Spontaneous nonsense mutation in the tuftelin 1 gene is associated with abnormal hair appearance and amelioration of glucose and lipid metabolism in the rat

Recently, we have identified a recessive mutation, an abnormal coat appearance in the BXH6 strain, a member of the HXB/BXH set of recombinant inbred (RI) strains. The RI strains were derived from the spontaneously hypertensive rat (SHR) and Brown Norway rat (BN-Lx) progenitors. Whole genome sequencing of the mutant rats identified the 195875980 G/A mutation in the tuftelin 1 (Tuft1) gene on chromosome 2, which resulted in a premature stop codon. Compared with wild-type BXH6 rats, BXH6-Tuft1 mutant rats exhibited lower body weight due to reduced visceral fat and ectopic fat accumulation in the liver and heart. Reduced adiposity was associated with decreased serum glucose and insulin and increased insulin-stimulated glycogenesis in skeletal muscle. In addition, mutant rats had lower serum monocyte chemoattractant protein-1 and leptin levels, indicative of reduced inflammation. Analysis of the liver proteome identified differentially expressed proteins from fatty acid metabolism and β-oxidation, peroxisomes, carbohydrate metabolism, inflammation, and proteasome pathways. These results provide evidence for the important role of the Tuft1 gene in the regulation of lipid and glucose metabolism and suggest underlying molecular mechanisms.NEW & NOTEWORTHY A new spontaneous mutation, abnormal hair appearance in the rat, has been identified as a nonfunctional tuftelin 1 (Tuft1) gene. The pleiotropic effects of this mutation regulate glucose and lipid metabolism. Analysis of the liver proteome revealed possible molecular mechanisms for the metabolic effects of the Tuft1 gene.

Loss of COX4I1 Leads to Combined Respiratory Chain Deficiency and Impaired Mitochondrial Protein Synthesis

The oxidative phosphorylation (OXPHOS) system localized in the inner mitochondrial membrane secures production of the majority of ATP in mammalian organisms. Individual OXPHOS complexes form supramolecular assemblies termed supercomplexes. The complexes are linked not only by their function but also by interdependency of individual complex biogenesis or maintenance. For instance, cytochrome c oxidase (cIV) or cytochrome bc1 complex (cIII) deficiencies affect the level of fully assembled NADH dehydrogenase (cI) in monomeric as well as supercomplex forms. It was hypothesized that cI is affected at the level of enzyme assembly as well as at the level of cI stability and maintenance. However, the true nature of interdependency between cI and cIV is not fully understood yet. We used a HEK293 cellular model where the COX4 subunit was completely knocked out, serving as an ideal system to study interdependency of cI and cIV, as early phases of cIV assembly process were disrupted. Total absence of cIV was accompanied by profound deficiency of cI, documented by decrease in the levels of cI subunits and significantly reduced amount of assembled cI. Supercomplexes assembled from cI, cIII, and cIV were missing in COX4I1 knock-out (KO) due to loss of cIV and decrease in cI amount. Pulse-chase metabolic labeling of mitochondrial DNA (mtDNA)-encoded proteins uncovered a decrease in the translation of cIV and cI subunits. Moreover, partial impairment of mitochondrial protein synthesis correlated with decreased content of mitochondrial ribosomal proteins. In addition, complexome profiling revealed accumulation of cI assembly intermediates, indicating that cI biogenesis, rather than stability, was affected. We propose that attenuation of mitochondrial protein synthesis caused by cIV deficiency represents one of the mechanisms, which may impair biogenesis of cI.

Role of Mitochondrial Glycerol-3-Phosphate Dehydrogenase in Metabolic Adaptations of Prostate Cancer

Prostate cancer is one of the most prominent cancers diagnosed in males. Contrasting with other cancer types, glucose utilization is not increased in prostate carcinoma cells as they employ different metabolic adaptations involving mitochondria as a source of energy and intermediates required for rapid cell growth. In this regard, prostate cancer cells were associated with higher activity of mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH), the key rate limiting component of the glycerophosphate shuttle, which connects mitochondrial and cytosolic processes and plays significant role in cellular bioenergetics. Our research focused on the role of mGPDH biogenesis and regulation in prostate cancer compared to healthy cells. We show that the 42 amino acid presequence is cleaved from N-terminus during mGPDH biogenesis. Only the processed form is part of the mGPDH dimer that is the prominent functional enzyme entity. We demonstrate that mGPDH overexpression enhances the wound healing ability in prostate cancer cells. As mGPDH is at the crossroad of glycolysis, lipogenesis and oxidative metabolism, regulation of its activity by intramitochondrial processing might represent rapid means of cellular metabolic adaptations.

