. Steady-state glucose and glutamine tracing studies were performed with LC-MS quantification of cellular metabolites. The therapeutic potential of nutritional supplements was evaluated by assessing their effect on proliferation and on the metabolomics profile. Successful therapies were then tested in a in vivo lethal rotenone model in zebrafish.

Results

OXPHOS dysfunction has a unique metabolic signature linked to an NAD+/NADH imbalance including depletion of TCA intermediates and aspartate, and increased levels of glycerol-3-phosphate. Supplementation with pyruvate and uridine fully rescues this altered metabolic profile and the subsequent proliferation deficit. Additionally, in zebrafish, the same nutritional treatment increases the survival after rotenone exposure.

Conclusions

Our findings reinforce the importance of the NAD+/NADH imbalance following OXPHOS dysfunction in PMD and open the door to new diagnostic and therapeutic tools for PMD.

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OXPHOS deficiency causes a distinct metabolic profile linked to a NAD+/NADH imbalance.

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Depleted intracellular aspartic acid is a potential biomarker for OXPHOS dysfunction.

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Therapy with pyruvate and uridine corrects the metabolic profile of OXPHOS deficiency.

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Pyruvate and uridine treatment increases survival in a lethal rotenone zebrafish model.

Neuroligin-mediated neurodevelopmental defects are induced by mitochondrial dysfunction and prevented by lutein in C. elegans

Complex-I-deficiency represents the most frequent pathogenetic cause of human mitochondriopathies. Therapeutic options for these neurodevelopmental life-threating disorders do not exist, partly due to the scarcity of appropriate model systems to study them. Caenorhabditis elegans is a genetically tractable model organism widely used to investigate neuronal pathologies. Here, we generate C. elegans models for mitochondriopathies and show that depletion of complex I subunits recapitulates biochemical, cellular and neurodevelopmental aspects of the human diseases. We exploit two models, nuo-5/NDUFS1- and lpd-5/NDUFS4-depleted animals, for a suppressor screening that identifies lutein for its ability to rescue animals’ neurodevelopmental deficits. We uncover overexpression of synaptic neuroligin as an evolutionarily conserved consequence of mitochondrial dysfunction, which we find to mediate an early cholinergic defect in C. elegans. We show lutein exerts its beneficial effects by restoring neuroligin expression independently from its antioxidant activity, thus pointing to a possible novel pathogenetic target for the human disease.

Mitochondrial deficiency causes rare incurable disorders. Here, the authors use C. elegans to study these diseases and find that the natural compound lutein prevents neurodevelopmental deficits, thus pointing to a possible therapeutic target for the human diseases.

Mitochondrial respiratory chain dysfunction alters ER sterol sensing and mevalonate pathway activity

Mitochondrial dysfunction induces a strong adaptive retrograde signaling response; however, many of the downstream effectors of this response remain to be discovered. Here, we studied the shared transcriptional responses to three different mitochondrial respiratory chain inhibitors in human primary skin fibroblasts using QuantSeq 3′-RNA-sequencing. We found that genes involved in the mevalonate pathway were concurrently downregulated, irrespective of the respiratory chain complex affected. Targeted metabolomics demonstrated that impaired mitochondrial respiration at any of the three affected complexes also had functional consequences on the mevalonate pathway, reducing levels of cholesterol precursor metabolites. A deeper study of complex I inhibition showed a reduced activity of endoplasmic reticulum–bound sterol-sensing enzymes through impaired processing of the transcription factor Sterol Regulatory Element-Binding Protein 2 and accelerated degradation of the endoplasmic reticulum cholesterol-sensors squalene epoxidase and HMG-CoA reductase. These adaptations of mevalonate pathway activity affected neither total intracellular cholesterol levels nor the cellular free (nonesterified) cholesterol pool. Finally, measurement of intracellular cholesterol using the fluorescent cholesterol binding dye filipin revealed that complex I inhibition elevated cholesterol on intracellular compartments. Taken together, our study shows that mitochondrial respiratory chain dysfunction elevates intracellular free cholesterol levels and therefore attenuates the expression of mevalonate pathway enzymes, which lowers endogenous cholesterol biosynthesis, disrupting the metabolic output of the mevalonate pathway. We conclude that intracellular disturbances in cholesterol homeostasis may alter systemic cholesterol management in diseases associated with declining mitochondrial function.

Mitochondrial dynamics regulators: implications for therapeutic intervention in cancer

Regardless of the recent advances in therapeutic developments, cancer is still among the primary causes of death globally, indicating the need for alternative therapeutic strategies. Mitochondria, a dynamic organelle, continuously undergo the fusion and fission processes to meet cell requirements. The balanced fission and fusion processes, referred to as mitochondrial dynamics, coordinate mitochondrial shape, size, number, energy metabolism, cell cycle, mitophagy, and apoptosis. An imbalance between these opposing events alters mitochondWangrial dynamics, affects the overall mitochondrial shape, and deregulates mitochondrial function. Emerging evidence indicates that alteration of mitochondrial dynamics contributes to various aspects of tumorigenesis and cancer progression. Therefore, targeting the mitochondrial dynamics regulator could be a potential therapeutic approach for cancer treatment. This review will address the role of imbalanced mitochondrial dynamics in mitochondrial dysfunction during cancer progression. We will outline the clinical significance of mitochondrial dynamics regulators in various cancer types with recent updates in cancer stemness and chemoresistance and its therapeutic potential and clinical utility as a predictive biomarker.

