Metabolic adaptation to the chronic loss of Ca2+ signaling induced by KO of IP3 receptors or the mitochondrial Ca2+ uniporter

Calcium signaling is essential for regulating many biological processes. Endoplasmic reticulum inositol trisphosphate receptors (IP₃Rs) and the mitochondrial Ca²⁺ uniporter (MCU) are key proteins that regulate intracellular Ca²⁺ concentration. Mitochondrial Ca²⁺ accumulation activates Ca²⁺-sensitive dehydrogenases of the tricarboxylic acid (TCA) cycle that maintain the biosynthetic and bioenergetic needs of both normal and cancer cells. However, the interplay between calcium signaling and metabolism is not well understood. In this study, we used human cancer cell lines (HEK293 and HeLa) with stable KOs of all three IP₃R isoforms (triple KO [TKO]) or MCU to examine metabolic and bioenergetic responses to the chronic loss of cytosolic and/or mitochondrial Ca²⁺ signaling. Our results show that TKO cells (exhibiting total loss of Ca²⁺ signaling) are viable, displaying a lower proliferation and oxygen consumption rate, with no significant changes in ATP levels, even when made to rely solely on the TCA cycle for energy production. MCU KO cells also maintained normal ATP levels but showed increased proliferation, oxygen consumption, and metabolism of both glucose and glutamine. However, MCU KO cells were unable to maintain ATP levels and died when relying solely on the TCA cycle for energy. We conclude that constitutive Ca²⁺ signaling is dispensable for the bioenergetic needs of both IP₃R TKO and MCU KO human cancer cells, likely because of adequate basal glycolytic and TCA cycle flux. However, in MCU KO cells, the higher energy expenditure associated with increased proliferation and oxygen consumption makes these cells more prone to bioenergetic failure under conditions of metabolic stress.     

Upregulation of autophagy decreases chlorine-induced mitochondrial injury and lung inflammation

The mechanisms of toxicity during exposure of the airways to chlorinated biomolecules generated during the course of inflammation and to chlorine (Cl₂) gas are poorly understood. We hypothesized that lung epithelial cell mitochondria are damaged by Cl₂ exposure and activation of autophagy mitigates this injury. To address this, NCI-H441 (human lung adenocarcinoma epithelial) cells were exposed to Cl₂ (100 ppm/15 min) and bioenergetics were assessed. One hour after Cl₂, cellular bioenergetic function and mitochondrial membrane potential were decreased. These changes were associated with increased MitoSOX signal, and treatment with the mitochondrial redox modulator MitoQ attenuated these bioenergetic defects. At 6 h postexposure, there was significant increase in autophagy, which was associated with an improvement of mitochondrial function. Pretreatment of H441 cells with trehalose (an autophagy activator) improved bioenergetic function, whereas 3-methyladenine (an autophagy inhibitor) resulted in increased bioenergetic dysfunction 1 h after Cl₂ exposure. These data indicate that Cl₂ induces bioenergetic dysfunction, and autophagy plays a protective role in vitro. Addition of trehalose (2 vol%) to the drinking water of C57BL/6 mice for 6 weeks, but not 1 week, before Cl₂ (400 ppm/30 min) decreased white blood cells in the bronchoalveolar lavage fluid at 6 h after Cl₂ by 70%. Acute administration of trehalose delivered through inhalation 24 and 1 h before the exposure decreased alveolar permeability but not cell infiltration. These data indicate that Cl₂ induces bioenergetic dysfunction associated with lung inflammation and suggests that autophagy plays a protective role.     

Fasting induces prominent proteomic changes in liver in very long chain Acyl-CoA dehydrogenase deficient mice