Cytochrome c Oxidase Subunit 4 Isoform Exchange Results in Modulation of Oxygen Affinity

Cytochrome c oxidase (COX) is regulated through tissue-, development- or environment-controlled expression of subunit isoforms. The COX4 subunit is thought to optimize respiratory chain function according to oxygen-controlled expression of its isoforms COX4i1 and COX4i2. However, biochemical mechanisms of regulation by the two variants are only partly understood. We created an HEK293-based knock-out cellular model devoid of both isoforms (COX4i1/2 KO). Subsequent knock-in of COX4i1 or COX4i2 generated cells with exclusive expression of respective isoform. Both isoforms complemented the respiratory defect of COX4i1/2 KO. The content, composition, and incorporation of COX into supercomplexes were comparable in COX4i1- and COX4i2-expressing cells. Also, COX activity, cytochrome c affinity, and respiratory rates were undistinguishable in cells expressing either isoform. Analysis of energy metabolism and the redox state in intact cells uncovered modestly increased preference for mitochondrial ATP production, consistent with the increased NADH pool oxidation and lower ROS in COX4i2-expressing cells in normoxia. Most remarkable changes were uncovered in COX oxygen kinetics. The p₅₀ (partial pressure of oxygen at half-maximal respiration) was increased twofold in COX4i2 versus COX4i1 cells, indicating decreased oxygen affinity of the COX4i2-containing enzyme. Our finding supports the key role of the COX4i2-containing enzyme in hypoxia-sensing pathways of energy metabolism.

Mutant Wars2 gene in spontaneously hypertensive rats impairs brown adipose tissue function and predisposes to visceral obesity

Brown adipose tissue (BAT) plays an important role in lipid and glucose metabolism in rodents and possibly also in humans. Identification of genes responsible for BAT function would shed light on underlying pathophysiological mechanisms of metabolic disturbances. Recent linkage analysis in the BXH/HXB recombinant inbred (RI) strains, derived from Brown Norway (BN) and spontaneously hypertensive rats (SHR), identified two closely linked quantitative trait loci (QTL) associated with glucose oxidation and glucose incorporation into BAT lipids in the vicinity of Wars2 (tryptophanyl tRNA synthetase 2 (mitochondrial)) gene on chromosome 2. The SHR harbors L53F WARS2 protein variant that was associated with reduced angiogenesis and Wars2 thus represents a prominent positional candidate gene. In the current study, we validated this candidate as a quantitative trait gene (QTG) using transgenic rescue experiment. SHR-Wars2 transgenic rats with wild type Wars2 gene when compared to SHR, showed more efficient mitochondrial proteosynthesis and increased mitochondrial respiration, which was associated with increased glucose oxidation and incorporation into BAT lipids, and with reduced weight of visceral fat. Correlation analyses in RI strains showed that increased activity of BAT was associated with amelioration of insulin resistance in muscle and white adipose tissue. In summary, these results demonstrate important role of Wars2 gene in regulating BAT function and consequently lipid and glucose metabolism.

Knockout of Tmem70 alters biogenesis of ATP synthase and leads to embryonal lethality in mice