The long non-coding RNA HOTAIRM1 promotes tumor aggressiveness and radiotherapy resistance in glioblastoma

Glioblastoma is the most common malignant primary brain tumor. To date, clinically relevant biomarkers are restricted to isocitrate dehydrogenase (IDH) gene 1 or 2 mutations and O6-methylguanine DNA methyltransferase (MGMT) promoter methylation. Long non-coding RNAs (lncRNAs) have been shown to contribute to glioblastoma pathogenesis and could potentially serve as novel biomarkers. The clinical significance of HOXA Transcript Antisense RNA, Myeloid-Specific 1 (HOTAIRM1) was determined by analyzing HOTAIRM1 in multiple glioblastoma gene expression data sets for associations with prognosis, as well as, IDH mutation and MGMT promoter methylation status. Finally, the role of HOTAIRM1 in glioblastoma biology and radiotherapy resistance was characterized in vitro and in vivo. We identified HOTAIRM1 as a candidate lncRNA whose up-regulation is significantly associated with shorter survival of glioblastoma patients, independent from IDH mutation and MGMT promoter methylation. Glioblastoma cell line models uniformly showed reduced cell viability, decreased invasive growth and diminished colony formation capacity upon HOTAIRM1 down-regulation. Integrated proteogenomic analyses revealed impaired mitochondrial function and determination of reactive oxygen species (ROS) levels confirmed increased ROS levels upon HOTAIRM1 knock-down. HOTAIRM1 knock-down decreased expression of transglutaminase 2 (TGM2), a candidate protein implicated in mitochondrial function, and knock-down of TGM2 mimicked the phenotype of HOTAIRM1 down-regulation in glioblastoma cells. Moreover, HOTAIRM1 modulates radiosensitivity of glioblastoma cells both in vitro and in vivo. Our data support a role for HOTAIRM1 as a driver of biological aggressiveness, radioresistance and poor outcome in glioblastoma. Targeting HOTAIRM1 may be a promising new therapeutic approach.

Mitochondrial Dynamics, ROS, and Cell Signaling: A Blended Overview

Mitochondria are key intracellular organelles involved not only in the metabolic state of the cell, but also in several cellular functions, such as proliferation, Calcium signaling, and lipid trafficking. Indeed, these organelles are characterized by continuous events of fission and fusion which contribute to the dynamic plasticity of their network, also strongly influenced by mitochondrial contacts with other subcellular organelles. Nevertheless, mitochondria release a major amount of reactive oxygen species (ROS) inside eukaryotic cells, which are reported to mediate a plethora of both physiological and pathological cellular functions, such as growth and proliferation, regulation of autophagy, apoptosis, and metastasis. Therefore, targeting mitochondrial ROS could be a promising strategy to overcome and hinder the development of diseases such as cancer, where malignant cells, possessing a higher amount of ROS with respect to healthy ones, could be specifically targeted by therapeutic treatments. In this review, we collected the ultimate findings on the blended interplay among mitochondrial shaping, mitochondrial ROS, and several signaling pathways, in order to contribute to the dissection of intracellular molecular mechanisms involved in the pathophysiology of eukaryotic cells, possibly improving future therapeutic approaches.

Mitochondria-targeted phenolic antioxidants induce ROS-protective pathways in primary human skin fibroblasts

Phytochemical antioxidants like gallic and caffeic acid are constituents of the normal human diet that display beneficial health effects, potentially via activating stress response pathways. Using primary human skin fibroblasts (PHSFs) as a model, we here investigated whether such pathways were induced by novel mitochondria-targeted variants of gallic acid (AntiOxBEN₂) and caffeic acid (AntiOxCIN₄). Both molecules reduced cell viability with similar kinetics and potency (72 h incubation, IC50 ~23 μM). At a relatively high but non-toxic concentration (12.5 μM), AntiOxBEN₂ and AntiOxCIN₄ increased ROS levels (at 24 h), followed by a decline (at 72 h). Further analysis at the 72 h timepoint demonstrated that AntiOxBEN₂ and AntiOxCIN₄ did not alter mitochondrial membrane potential (Δψ), but increased cellular glutathione (GSH) levels, mitochondrial NAD(P)H autofluorescence, and mitochondrial superoxide dismutase 2 (SOD2) protein levels. In contrast, cytosolic SOD1 protein levels were not affected. AntiOxBEN₂ and AntiOxCIN₄ both stimulated the gene expression of Nuclear factor erythroid 2-related factor 2 (NRF2; a master regulator of the cellular antioxidant response toward oxidative stress). AntiOxBEN2 and ANtiOxCIN4 differentially affected the gene expression of the antioxidants Heme oxygenase 1 (HMOX1) and NAD(P)H dehydrogenase (quinone) 1 (NQO1). Both antioxidants did not protect from cell death induced by GSH depletion and AntiOxBEN₂ (but not AntiOxCIN₄) antagonized hydrogen peroxide-induced cell death. We conclude that AntiOxBEN₂ and AntiOxCIN₄ increase ROS levels, which stimulates NRF2 expression and, as a consequence, SOD2 and GSH levels. This highlights that AntiOxBEN₂ and AntiOxCIN₄ can act as prooxidants thereby activating endogenous ROS-protective pathways.

Enhanced mitophagy in bronchial fibroblasts from severe asthmatic patients

Background

Sub-epithelial fibrosis is a characteristic feature of airway remodeling in asthma which correlates with disease severity. Current asthma medications are ineffective in treating fibrosis. In this study, we aimed to investigate the mitochondrial phenotype in fibroblasts isolated from airway biopsies of non-asthmatic and severe asthmatic subjects by examining mitophagy as a mechanism contributing to fibroblast persistence and thereby, fibrosis in severe asthma.

Methods

Bioinformatics analysis of publicly available transcriptomic data was performed to identify the top enriched pathways in asthmatic fibroblasts. Endogenous expression of mitophagy markers in severe asthmatic and non-asthmatic fibroblasts was determined using qRT-PCR, western blot and immunofluorescence. Mitophagy flux was examined by using lysosomal protease inhibitors, E64d and pepstatin A. Mitochondrial membrane potential and metabolic activity were also evaluated using JC-1 assay and MTT assay, respectively.