Very long chain acyl-CoA dehydrogenase (VLCAD) deficiency (VLCADD) is a clinically heterogeneous disorder of mitochondrial fatty acid β-oxidation usually identified through newborn screening. Genotype-phenotype correlations have been defined, but considerable clinical heterogeneity still exists. Symptoms are often induced by physiological stress such as fasting or intercurrent illness, setting it as an important example of environmental effects altering clinical course in an individual with a genetic disease. However, neither the cellular changes that predispose to this phenomenon nor the alterations it induces are well characterized. We examined the effects of fasting in a knockout mouse model to explore changes in global mitochondria protein profiles in liver and to investigate the physiologically relevant changes that lead to the clinical presentations. An isobaric tags for relative and absolute quantification (iTRAQ) labeling approach was employed to examine mitochondrial proteome changes in VLCAD deficient compared to wild type mice in the fed and fasted states. We identified numerous proteomic changes associated with the gene defect and fasting within relevant metabolic pathways. Few changes induced by fasting were shared between the VLCAD deficient and wild type mice, with more alterations found in the deficient mice on fasting. Particularly, fasting in the deficient mice could reverse the protective response in oxidative phosphorylation pathway seen in wild type animals. In addition, we found that changes in chaperone proteins including heat shock protein 60 (HSP60) and 10 (HSP10) during fasting differed between the two genotypes, highlighting the importance of these proteins in VLCAD deficiency. Finally, the effects on the liver proteome imposed by changes in fasted VLCAD deficient mice indicates that this environmental factor may be an inducer of both cellular and physiological changes.     

The low conductance mitochondrial permeability transition pore confers excitability and CICR wave propagation in a computational model

Mitochondria have long been known to sequester cytosolic Ca²⁺ and even to shape intracellular patterns of endoplasmic reticulum-based Ca²⁺ signaling. Evidence suggests that the mitochondrial network is an excitable medium which can demonstrate independent Ca²⁺ induced Ca²⁺ release via the mitochondrial permeability transition. The role of this excitability remains unclear, but mitochondrial Ca²⁺ handling appears to be a crucial element in diverse diseases as diabetes, neurodegeneration and cardiac dysfunction that also have bioenergetic components. In this paper, we extend the modular Magnus-Keizer computational model for respiration-driven Ca²⁺ handling to include a permeability transition based on a channel-like pore mechanism. We demonstrate both excitability and Ca²⁺ wave propagation accompanied by depolarizations qualitatively similar to those reported in cell and isolated mitochondria preparations. These waves depend on the energy state of the mitochondria, as well as other elements of mitochondrial physiology. Our results support the concept that mitochondria can transmit state dependent signals about their function across the mitochondrial network. Our model provides the tools for predictions about the internal physiology that leads to this qualitatively different Ca²⁺ excitability seen in mitochondria.     

Mechanism and regulation of the mitochondrial ATP-Mg/Pi carrier

The mitochondrial ATP-Mg/P_i carrier functions to modulate the matrix adenine nucleotide pool size (ATP + ADP + AMP). Micromolar Ca²⁺ is required to activate the carrier. Net adenine nucleotide transport occurs as an electroneutral divalent exchange of ATP-Mg^2– for HPO ₄ ^2– . A steady-state adenine nucleotide pool size is attained when the HPO ₄ ^2– and ATP-Mg^2– matrix/cytoplasm concentration ratios are the same. This means that ATP-Mg^2– can be accumulated against a concentration gradient in proportion to the [HPO ₄ ^2– ] gradient that is normally maintained by the P_i/OH^– carrier. In liver, changes in matrix adenine nucleotide concentrations that are brought about by the ATP-Mg/P_i carrier can affect the activity of adenine nucleotide-dependent enzymes that are in the mitochondrial compartment. These enzymes in turn contribute to the overall regulation of bioenergetic function, flux through the gluconeogenesis and urea synthesis pathways, and organelle biogenesis. The ATP-Mg/P_i carrier is distinct from other mitochondrial transport systems with respect to kinetics and to substrate and inhibitor sensitivity. It is the only carrier regulated by Ca²⁺. This carrier is present in kidney and liver mitochondria, but not in heart.     