TMEM70, a 21-kDa protein localized in the inner mitochondrial membrane, has been shown to facilitate the biogenesis of mammalian F1Fo ATP synthase. Mutations of the TMEM70 gene represent the most frequent cause of isolated ATP synthase deficiency resulting in a severe mitochondrial disease presenting as neonatal encephalo-cardiomyopathy (OMIM 604273). To better understand the biological role of this factor, we generated Tmem70-deficient mice and found that the homozygous Tmem70-/- knockouts exhibited profound growth retardation and embryonic lethality at ∼9.5 days post coitum. Blue-Native electrophoresis demonstrated an isolated deficiency in fully assembled ATP synthase in the Tmem70-/- embryos (80% decrease) and a marked accumulation of F1 complexes indicative of impairment in ATP synthase biogenesis that was stalled at the early stage, following the formation of F1 oligomer. Consequently, a decrease in ADP-stimulated State 3 respiration, respiratory control ratio and ATP/ADP ratios, indicated compromised mitochondrial ATP production. Tmem70-/- embryos exhibited delayed development of the cardiovascular system and a disturbed heart mitochondrial ultrastructure, with concentric or irregular cristae structures. Tmem70+/- heterozygous mice were fully viable and displayed normal postnatal growth and development of the mitochondrial oxidative phosphorylation system. Nevertheless, they presented with mild deterioration of heart function. Our results demonstrated that Tmem70 knockout in the mouse results in embryonic lethality due to the lack of ATP synthase and impairment of mitochondrial energy provision. This is analogous to TMEM70 dysfunction in humans and verifies the crucial role of this factor in the biosynthesis and assembly of mammalian ATP synthase.

Data on cytochrome c oxidase assembly in mice and human fibroblasts or tissues induced by SURF1 defect

This paper describes data related to a research article entitled “Tissue- and species-specific differences in cytochrome c oxidase assembly induced by SURF1 defects” [1]. This paper includes data of the quantitative analysis of individual forms of respiratory chain complexes I, III and IV present in SURF1 knockout (SURF1^−/−) and control (SURF1^+/+) mouse fibroblasts and tissues and in fibroblasts of human control and patients with SURF1 gene mutation. Also it includes data demonstrating response of complex IV, cytochrome c oxidase (COX), to reversible inhibition of mitochondrial translation in SURF1^−/− mouse and SURF1 patient fibroblast cell lines.

Tissue- and species-specific differences in cytochrome c oxidase assembly induced by SURF1 defects

Mitochondrial protein SURF1 is a specific assembly factor of cytochrome c oxidase (COX), but its function is poorly understood. SURF1 gene mutations cause a severe COX deficiency manifesting as the Leigh syndrome in humans, whereas in mice SURF1^−/− knockout leads only to a mild COX defect. We used SURF1^−/− mouse model for detailed analysis of disturbed COX assembly and COX ability to incorporate into respiratory supercomplexes (SCs) in different tissues and fibroblasts. Furthermore, we compared fibroblasts from SURF1^−/− mouse and SURF1 patients to reveal interspecies differences in kinetics of COX biogenesis using 2D electrophoresis, immunodetection, arrest of mitochondrial proteosynthesis and pulse-chase metabolic labeling.

The crucial differences observed are an accumulation of abundant COX1 assembly intermediates, low content of COX monomer and preferential recruitment of COX into I–III₂–IV_n SCs in SURF1 patient fibroblasts, whereas SURF1^−/− mouse fibroblasts were characterized by low content of COX1 assembly intermediates and milder decrease in COX monomer, which appeared more stable. This pattern was even less pronounced in SURF1^−/− mouse liver and brain. Both the control and SURF1^−/− mice revealed only negligible formation of the I–III₂–IV_n SCs and marked tissue differences in the contents of COX dimer and III₂–IV SCs, also less noticeable in liver and brain than in heart and muscle. Our studies support the view that COX assembly is much more dependent on SURF1 in humans than in mice. We also demonstrate markedly lower ability of mouse COX to form I–III₂–IV_n supercomplexes, pointing to tissue-specific and species-specific differences in COX biogenesis.

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In SURF1 −/− mouse the decrease of COX amount and activity was tissue/cell specific.

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Assembly kinetics proceeded to the level of stable COX monomer in SURF1 −/− mouse.

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COX assembly intermediates were faster degraded/depleted in time in SURF1 −/− mouse.

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COX was preferentially recruited in supercomplex I–III₂–IV₁ in SURF1 patient cells.

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Newly synthesized COX monomer was unstable and rapidly degraded in SURF1 patient.