Results

Bioinformatics analysis revealed the enrichment of Pink/Parkin-mediated mitophagy in asthmatic fibroblasts compared to healthy controls. In severe asthmatic fibroblasts, the differential expression of mitophagy genes, PINK1 and PRKN, was accompanied by the accumulation of PINK1, Parkin and other mitophagy proteins at baseline. The further accumulation of endogenous LC3BII, p62 and PINK1 in the presence of E64d and pepstatin A in severe asthmatic fibroblasts reinforced their enhanced mitophagy flux. Significantly reduced mitochondrial membrane potential and metabolic activity were also demonstrated at baseline confirming the impairment in mitochondrial function in severe asthmatic fibroblasts. Interestingly, these fibroblasts displayed neither an apoptotic nor senescent phenotype but a pro-fibrotic phenotype with an adaptive survival mechanism triggered by increased AMPKα phosphorylation and mitochondrial biogenesis.

Conclusions

Our results demonstrated a role for mitophagy in the pathogenesis of severe asthma where the enhanced turnover of damaged mitochondria may contribute to fibrosis in severe asthma by promoting the persistence and pro-fibrotic phenotype of fibroblasts.

Doxorubicin-induced p53 interferes with mitophagy in cardiac fibroblasts

Anthracyclines are the critical component in a majority of pediatric chemotherapy regimens due to their broad anticancer efficacy. Unfortunately, the vast majority of long-term childhood cancer survivors will develop a chronic health condition caused by their successful treatments and severe cardiac disease is a common life-threatening outcome that is unequivocally linked to previous anthracycline exposure. The intricacies of how anthracyclines such as doxorubicin, damage the heart and initiate a disease process that progresses over multiple decades is not fully understood. One area left largely unstudied is the role of the cardiac fibroblast, a key cell type in cardiac maturation and injury response. In this study, we demonstrate the effect of doxorubicin on cardiac fibroblast function in the presence and absence of the critical DNA damage response protein p53. In wildtype cardiac fibroblasts, doxorubicin-induced damage correlated with decreased proliferation and migration, cell cycle arrest, and a dilated cardiomyopathy gene expression profile. Interestingly, these doxorubicin-induced changes were completely or partially restored in p53-/- cardiac fibroblasts. Moreover, in wildtype cardiac fibroblasts, doxorubicin produced DNA damage and mitochondrial dysfunction, both of which are well-characterized cell stress responses induced by cytotoxic chemotherapy and varied forms of heart injury. A 3-fold increase in p53 (p = 0.004) prevented the completion of mitophagy (p = 0.032) through sequestration of Parkin. Interactions between p53 and Parkin increased in doxorubicin-treated cardiac fibroblasts (p = 0.0003). Finally, Parkin was unable to localize to the mitochondria in wildtype cardiac fibroblasts, but mitochondrial localization was restored in p53-/- cardiac fibroblasts. These findings strongly suggest that cardiac fibroblasts are an important myocardial cell type that merits further study in the context of doxorubicin treatment. A more robust knowledge of the role cardiac fibroblasts play in the development of doxorubicin-induced cardiotoxicity will lead to novel clinical strategies that will improve the quality of life of cancer survivors.

Impaired mitochondrial complex I function as a candidate driver in the biological stress response and a concomitant stress-induced brain metabolic reprogramming in male mice

Mitochondria play a critical role in bioenergetics, enabling stress adaptation, and therefore, are central in biological stress responses and stress-related complex psychopathologies. To investigate the effect of mitochondrial dysfunction on the stress response and the impact on various biological domains linked to the pathobiology of depression, a novel mouse model was created. These mice harbor a gene trap in the first intron of the Ndufs4 gene (Ndufs4GT/GT mice), encoding the NDUFS4 protein, a structural component of complex I (CI), the first enzyme of the mitochondrial electron transport chain. We performed a comprehensive behavioral screening with a broad range of behavioral, physiological, and endocrine markers, high-resolution ex vivo brain imaging, brain immunohistochemistry, and multi-platform targeted mass spectrometry-based metabolomics. Ndufs4GT/GT mice presented with a 25% reduction of CI activity in the hippocampus, resulting in a relatively mild phenotype of reduced body weight, increased physical activity, decreased neurogenesis and neuroinflammation compared to WT littermates. Brain metabolite profiling revealed characteristic biosignatures discriminating Ndufs4GT/GT from WT mice. Specifically, we observed a reversed TCA cycle flux and rewiring of amino acid metabolism in the prefrontal cortex. Next, exposing mice to chronic variable stress (a model for depression-like behavior), we found that Ndufs4GT/GT mice showed altered stress response and coping strategies with a robust stress-associated reprogramming of amino acid metabolism. Our data suggest that impaired mitochondrial CI function is a candidate driver for altered stress reactivity and stress-induced brain metabolic reprogramming. These changes result in unique phenomic and metabolomic signatures distinguishing groups based on their mitochondrial genotype.

Mitochondrial dysfunction, AMPK activation and peroxisomal metabolism: A coherent scenario for non-canonical 3-methylglutaconic acidurias

The occurrence of 3-methylglutaconic aciduria (3-MGA) is a well understood phenomenon in leucine oxidation and ketogenesis disorders (primary 3-MGAs). In contrast, its genesis in non-canonical (secondary) 3-MGAs, a growing-up group of disorders encompassing more than a dozen of inherited metabolic diseases, is a mystery still remaining unresolved for three decades. To puzzle out this anthologic problem of metabolism, three clues were considered: (i) the variety of disorders suggests a common cellular target at the cross-road of metabolic and signaling pathways, (ii) the response to leucine loading test only discriminative for primary but not secondary 3-MGAs suggests these latter are disorders of extramitochondrial HMG-CoA metabolism as also attested by their failure to increase 3-hydroxyisovalerate, a mitochondrial metabolite accumulating only in primary 3-MGAs, (iii) the peroxisome is an extramitochondrial site possessing its own pool and displaying metabolism of HMG-CoA, suggesting its possible involvement in producing extramitochondrial 3-methylglutaconate (3-MG). Following these clues provides a unifying common basis to non-canonical 3-MGAs: constitutive mitochondrial dysfunction induces AMPK activation which, by inhibiting early steps in cholesterol and fatty acid syntheses, pipelines cytoplasmic acetyl-CoA to peroxisomes where a rise in HMG-CoA followed by local dehydration and hydrolysis may lead to 3-MGA yield. Additional contributors are considered, notably for 3-MGAs associated with hyperammonemia, and to a lesser extent in CLPB deficiency. Metabolic and signaling itineraries followed by the proposed scenario are essentially sketched, being provided with compelling evidence from the literature coming in their support.