Nuclear-encoded NCX3 and AKAP121: Two novel modulators of mitochondrial calcium efflux in normoxic and hypoxic neurons

Mitochondria are highly dynamic organelles extremely important for cell survival. Their structure resembles that of prokaryotic cells since they are composed with two membranes, the inner (IMM) and the outer mitochondrial membrane (OMM) delimitating the intermembrane space (IMS) and the matrix which contains mitochondrial DNA (mtDNA). This structure is strictly related to mitochondrial function since they produce the most of the cellular ATP through the oxidative phosphorylation which generate the electrochemical gradient at the two sides of the inner mitochondrial membrane an essential requirement for mitochondrial function. Cells of highly metabolic demand like those composing muscle, liver and brain, are particularly dependent on mitochondria for their activities. Mitochondria undergo to continual changes in morphology since, they fuse and divide, branch and fragment, swell and extend. Importantly, they move throughout the cell to deliver ATP and other metabolites where they are mostly required. Along with the capability to control energy metabolism, mitochondria play a critical role in the regulation of many physiological processes such as programmed cell death, autophagy, redox signalling, and stem cells reprogramming. All these phenomena are regulated by Ca²⁺ ions within this organelle. This review will discuss the molecular mechanisms regulating mitochondrial calcium cycling in physiological and pathological conditions with particular regard to their impact on mitochondrial dynamics and function during ischemia. Particular emphasis will be devoted to the role played by NCX3 and AKAP121 as new molecular targets for mitochondrial function and dysfunction.     

Effects of a high fat diet on liver mitochondria: increased ATP-sensitive K<Superscript>+</Superscript> channel activity and reactive oxygen species generation

High fat diets are extensively associated with health complications within the spectrum of the metabolic syndrome. Some of the most prevalent of these pathologies, often observed early in the development of high-fat dietary complications, are non-alcoholic fatty liver diseases. Mitochondrial bioenergetics and redox state changes are also widely associated with alterations within the metabolic syndrome. We investigated the mitochondrial effects of a high fat diet leading to non-alcoholic fatty liver disease in mice. We found that the diet does not substantially alter respiratory rates, ADP/O ratios or membrane potentials of isolated liver mitochondria. However, H₂O₂ release using different substrates and ATP-sensitive K⁺ transport activities are increased in mitochondria from animals on high fat diets. The increase in H₂O₂ release rates was observed with different respiratory substrates and was not altered by modulators of mitochondrial ATP-sensitive K⁺ channels, indicating it was not related to an observed increase in K⁺ transport. Altogether, we demonstrate that mitochondria from animals with diet-induced steatosis do not present significant bioenergetic changes, but display altered ion transport and increased oxidant generation. This is the first evidence, to our knowledge, that ATP-sensitive K⁺ transport in mitochondria can be modulated by diet.     

Shift in brain metabolism in late onset Alzheimer's disease: implications for biomarkers and therapeutic interventions

Alzheimer's is a neurodegenerative disease with a complex and progressive pathological phenotype characterized first by hypometabolism and impaired mitochondrial bioenergetics followed by pathological burden. Increasing evidence indicates an antecedent and potentially causal role of mitochondrial bioenergetic deficits and brain hypometabolism coupled with increased mitochondrial oxidative stress in AD pathogenesis. Compromised mitochondrial bioenergetics lead to over-production of and mitochondrial accumulation of β-amyloid, which is coupled with oxidative stress. Collectively, this results in a shift in brain metabolic profile from glucose-driven bioenergetics towards a compensatory, but less efficient, ketogenic pathway. We propose that the compensatory shift from a primarily aerobic glycolysis pathway to a ketogenic/fatty acid β-oxidation pathway eventually leads to white matter degeneration. The essential role of mitochondrial bioenergetics and the unique trajectory of compensatory metabolic adaptations in brain enable a bioenergetic-centric strategy for development of biomarkers. From a therapeutic perspective, this trajectory of alterations in brain metabolic capacity enables disease-stage specific strategies to target brain metabolism for disease prevention and treatment. A combination of nutraceutical and pharmaceutical interventions that enhance glucose-driven metabolic activity and potentiate mitochondrial bioenergetic function could prevent the antecedent decline in brain glucose metabolism, promote healthy aging and prevent AD. Alternatively, during the prodromal incipient phase of AD, sustained activation of ketogenic metabolic pathways coupled with supplementation of the alternative fuel source, ketone bodies, could sustain mitochondrial bioenergetic function to prevent or delay further progression of the disease.     