Effects of mtDNA in SHR-mtF344 versus SHR conplastic strains on reduced OXPHOS enzyme levels, insulin resistance, cardiac hypertrophy, and systolic dysfunction

Common inbred strains of the laboratory rat can be divided into four major mitochondrial DNA (mtDNA) haplotype groups represented by the BN, F344, LEW, and SHR strains. In the current study, we investigated the metabolic and hemodynamic effects of the SHR vs. F344 mtDNA by comparing the SHR vs. SHR-mt(F344) conplastic strains that are genetically identical except for their mitochondrial genomes. Altogether 13 amino acid substitutions in protein coding genes, seven single nucleotide polymorphisms in tRNA genes, and 12 single nucleotide changes in rRNA genes were detected in F344 mtDNA compared with SHR mtDNA. Analysis of oxidative phosphorylation system (OXPHOS) in heart left ventricles (LV), muscle, and liver revealed reduced activity and content of several respiratory chain complexes in SHR-mt(F344) conplastic rats compared with the SHR strain. Lower function of OXPHOS in LV of conplastic rats was associated with significantly increased relative ventricular mass and reduced fractional shortening that was independent of blood pressure. In addition, conplastic rats exhibited reduced sensitivity of skeletal muscles to insulin action and impaired glucose tolerance. These results provide evidence that inherited alterations in mitochondrial genome, in the absence of variation in the nuclear genome and other confounding factors, predispose to insulin resistance, cardiac hypertrophy and systolic dysfunction.

ROS production in brown adipose tissue mitochondria: the question of UCP1-dependence

Whether active UCP1 can reduce ROS production in brown-fat mitochondria is presently not settled. The issue is of principal significance, as it can be seen as a proof- or disproof-of-principle concerning the ability of any protein to diminish ROS production through membrane depolarization. We therefore undertook a comprehensive investigation of the significance of UCP1 for ROS production, by comparing the ROS production in brown-fat mitochondria isolated from wildtype mice (that display membrane depolarization) or from UCP1(-/-) mice (with a high membrane potential). We tested the significance of UCP1 for glycerol-3-phosphate-supported ROS production by three methods (fluorescent dihydroethidium and the ESR probe PHH for superoxide, and fluorescent Amplex Red for hydrogen peroxide), and followed ROS production also with succinate, acyl-CoA or pyruvate as substrate. We studied the effects of the reverse electron flow inhibitor rotenone, the UCP1 activity inhibitor GDP, and the uncoupler FCCP. We also examined the effect of a physiologically induced increase in UCP1 amount. We noted GDP effects that were not UCP1-related. We conclude that only ROS production supported by exogenously added succinate was affected by the presence of active UCP1; ROS production supported by any other tested substrate (including endogenously generated succinate) was unaffected. This conclusion indicates that UCP1 is not involved in control of ROS production in brown-fat mitochondria. Extrapolation of these data to other tissues would imply that membrane depolarization may not necessarily decrease physiologically relevant ROS production. This article is a part of a Special Issue entitled: 18th European Bioenergetics Conference (Biochim. Biophys. Acta, Volume 1837, Issue 7, July 2014).

Nonsynonymous variants in mt-Nd2, mt-Nd4, and mt-Nd5 are linked to effects on oxidative phosphorylation and insulin sensitivity in rat conplastic strains

Common inbred strains of the laboratory rat can be divided into four different mitochondrial DNA haplotype groups represented by the SHR, BN, LEW, and F344 strains. In the current study, we investigated the metabolic and hemodynamic effects of the SHR vs. LEW mitochondrial genomes by comparing the SHR to a new SHR conplastic strain, SHR-mt(LEW); these strains are genetically identical except for their mitochondrial genomes. Complete mitochondrial DNA (mtDNA) sequence analysis comparing the SHR and LEW strains revealed gene variants encoding amino acid substitutions limited to a single mitochondrial enzyme complex, NADH dehydrogenase (complex I), affecting subunits 2, 4, and 5. Two of the variants in the mt-Nd4 subunit gene are located close to variants known to be associated with exercise intolerance and diabetes mellitus in humans. No variants were found in tRNA or rRNA genes. These variants in mt-Nd2, mt-Nd4, and mt-Nd5 in the SHR-mt(LEW) conplastic strain were linked to reductions in oxidative and nonoxidative glucose metabolism in skeletal muscle. In addition, SHR-mt(LEW) conplastic rats showed increased serum nonesterified fatty acid levels and resistance to insulin stimulated incorporation of glucose into adipose tissue lipids. These results provide evidence that inherited variation in mitochondrial genes encoding respiratory chain complex I subunits, in the absence of variation in the nuclear genome and other confounding factors, can influence glucose and lipid metabolism when expressed on the nuclear genetic background of the SHR strain.