Sorafenib-Induced Apoptosis in Hepatocellular Carcinoma Is Reversed by SIRT1

Sorafenib is a multi-kinase inhibitor and one of the few systemic treatment options for patients with advanced hepatocellular carcinomas (HCCs). Resistance to sorafenib develops frequently and could be mediated by the nicotinamide adenine dinucleotide (NAD)-dependent deacetylase sirtuin (SIRT)1. We aimed to test whether sorafenib efficacy is influenced by cellular NAD levels and NAD-dependent SIRT1 function. We analyzed sorafenib effects on apoptosis induction, NAD salvage, mitochondrial function, and related signaling pathways in HCC cell lines (HepG2, Hep3B, und HUH7) overexpressing SIRT1 or supplemented with the NAD metabolite nicotinamide mononucleotide (NMN) compared to controls. Treatment of HCC cell lines with sorafenib dose-dependently induced apoptosis and a significant decrease in cellular NAD concentrations. The SIRT1 protein was downregulated in HUH7 cells but not in Hep3B cells. After sorafenib treatment, mitochondrial respiration in permeabilized cells was lower, citrate synthase activity was attenuated, and cellular adenosine triphosphate (ATP) levels were decreased. Concomitant to increased phosphorylation of adenosine monophosphate (AMP)-activated protein kinase (AMPK), sorafenib treatment led to decreased activity of the mechanistic target of rapamycin (mTOR), indicative of energy deprivation. Transient overexpression of SIRT1, as well as NAD repletion by NMN, decreased sorafenib-induced apoptosis. We can, therefore, conclude that sorafenib influences the NAD/SIRT1/AMPK axis. Overexpression of SIRT1 could be an underlying mechanism of resistance to sorafenib treatment in HCC.

Rescue from galactose-induced death of Leigh Syndrome patient cells by pyruvate and NAD

Cell models of mitochondrial complex I (CI) deficiency display activation of glycolysis to compensate for the loss in mitochondrial ATP production. This adaptation can mask other relevant deficiency-induced aberrations in cell physiology. Here we investigated the viability, mitochondrial morphofunction, ROS levels and ATP homeostasis of primary skin fibroblasts from Leigh Syndrome (LS) patients with isolated CI deficiency. These cell lines harbored mutations in nuclear DNA (nDNA)-encoded CI genes (NDUFS7, NDUFS8, NDUFV1) and, to prevent glycolysis upregulation, were cultured in a pyruvate-free medium in which glucose was replaced by galactose. Following optimization of the cell culture protocol, LS fibroblasts died in the galactose medium, whereas control cells did not. LS cell death was dose-dependently inhibited by pyruvate, malate, oxaloacetate, α-ketoglutarate, aspartate, and exogenous NAD⁺ (eNAD), but not by lactate, succinate, α-ketobutyrate, and uridine. Pyruvate and eNAD increased the cellular NAD⁺ content in galactose-treated LS cells to a different extent and co-incubation studies revealed that pyruvate-induced rescue was not primarily mediated by NAD⁺. Functionally, in LS cells glucose-by-galactose replacement increased mitochondrial fragmentation and mass, depolarized the mitochondrial membrane potential (Δψ), increased H₂DCFDA-oxidizing ROS levels, increased mitochondrial ATP generation, and reduced the total cellular ATP content. These aberrations were differentially rescued by pyruvate and eNAD, supporting the conclusion that these compounds rescue galactose-induced LS cell death via different mechanisms. These findings establish a cell-based strategy for intervention testing and enhance our understanding of CI deficiency pathophysiology.

The Spectrum of Neurological and White Matter Changes and Premutation Status Categories of Older Male Carriers of the FMR1 Alleles Are Linked to Genetic (CGG and FMR1 mRNA) and Cellular Stress (AMPK) Markers

The fragile X premutation (PM) allele contains a CGG expansion of 55-200 repeats in the FMR1 gene's promoter. Male PM carriers have an elevated risk of developing neurological and psychiatric changes, including an approximately 50% risk of the fragile X-associated tremor/ataxia syndrome (FXTAS). The aim of this study was to assess the relationships of regional white matter hyperintensities (wmhs) semi-quantitative scores, clinical status, motor (UPDRS, ICARS, Tremor) scales, and cognitive impairments, with FMR1-specific genetic changes, in a sample of 32 unselected male PM carriers aged 39-81 years. Half of these individuals were affected with FXTAS, while the non-FXTAS group comprised subcategories of non-affected individuals and individuals affected with non-syndromic changes. The dynamics of pathological processes at the cellular level relevant to the clinical status of PM carriers was investigated using the enzyme AMP-activated protein kinase (AMPK), which is a highly sensitive cellular stress-sensing alarm protein. This enzyme, as well as genetic markers - CGG repeat number and the levels of the FMR1 mRNA - were assessed in blood lymphoblasts. The results showed that the repeat distribution for FXTAS individuals peaked at 85-90 CGGs; non-FXTAS carriers were distributed within the lowest end of the PM repeat range, and non-syndromic carriers assumed an intermediate position. The size of the CGG expansion was significantly correlated, across all three categories, with infratentorial and total wmhs and with all motor scores, and the FMR1 mRNA levels with all the wmh scores, whilst AMPK activity showed considerable elevation in the non-FXTAS combined group, decreasing in the FXTAS group, proportionally to increasing severity of the wmhs and tremor/ataxia. We conclude that the size of the CGG expansion relates to the risk for FXTAS, to severity of infratentorial wmhs lesions, and to all three motor scale scores. FMR1 mRNA shows a strong association with the extent of wmhs, which is the most sensitive marker of the pathological process. However, the AMPK activity findings - suggestive of a role of this enzyme in the risk of FXTAS - need to be verified and expanded in future studies using larger samples and longitudinal assessment.