Bioenergetic parameters of lamprey and frog liver mitochondria during metabolic depression and activity

The objective of this study is to elucidate the role of mitochondria in reversible metabolic depression of hepatocytes of the Baltic lamprey (Lampetra fluviatilis) taking place in the last year of its life cycle and to compare their main bioenergetic parameters with those of the frog (Rana temporaria) and the white outbred mouse (Mus musculus). Using isolated mitochondria as a model, we have revealed significant seasonal variations in the main bioenergetic parameters of the lamprey liver. These changes indicate that the metabolic depression is mediated by prolonged reversible alterations of mitochondrial functions, which manifest in low activity of the mitochondrial respiratory chain, low oxidative phosphorylation, low content of mitochondrial adenine nucleotides, high level of reduced mitochondrial pyridine nucleotides and leaky mitochondrial membranes observed in winter. The enhanced ion membrane permeability of winter lamprey liver mitochondria is found to be sensitive to EGTA and to cyclosporine A in combination with ADP and Mg²⁺ and is likely mediated opening the mitochondrial permeability transition pore in its low conductance state. The sharp activation of oxidation and phosphorylation in the lamprey liver mitochondria followed by spawning and death of the animal is observed in spring. The possible causes of the phenomenon and the differences obtained between lamprey, frog and mouse are under discussion.     

Decline in mitochondrial bioenergetics and shift to ketogenic profile in brain during reproductive senescence

Background

We have previously demonstrated that mitochondrial bioenergetic deficits precede Alzheimer's pathology in the female triple transgenic Alzheimer's (3xTgAD) mouse model. Herein, we sought to determine the impact of reproductive senescence on mitochondrial function in the normal non-transgenic (nonTg) and 3xTgAD female mouse model of AD.

Methods

Both nonTg and 3xTgAD female mice at 3, 6, 9, and 12 months of age were sacrificed and mitochondrial bioenergetic profile as well as oxidative stress markers were analyzed.

Results

In both nonTg and 3xTgAD mice, reproductive senescence paralleled a significant decline in PDH, and Complex IV cytochrome c oxidase activity and mitochondrial respiration. During the reproductive senescence transition, both nonTg and 3xTgAD mice exhibited greater individual variability in bioenergetic parameters suggestive of divergent bioenergetic phenotypes. Following transition through reproductive senescence, enzymes required for long-chain fatty acid (HADHA) and ketone body (SCOT) metabolism were significantly increased and variability in cytochrome c oxidase (Complex IV) collapsed to cluster at a ∼ 40% decline in both the nonTg and 3xTgAD brain which was indicative of alternative fuel generation with concomitant decline in ATP generation.

Conclusions

These data indicate that reproductive senescence in the normal nonTg female brain parallels the shift to ketogenic/fatty acid substrate phenotype with concomitant decline in mitochondrial function and exacerbation of bioenergetic deficits in the 3xTgAD brain.

General significance

These findings provide a plausible mechanism for increased life-time risk of AD in postmenopausal women and suggest an optimal window of opportunity to prevent or delay decline in bioenergetics during reproductive senescence.     

Mutation of mitochondrial ATP8 gene improves hepatic energy status in a murine model of acute endotoxemic liver failure

AimsMitochondria not only generate and modulate bioenergy but also serve as biosensors for oxidative stress, and eventually become effector organelles for cell viability. Therefore, the implications of mitochondrial (dys)function in the development of multiple organ failure are profound. We investigated whether a mutation in the ATPase subunit-8 gene affects the course of endotoxemic acute liver failure.Main methodsC57BL/6J (ATP8 wild type) and C57BL/6J-mt^FVB/N (ATP8 mutant) mice were challenged with d-galactosamine (GalN) and Escherichia coli lipopolysaccharide (LPS) for induction of acute liver failure, and studied 6 h thereafter. Control mice received physiological saline only. Analysis included in vivo fluorescence microscopy of hepatic microcirculation and levels of hepatocellular apoptosis, hepatic adenosine nucleotides and oxidative stress. Additionally, survival rates were assessed.Key findingsInduction of endotoxemic liver failure provoked marked liver damage, which was coexistent with a drop of total adenosine nucleotide levels and increased oxidative stress. Of interest, oxidative stress was higher in the GalN/LPS challenged ATP8 mutants compared to wild types. Concomitantly, adenosine triphosphate (ATP) levels in livers of mice carrying the ATP8 mutation remained higher than those in wild type mice. As net result, ATP8 mutants showed lower transaminase release and a tendency to better survival rate upon GalN/LPS exposure compared to wild types.SignificanceOur findings demonstrate that mutation in the ATPase subunit-8 partially protects mice against endotoxemic stress, most probably due to better hepatic energy status despite elevated oxidative stress. Thus, modulating mitochondrial function to preserve bioenergetic status may be an effective strategy to protect against sepsis-induced multiorgan dysfunction.     