The Mitochondrial Isoform of FASTK Modulates Nonopsonic Phagocytosis of Bacteria by Macrophages via Regulation of Respiratory Complex I

Phagocytosis is a pivotal process by which innate immune cells eliminate bacteria. In this study, we explore novel regulatory mechanisms of phagocytosis driven by the mitochondria. Fas-activated serine/threonine kinase (FASTK) is an RNA-binding protein with two isoforms, one localized to the mitochondria (mitoFASTK) and the other isoform to cytosol and nucleus. The mitoFASTK isoform has been reported to be necessary for the biogenesis of the mitochondrial ND6 mRNA, which encodes an essential subunit of mitochondrial respiratory complex I (CI, NADH:ubiquinone oxidoreductase). This study investigates the role and the mechanisms of action of FASTK in phagocytosis. Macrophages from FASTK^─/─ mice exhibited a marked increase in nonopsonic phagocytosis of bacteria. As expected, CI activity was specifically reduced by almost 50% in those cells. To explore if decreased CI activity could underlie the phagocytic phenotype, we tested the effect of CI inhibition on phagocytosis. Indeed, treatment with CI inhibitor rotenone or short hairpin RNAs against two CI subunits (NDUFS3 and NDUFS4) resulted in a marked increase in nonopsonic phagocytosis of bacteria. Importantly, re-expression of mitoFASTK in FASTK-depleted macrophages was sufficient to rescue the phagocytic phenotype. In addition, we also report that the decrease in CI activity in FASTK^─/─ macrophages is associated with an increase in phosphorylation of the energy sensor AMP-activated protein kinase (AMPK) and that its inhibition using Compound C reverted the phagocytosis phenotype. Taken together, our results clearly demonstrate for the first time, to our knowledge, that mitoFASTK plays a negative regulatory role on nonopsonic phagocytosis of bacteria in macrophages through its action on CI activity.

Cell-Based Methods for Determination of Efficacy for Candidate Therapeutics in the Clinical Management of Cancer

Determination of therapeutic efficacy is a major challenge in developing treatment options for cancer. Prior to in vivo studies, candidate therapeutics are evaluated using cell-based in vitro methods to assess their anti-cancer potential. This review describes the utility and limitations of evaluating therapeutic efficacy using human tumor-derived cell lines. Indicators for therapeutic efficacy using tumor-derived cell lines include cell viability, cell proliferation, colony formation, cytotoxicity, cytostasis, induction of apoptosis, and cell cycle arrest. Cell panel screens, 3D tumor spheroid models, drug-drug/drug-radiation combinatorial analysis, and invasion/migration assays reveal analogous in vitro information. In animal models, cellular assays can assess tumor micro-environment and therapeutic delivery. The utility of tumor-derived cell lines for efficacy determination is manifest in numerous commercially approved drugs that have been applied in clinical management of cancer. Studies reveal most tumor-derived cell lines preserve the genomic signature of the primary tumor source and cell line-based data is highly predictive of subsequent clinical studies. However, cell-based data often disregards natural system components, resulting in cell autonomous outcomes. While 3D cell culture platforms can counter such limitations, they require additional time and cost. Despite the limitations, cell-based methods remain essential in early stages of anti-cancer drug development.

NMR metabolomics of fibroblasts with inherited mitochondrial Complex I mutation reveals treatment-reversible lipid and amino acid metabolism alterations

INTRODUCTION

Elucidating molecular alterations due to mitochondrial Complex I (CI) mutations may help to understand CI deficiency (CID), not only in mitochondriopathies but also as it is caused by drugs or associated to many diseases.

OBJECTIVES

CID metabolic expression was investigated in Leber's hereditary optic neuropathy (LHON) caused by an inherited mutation of CI.

METHODS

NMR-based metabolomics analysis was performed in intact skin fibroblasts from LHON patients. It used several datasets: one-dimensional ¹H-NMR spectra, two-dimensional ¹H-NMR spectra and quantified metabolites. Spectra were analysed using orthogonal partial least squares-discriminant analysis (OPLS-DA), and quantified metabolites using univariate statistics. The response to idebenone (IDE) and resveratrol (RSV), two agents improving CI activity and mitochondrial functions was evaluated.

RESULTS

LHON fibroblasts had decreased CI activity (- 43%, p < 0.01). Metabolomics revealed prominent alterations in LHON including the increase of fatty acids (FA), polyunsaturated FA and phosphatidylcholine with a variable importance in the prediction (VIP) > 1 in OPLS-DA, p < 0.01 in univariate statistics, and the decrease of amino acids (AA), predominantly glycine, glutamate, glutamine (VIP > 1) and alanine (VIP > 1, p < 0.05). In LHON, treatment with IDE and RSV increased CI activity (+ 40 and + 44%, p < 0.05). IDE decreased FA, polyunsaturated FA and phosphatidylcholine (p < 0.05), but did not modified AA levels. RSV decreased polyunsaturated FA, and increased several AA (VIP > 1 and/or p < 0.05).

CONCLUSION

LHON fibroblasts display lipid and amino acid metabolism alterations that are reversed by mitochondria-targeted treatments, and can be related to adaptive changes. Findings bring insights into molecular changes induced by CI mutation and, beyond, CID of other origins.

Extracellular acidification induces ROS- and mPTP-mediated death in HEK293 cells

The extracellular pH (pHe) is a key determinant of the cellular (micro)environment and needs to be maintained within strict boundaries to allow normal cell function. Here we used HEK293 cells to study the effects of pHe acidification (24 h), induced by mitochondrial inhibitors (rotenone, antimycin A) and/or extracellular HCl addition. Lowering pHe from 7.2 to 5.8 reduced cell viability by 70% and was paralleled by a decrease in cytosolic pH (pHc), hyperpolarization of the mitochondrial membrane potential (Δψ), increased levels of hydroethidine-oxidizing ROS and stimulation of protein carbonylation. Co-treatment with the antioxidant α-tocopherol, the mitochondrial permeability transition pore (mPTP) desensitizer cyclosporin A and Necrostatin-1, a combined inhibitor of Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and Indoleamine 2,3-dioxygenase (IDO), prevented acidification-induced cell death. In contrast, the caspase inhibitor zVAD.fmk and the ferroptosis inhibitor Ferrostatin-1 were ineffective. We conclude that extracellular acidification induces necroptotic cell death in HEK293 cells and that the latter involves intracellular acidification, mitochondrial functional impairment, increased ROS levels, mPTP opening and protein carbonylation. These findings suggest that acidosis of the extracellular environment (as observed in mitochondrial disorders, ischemia, acute inflammation and cancer) can induce cell death via a ROS- and mPTP opening-mediated pathogenic mechanism.

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Extracellular acidification induces mitochondrial dysfunction.

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Extracellular acidification increases intracellular ROS levels.

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Extracellular acidification stimulates protein carbonylation.

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Extracellular acidification induces mPTP opening- and ROS-dependent cell death.

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Acidosis-induced oxidative stress likely contributes to various pathologies.

How AMPK and PKA Interplay to Regulate Mitochondrial Function and Survival in Models of Ischemia and Diabetes

Adenosine monophosphate-activated protein kinase (AMPK) is a conserved, redox-activated master regulator of cell metabolism. In the presence of oxidative stress, AMPK promotes cytoprotection by enhancing the conservation of energy by suppressing protein translation and by stimulating autophagy. AMPK interplays with protein kinase A (PKA) to regulate oxidative stress, mitochondrial function, and cell survival. AMPK and dual-specificity A-kinase anchoring protein 1 (D-AKAP1), a mitochondrial-directed scaffold of PKA, interact to regulate mitochondrial function and oxidative stress in cardiac and endothelial cells. Ischemia and diabetes, a chronic disease that increases the onset of cardiovascular diseases, suppress the cardioprotective effects of AMPK and PKA. Here, we review the molecular mechanisms by which AMPK and D-AKAP1/PKA interplay to regulate mitochondrial function, oxidative stress, and signaling pathways that prime endothelial cells, cardiac cells, and neurons for cytoprotection against oxidative stress. We discuss recent literature showing how temporal dynamics and localization of activated AMPK and PKA holoenzymes play a crucial role in governing cellular bioenergetics and cell survival in models of ischemia, cardiovascular diseases, and diabetes. Finally, we propose therapeutic strategies that tout localized PKA and AMPK signaling to reverse mitochondrial dysfunction, oxidative stress, and death of neurons and cardiac and endothelial cells during ischemia and diabetes.

Mitochondrial disorders in children: toward development of small-molecule treatment strategies

This review presents our current understanding of the pathophysiology and potential treatment strategies with respect to mitochondrial disease in children. We focus on pathologies due to mutations in nuclear DNA‐encoded structural and assembly factors of the mitochondrial oxidative phosphorylation (OXPHOS) system, with a particular emphasis on isolated mitochondrial complex I deficiency. Following a brief introduction into mitochondrial disease and OXPHOS function, an overview is provided of the diagnostic process in children with mitochondrial disorders. This includes the impact of whole‐exome sequencing and relevance of cellular complementation studies. Next, we briefly present how OXPHOS mutations can affect cellular parameters, primarily based on studies in patient‐derived fibroblasts, and how this information can be used for the rational design of small‐molecule treatment strategies. Finally, we discuss clinical trial design and provide an overview of small molecules that are currently being developed for treatment of mitochondrial disease.

[High resolution respirometry in diagnostic of mitochondrial disorders caused by mitochondrial complex I deficiency]

Complex I (CI) deficiency is one of the most common defects in the OXPHOS system; it represents more than 30% cases of mitochondrial diseases. The group is characterized by clinical and genetic heterogeneity and comprise several nosological forms. The most prevalent phenotypes for CI are LHON and Leigh syndrome. In this study we have analyzed skin fibroblasts from 11 patients with mutations in mtDNA, which cause LHON or Leigh-like phenotypes: m.11778 G>A (n=3), m.3460 A>G (n=2), m.3635 G>A (n=1), m.3308 T>G (n=2), m.3472 T>C (n=1) and 2 patients with earlier unknown substitutions m.3945 C>A and m.14441T>C. High-resolution respirometry (HRR) on the Oxygraph-2k instrument ("Oroboros corp.", Austria) was performed for complex analysis of the mitochondrial respiratory function in intact and permeabilized fibroblasts of patients and healthy controls. Flux control rations in intact cells R/E, (R-L)/E (p<0.05) were raised compared to the control. Rates of R, E, L normalized on the CS were statistically varied between patients and controls. In permeabilized fibroblasts we observed differences in CII/E, Rot/E, R/CII, CI/CII (p<0.05) between groups. These data highlight the dysfunction of the OXPHOS system and particularly CI. Increased citrate synthase level and decreased CI/CII ratio indicate compensatory metabolic response to respiratory chain dysfunction. Our results show applicability of HRR in revealing the biochemical abnormalities of complex I in fibroblasts of patients with LHON and Leigh-like syndrome. We also suggest HRR to be a useful method for inspection of other mutations causing complex I deficiency.

Modulation of oxidative phosphorylation and redox homeostasis in mitochondrial NDUFS4 deficiency via mesenchymal stem cells

Background

Disorders of the oxidative phosphorylation (OXPHOS) system represent a large group among the inborn errors of metabolism. The most frequently observed biochemical defect is isolated deficiency of mitochondrial complex I (CI). No effective treatment strategies for CI deficiency are so far available. The purpose of this study was to investigate whether and how mesenchymal stem cells (MSCs) are able to modulate metabolic function in fibroblast cell models of CI deficiency.

Methods

We used human and murine fibroblasts with a defect in the nuclear DNA encoded NDUFS4 subunit of CI. Fibroblasts were co-cultured with MSCs under different stress conditions and intercellular mitochondrial transfer was assessed by flow cytometry and fluorescence microscopy. Reactive oxygen species (ROS) levels were measured using MitoSOX-Red. Protein levels of CI were analysed by blue native polyacrylamide gel electrophoresis (BN-PAGE).