Impaired mitochondrial oxidative phosphorylation in skeletal muscle of the dystrophin-deficient mdx mouse

The mdx mouse, an animal model of the Duchenne muscular dystrophy, was used for the investigation of changes in mitochondrial function associated with dystrophin deficiency. Enzymatic analysis of skeletal muscle showed an approximately 50% decrease in the activity of all respiratory chain-linked enzymes in musculus quadriceps of adult mdx mice as compared with controls, while in cardiac muscle no difference was observed. The activities of cytosolic and mitochondrial matrix enzymes were not significantly different from the control values in both cardiac and skeletal muscles. In saponin-permeabilized skeletal muscle fibers of mdx mice the maximal rates of mitochondrial respiration were about two times lower than those of controls. These changes were also demonstrated on the level of isolated mitochondria. Mdx muscle mitochondria had only 60% of maximal respiration activities of control mice skeletal muscle mitochondria and contained only about 60% of hemoproteins of mitochondrial inner membrane. Similar findings were observed in a skeletal muscle biopsy of a Duchenne muscular dystrophy patient. These data strongly suggest that a specific decrease in the amount of all mitochondrial inner membrane enzymes, most probably as result of Ca²⁺ overload of muscle fibers, is the reason for the bioenergetic deficits in dystrophin-deficient skeletal muscle.     

Mfn2 localization in the ER is necessary for its bioenergetic function and neuritic development

Mfn2 is a mitochondrial fusion protein with bioenergetic functions implicated in the pathophysiology of neuronal and metabolic disorders. Understanding the bioenergetic mechanism of Mfn2 may aid in designing therapeutic approaches for these disorders. Here we show using endoplasmic reticulum (ER) or mitochondria‐targeted Mfn2 that Mfn2 stimulation of the mitochondrial metabolism requires its localization in the ER, which is independent of its fusion function. ER‐located Mfn2 interacts with mitochondrial Mfn1/2 to tether the ER and mitochondria together, allowing Ca²⁺ transfer from the ER to mitochondria to enhance mitochondrial bioenergetics. The physiological relevance of these findings is shown during neurite outgrowth, when there is an increase in Mfn2‐dependent ER‐mitochondria contact that is necessary for correct neuronal arbor growth. Reduced neuritic growth in Mfn2 KO neurons is recovered by the expression of ER‐targeted Mfn2 or an artificial ER‐mitochondria tether, indicating that manipulation of ER‐mitochondria contacts could be used to treat pathologic conditions involving Mfn2.     

Differentiation of SH-SY5Y cells to a neuronal phenotype changes cellular bioenergetics and the response to oxidative stress

Cell differentiation is associated with changes in metabolism and function. Understanding these changes during differentiation is important in the context of stem cell research, cancer, and neurodegenerative diseases. An early event in neurodegenerative diseases is the alteration of mitochondrial function and increased oxidative stress. Studies using both undifferentiated and differentiated SH-SY5Y neuroblastoma cells have shown distinct responses to cellular stressors; however, the mechanisms remain unclear. We hypothesized that because the regulation of glycolysis and oxidative phosphorylation is modulated during cellular differentiation, this would change bioenergetic function and the response to oxidative stress. To test this, we used retinoic acid (RA) to induce differentiation of SH-SY5Y cells and assessed changes in cellular bioenergetics using extracellular flux analysis. After exposure to RA, the SH-SY5Y cells had an increased mitochondrial membrane potential, without changing mitochondrial number. Differentiated cells exhibited greater stimulation of mitochondrial respiration with uncoupling and an increased bioenergetic reserve capacity. The increased reserve capacity in the differentiated cells was suppressed by the inhibitor of glycolysis 2-deoxy-d-glucose. Furthermore, we found that differentiated cells were substantially more resistant to cytotoxicity and mitochondrial dysfunction induced by the reactive lipid species 4-hydroxynonenal or the reactive oxygen species generator 2,3-dimethoxy-1,4-naphthoquinone. We then analyzed the levels of selected mitochondrial proteins and found an increase in complex IV subunits, which we propose contributes to the increase in reserve capacity in the differentiated cells. Furthermore, we found an increase in MnSOD that could, at least in part, account for the increased resistance to oxidative stress. Our findings suggest that profound changes in mitochondrial metabolism and antioxidant defenses occur upon differentiation of neuroblastoma cells to a neuron-like phenotype.     