Results

Direct cellular interactions and mitochondrial transfer between MSCs and human as well as mouse fibroblast cell lines were demonstrated. Mitochondrial transfer was visible in 13.2% and 6% of fibroblasts (e.g. fibroblasts containing MSC mitochondria) for human and mouse cell lines, respectively. The transfer rate could be further stimulated via treatment of cells with TNF-α. MSCs effectively lowered cellular ROS production in NDUFS4-deficient fibroblast cell lines (either directly via co-culture or indirectly via incubation of cell lines with cell-free MSC supernatant). However, CI protein expression and activity were not rescued by MSC treatment.

Conclusion

This study demonstrates the interplay between MSCs and fibroblast cell models of isolated CI deficiency including transfer of mitochondria as well as modulation of cellular ROS levels. Further exploration of these cellular interactions might help to develop MSC-based treatment strategies for human CI deficiency.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0601-7) contains supplementary material, which is available to authorized users.

Mitochondrial complex I inhibition triggers a mitophagy-dependent ROS increase leading to necroptosis and ferroptosis in melanoma cells

Inhibition of complex I (CI) of the mitochondrial respiratory chain by BAY 87-2243 (‘BAY') triggers death of BRAF^V600E melanoma cell lines and inhibits in vivo tumor growth. Here we studied the mechanism by which this inhibition induces melanoma cell death. BAY treatment depolarized the mitochondrial membrane potential (Δψ), increased cellular ROS levels, stimulated lipid peroxidation and reduced glutathione levels. These effects were paralleled by increased opening of the mitochondrial permeability transition pore (mPTP) and stimulation of autophagosome formation and mitophagy. BAY-induced cell death was not due to glucose shortage and inhibited by the antioxidant α-tocopherol and the mPTP inhibitor cyclosporin A. Tumor necrosis factor receptor-associated protein 1 (TRAP1) overexpression in BAY-treated cells lowered ROS levels and inhibited mPTP opening and cell death, whereas the latter was potentiated by TRAP1 knockdown. Knockdown of autophagy-related 5 (ATG5) inhibited the BAY-stimulated autophagosome formation, cellular ROS increase and cell death. Knockdown of phosphatase and tensin homolog-induced putative kinase 1 (PINK1) inhibited the BAY-induced Δψ depolarization, mitophagy stimulation, ROS increase and cell death. Dynamin-related protein 1 (Drp1) knockdown induced mitochondrial filamentation and inhibited BAY-induced cell death. The latter was insensitive to the pancaspase inhibitor z-VAD-FMK, but reduced by necroptosis inhibitors (necrostatin-1, necrostatin-1s)) and knockdown of key necroptosis proteins (receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and mixed lineage kinase domain-like (MLKL)). BAY-induced cell death was also reduced by the ferroptosis inhibitor ferrostatin-1 and overexpression of the ferroptosis-inhibiting protein glutathione peroxidase 4 (GPX4). This overexpression also inhibited the BAY-induced ROS increase and lipid peroxidation. Conversely, GPX4 knockdown potentiated BAY-induced cell death. We propose a chain of events in which: (i) CI inhibition induces mPTP opening and Δψ depolarization, that (ii) stimulate autophagosome formation, mitophagy and an associated ROS increase, leading to (iii) activation of combined necroptotic/ferroptotic cell death.

Acute stimulation of glucose influx upon mitoenergetic dysfunction requires LKB1, AMPK, Sirt2 and mTOR-RAPTOR

Mitochondria play a central role in cellular energy production, and their dysfunction can trigger a compensatory increase in glycolytic flux to sustain cellular ATP levels. Here, we studied the mechanism of this homeostatic phenomenon in C2C12 myoblasts. Acute (30 min) mitoenergetic dysfunction induced by the mitochondrial inhibitors piericidin A and antimycin A stimulated Glut1-mediated glucose uptake without altering Glut1 (also known as SLC2A1) mRNA or plasma membrane levels. The serine/threonine liver kinase B1 (LKB1; also known as STK11) and AMP-activated protein kinase (AMPK) played a central role in this stimulation. In contrast, ataxia-telangiectasia mutated (ATM; a potential AMPK kinase) and hydroethidium (HEt)-oxidizing reactive oxygen species (ROS; increased in piericidin-A- and antimycin-A-treated cells) appeared not to be involved in the stimulation of glucose uptake. Treatment with mitochondrial inhibitors increased NAD⁺ and NADH levels (associated with a lower NAD⁺:NADH ratio) but did not affect the level of Glut1 acetylation. Stimulation of glucose uptake was greatly reduced by chemical inhibition of Sirt2 or mTOR-RAPTOR. We propose that mitochondrial dysfunction triggers LKB1-mediated AMPK activation, which stimulates Sirt2 phosphorylation, leading to activation of mTOR-RAPTOR and Glut1-mediated glucose uptake.

Augmentation of glycolytic metabolism by meclizine is indispensable for protection of dorsal root ganglion neurons from hypoxia-induced mitochondrial compromise

To meet energy demands, dorsal root ganglion (DRG) neurons harbor high mitochondrial content, which renders them acutely vulnerable to disruptions of energy homeostasis. While neurons typically rely on mitochondrial energy production and have not been associated with metabolic plasticity, new studies reveal that meclizine, a drug, recently linked to modulations of energy metabolism, protects neurons from insults that disrupt energy homeostasis. We show that meclizine rapidly enhances glycolysis in DRG neurons and that glycolytic metabolism is indispensable for meclizine-exerted protection of DRG neurons from hypoxic stress. We report that supplementation of meclizine during hypoxic exposure prevents ATP depletion, preserves NADPH and glutathione stores, curbs reactive oxygen species (ROS) and attenuates mitochondrial clustering in DRG neurites. Using extracellular flux analyzer, we show that in cultured DRG neurons meclizine mitigates hypoxia-induced loss of mitochondrial respiratory capacity. Respiratory capacity is a measure of mitochondrial fitness and cell ability to meet fluctuating energy demands and therefore, a key determinant of cellular fate. While meclizine is an 'old' drug with long record of clinical use, its ability to modulate energy metabolism has been uncovered only recently. Our findings documenting neuroprotection by meclizine in a setting of hypoxic stress reveal previously unappreciated metabolic plasticity of DRG neurons as well as potential for pharmacological harnessing of the newly discovered metabolic plasticity for protection of peripheral nervous system under mitochondria compromising conditions.

Mitoenergetic Dysfunction Triggers a Rapid Compensatory Increase in Steady-State Glucose Flux

ATP can be produced in the cytosol by glycolytic conversion of glucose (GLC) into pyruvate. The latter can be metabolized into lactate, which is released by the cell, or taken up by mitochondria to fuel ATP production by the tricarboxylic acid cycle and oxidative phosphorylation (OXPHOS) system. Altering the balance between glycolytic and mitochondrial ATP generation is crucial for cell survival during mitoenergetic dysfunction, which is observed in a large variety of human disorders including cancer. To gain insight into the kinetic properties of this adaptive mechanism we determined here how acute (30 min) inhibition of OXPHOS affected cytosolic GLC homeostasis. GLC dynamics were analyzed in single living C2C12 myoblasts expressing the fluorescent biosensor FLII(12)Pglu-700μδ6 (FLII). Following in situ FLII calibration, the kinetic properties of GLC uptake (V1) and GLC consumption (V2) were determined independently and used to construct a minimal mathematical model of cytosolic GLC dynamics. After validating the model, it was applied to quantitatively predict V1 and V2 at steady-state (i.e., when V1 = V2 = Vsteady-state) in the absence and presence of OXPHOS inhibitors. Integrating model predictions with experimental data on lactate production, cell volume, and O2 consumption revealed that glycolysis and mitochondria equally contribute to cellular ATP production in control myoblasts. Inhibition of OXPHOS induced a twofold increase in Vsteady-state and glycolytic ATP production flux. Both in the absence and presence of OXPHOS inhibitors, GLC was consumed at near maximal rates, meaning that GLC consumption is rate-limiting under steady-state conditions. Taken together, we demonstrate here that OXPHOS inhibition increases steady-state GLC uptake and consumption in C2C12 myoblasts. This activation fully compensates for the reduction in mitochondrial ATP production, thereby maintaining the balance between cellular ATP supply and demand.

Human thioredoxin 2 deficiency impairs mitochondrial redox homeostasis and causes early-onset neurodegeneration

Thioredoxin 2 (TXN2; also known as Trx2) is a small mitochondrial redox protein essential for the control of mitochondrial reactive oxygen species homeostasis, apoptosis regulation and cell viability. Exome sequencing in a 16-year-old adolescent suffering from an infantile-onset neurodegenerative disorder with severe cerebellar atrophy, epilepsy, dystonia, optic atrophy, and peripheral neuropathy, uncovered a homozygous stop mutation in TXN2. Analysis of patient-derived fibroblasts demonstrated absence of TXN2 protein, increased reactive oxygen species levels, impaired oxidative stress defence and oxidative phosphorylation dysfunction. Reconstitution of TXN2 expression restored all these parameters, indicating the causal role of TXN2 mutation in disease development. Supplementation with antioxidants effectively suppressed cellular reactive oxygen species production, improved cell viability and mitigated clinical symptoms during short-term follow-up. In conclusion, our report on a patient with TXN2 deficiency suggests an important role of reactive oxygen species homeostasis for human neuronal maintenance and energy metabolism.

Targeting mitochondrial complex I using BAY 87-2243 reduces melanoma tumor growth

Background

Numerous studies have demonstrated that functional mitochondria are required for tumorigenesis, suggesting that mitochondrial oxidative phosphorylation (OXPHOS) might be a potential target for cancer therapy. In this study, we investigated the effects of BAY 87-2243, a small molecule that inhibits the first OXPHOS enzyme (complex I), in melanoma in vitro and in vivo.

Results

BAY 87-2243 decreased mitochondrial oxygen consumption and induced partial depolarization of the mitochondrial membrane potential. This was associated with increased reactive oxygen species (ROS) levels, lowering of total cellular ATP levels, activation of AMP-activated protein kinase (AMPK), and reduced cell viability. The latter was rescued by the antioxidant vitamin E and high extracellular glucose levels (25 mM), indicating the involvement of ROS-induced cell death and a dependence on glycolysis for cell survival upon BAY 87-2243 treatment. BAY 87-2243 significantly reduced tumor growth in various BRAF mutant melanoma mouse xenografts and patient-derived melanoma mouse models. Furthermore, we provide evidence that inhibition of mutated BRAF using the specific small molecule inhibitor vemurafenib increased the OXPHOS dependency of BRAF mutant melanoma cells. As a consequence, the combination of both inhibitors augmented the anti-tumor effect of BAY 87-2243 in a BRAF mutant melanoma mouse xenograft model.

Conclusions

Taken together, our results suggest that complex I inhibition has potential clinical applications as a single agent in melanoma and also might be efficacious in combination with BRAF inhibitors in the treatment of patients with BRAF mutant melanoma.

Electronic supplementary material

The online version of this article (doi:10.1186/s40170-015-0138-0) contains supplementary material, which is available to authorized users.

Redox Homeostasis and Mitochondrial Dynamics

Within living cells, mitochondria are considered relevant sources of reactive oxygen species (ROS) and are exposed to reactive nitrogen species (RNS). During the last decade, accumulating evidence suggests that mitochondrial (dys)function, ROS/RNS levels, and aberrations in mitochondrial morphology are interconnected, albeit in a cell- and context-dependent manner. Here it is hypothesized that ROS and RNS are involved in the short-term regulation of mitochondrial morphology and function via non-transcriptional pathways. We review the evidence for such a mechanism and propose that it allows homeostatic control of mitochondrial function and morphology by redox signaling.