Mitochondrial dysfunction in brain aging: role of oxidative stress and cardiolipin

Aging is a biological process characterized by impairment of cellular bioenergetic function, increased oxidative stress, attenuated ability to respond to stresses, increased risk of contracting age-associated disorders that affects many tissues, with a more marked effect on brain and heart function. Oxidative stress is widely thought to underpin many aging processes. The mitochondrion is considered the most important cellular organelle to contribute to the aging process, mainly through respiratory chain dysfunction and formation of reactive oxygen species, leading to damage to mitochondrial proteins, lipids and mitochondrial DNA. Furthermore, exposure to oxidants, especially in the presence of Ca(2+), can induce the mitochondrial permeability transition with deleterious effects on mitochondrial function. Cardiolipin plays a central role in several mitochondrial bioenergetic processes as well as in mitochondrial-dependent steps in apoptosis and mitochondrial membrane stability and dynamics. Alterations to cardiolipin structure, content and acyl chain profile have been associated with mitochondrial dysfunction in multiple tissues in several physiopathological conditions and aging. In this review, we focus on the role played by oxidative stress and cardiolipin in mitochondrial bioenergetic alterations associated with brain aging.     

Roles of sirtuins in the regulation of antioxidant defense and bioenergetic function of mitochondria under oxidative stress

Abstract

In addition to serving as the power house of mammalian cells, mitochondria are crucial for the maintenance of cellular homeostasis in response to physiological or environmental changes. Several lines of evidence suggest that posttranslational modification (PTM) of proteins plays a pivotal role in the regulation of the bioenergetic function of mitochondria. Among them, reversible lysine acetylation of mitochondrial proteins has been established as one of the key mechanisms in cellular response to energy demand by modulating the flux of a number of key metabolic pathways. In this article, we focus on the role of Sirt3-mediated deacetylation in: (1) flexibility of energy metabolism, (2) activation of antioxidant defense, and (3) maintenance of cellular redox status in response to dietary challenge and oxidative stress. We suggest that oxidative stress-elicited down-regulation of Sirt3 plays a role in the pathophysiology of diabetes, cardiac hypotrophy, mitochondrial diseases, and age-related diseases. Besides, the physiological role of newly identified lysine acylation mediated by Sirt5 and its biochemical effects on oxidative metabolism are also discussed. Moreover, we have integrated the regulatory function of several protein kinases that are involved in the phosphorylation of mitochondrial enzymes during oxidative stress. Finally, the functional consequence of the synergistic regulation through diverse protein modifications is emphasized on the maintenance of the bioenergetic homeostasis and metabolic adaptation of the animal and human cells. Together, we have provided an updated review of PTM in mitochondrial biology and their implications in aging and human diseases through an intricate regulation of energy metabolism under oxidative stress.     

Regulation of vascular smooth muscle cell bioenergetic function by protein glutathiolation

Protein thiolation by glutathione is a reversible and regulated post-translational modification that is increased in response to oxidants and nitric oxide. Because many mitochondrial enzymes contain critical thiol residues, it has been hypothesized that thiolation reactions regulate cell metabolism and survival. However, it has been difficult to differentiate the biological effects due to protein thiolation from other oxidative protein modifications. In this study, we used diamide to titrate protein glutathiolation and examined its impact on glycolysis, mitochondrial function, and cell death in rat aortic smooth muscle cells. Treatment of cells with diamide increased protein glutathiolation in a concentration-dependent manner and had comparably little effect on protein-protein disulfide formation. Diamide increased mitochondrial proton leak and decreased ATP-linked mitochondrial oxygen consumption and cellular bioenergetic reserve capacity. Concentrations of diamide above 200 μM promoted acute bioenergetic failure and caused cell death, whereas lower concentrations of diamide led to a prolonged increase in glycolytic flux and were not associated with loss of cell viability. Depletion of glutathione using buthionine sulfoximine had no effect on basal protein thiolation or cellular bioenergetics but decreased diamide-induced protein glutathiolation and sensitized the cells to bioenergetic dysfunction and death. The effects of diamide on cell metabolism and viability were fully reversible upon addition of dithiothreitol. These data suggest that protein thiolation modulates key metabolic processes in both the mitochondria and cytosol.     

The antiestrogen 4-hydroxytamoxifen protects against isotretinoin-induced permeability transition and bioenergetic dysfunction of liver mitochondria: comparison with tamoxifen

The combination of isotretinoin (13-cis-retinoic acid) with antiestrogens seems to be a promising strategy for cancer chemotherapy. The aim of the study was to evaluate the effects of isotretinoin alone or in combination with 4-hydroxytamoxifen (OHTAM) and with its prodrug tamoxifen (TAM), on the functions of rat liver mitochondria, i.e., mitochondrial permeability transition (MPT), bioenergetic functions and adenine nucleotide translocase (ANT). Isotretinoin (5 nmol/mg protein) induced the Ca²⁺-dependent MPT pore opening in mitochondria energized with succinate, which was prevented by OHTAM, cyclosporine A, TAM and ANT ligands. When mitochondria were energized with glutamate/malate and in the absence of added Ca²⁺ isotretinoin decreased the state 3 respiration, the ATP levels, the active ANT content and increased the lag phase of the phosphorylation cycle, demonstrating that isotretinoin decreased the mitochondrial phosphorylation efficiency. These changes of isotretinoin in bioenergetic parameters were not significant in the presence of succinate. The effects of isotretinoin at 5 nmol/mg protein on the Ca²⁺-dependent MPT and phosphorylative efficacy may be related with interactions with the ANT. Above 10 nmol/mg protein isotretinoin strongly diminished the active ANT content, decreased the Δψ, inhibited the complex I and induced proton leak through the Fo fraction of complex V. The combination of OHTAM with isotretinoin only induced significant changes in the energy production systems at concentrations ≥5 nmol isotretinoin/mg protein. Therefore, our results suggest that isotretinoin-associated liver toxicity is possibly related with mitochondrial dysfunctions and that the combination with OHTAM may contribute to decrease its toxicity.     

The role of mitochondrial electron transport in tumorigenesis and metastasis

Background

Tumor formation and spread via the circulatory and lymphatic drainage systems is associated with metabolic reprogramming that often includes increased glycolytic metabolism relative to mitochondrial energy production. However, cells within a tumor are not identical due to genetic change, clonal evolution and layers of epigenetic reprogramming. In addition, cell hierarchy impinges on metabolic status while tumor cell phenotype and metabolic status will be influenced by the local microenvironment including stromal cells, developing blood and lymphatic vessels and innate and adaptive immune cells. Mitochondrial mutations and changes in mitochondrial electron transport contribute to metabolic remodeling in cancer in ways that are poorly understood.

Scope of Review

This review concerns the role of mitochondria, mitochondrial mutations and mitochondrial electron transport function in tumorigenesis and metastasis.

Major Conclusions

It is concluded that mitochondrial electron transport is required for tumor initiation, growth and metastasis. Nevertheless, defects in mitochondrial electron transport that compromise mitochondrial energy metabolism can contribute to tumor formation and spread. These apparently contradictory phenomena can be reconciled by cells in individual tumors in a particular environment adapting dynamically to optimally balance mitochondrial genome changes and bioenergetic status.

General Significance

Tumors are complex evolving biological systems characterized by genetic and adaptive epigenetic changes. Understanding the complexity of these changes in terms of bioenergetics and metabolic changes will permit the development of better combination anticancer therapies. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.