Succinimidyl oleate, established inhibitor of CD36/FAT translocase inhibits complex III of mitochondrial respiratory chain

The functional role of CD36 protein detected in mitochondrial fractions in long chain fatty acid (LCFA) oxidation is unclear due to conflicting results obtained in Cd36 knockout mice and experiments using sulfo-N-succinimidyl oleate (SSO) for inhibition of CD36 mediated LCFA transport. We investigated effect of SSO on mitochondrial respiration and found that SSO substantially inhibits not only LCFA oxidation, but also oxidation of flavoprotein- and NADH-dependent substrates and generation of mitochondrial membrane potential. Experiments in rat liver, heart and kidney mitochondria demonstrated a direct effect on mitochondrial respiratory chain with the most pronounced inhibition of the complex III (IC(50) 4microM SSO). The results presented here show that SSO is a potent and irreversible inhibitor of mitochondrial respiratory chain.

Molecular mechanisms of silybin and 2,3-dehydrosilybin antiradical activity--role of individual hydroxyl groups

The flavonolignans silybin (1) and 2,3-dehydrosilybin (2) are important natural compounds with multiple biological activities operating at various cell levels. Many of these effects are connected with their radical-scavenging activities. The molecular mechanisms of the antioxidant activity of these compounds and even the functional groups responsible for this activity are not yet well known. Their mechanism can be inferred from the structures of the dimeric products obtained from radical-mediated reactions of selectively methylated derivatives of 1 and 2. The radical oxidation of 1 methylated at 7-OH and 2 methylated at both 3-OH and 7-OH yields C-C and C-O dimers that enable the molecular mechanism of their E-ring interaction with radicals to be elucidated and shows the importance of the 20-OH group in this respect. The pivotal role of the 3-OH group in the radical-scavenging activity of 2 was confirmed through the formation of another type of dimer from its selectively methylated derivative.

TMEM70 mutations cause isolated ATP synthase deficiency and neonatal mitochondrial encephalocardiomyopathy

We carried out whole-genome homozygosity mapping, gene expression analysis and DNA sequencing in individuals with isolated mitochondrial ATP synthase deficiency and identified disease-causing mutations in TMEM70. Complementation of the cell lines of these individuals with wild-type TMEM70 restored biogenesis and metabolic function of the enzyme complex. Our results show that TMEM70 is involved in mitochondrial ATP synthase biogenesis in higher eukaryotes.

High efficiency of ROS production by glycerophosphate dehydrogenase in mammalian mitochondria

We investigated hydrogen peroxide production in mitochondria with low (liver, heart, brain) and high (brown adipose tissue, BAT) content of glycerophosphate dehydrogenase (mGPDH). ROS production at state 4 due to electron backflow from mGPDH was low, but after inhibition of electron transport with antimycin A high rates of mGPDH-dependent ROS production were observed in liver, heart and brain mitochondria. When this ROS production was related to activity of mGPDH, many-fold higher ROS production was found in contrast to succinate- (39-, 28-, 3-fold) or pyruvate plus malate-dependent ROS production (32-, 96-, 5-fold). This specific rate of mGPDH-dependent ROS production was also exceedingly higher (28-, 66-, 22-fold) compared to that in BAT. mGPDH-dependent ROS production was localized to the dehydrogenase+CoQ and complex III, the latter being the highest in all mitochondria but BAT. Our results demonstrate high efficiency of mGPDH-dependent ROS production in mammalian mitochondria with a low content of mGPDH and suggest its endogenous inhibition in BAT.

Direct linkage of mitochondrial genome variation to risk factors for type 2 diabetes in conplastic strains

Recently, the relationship of mitochondrial DNA (mtDNA) variants to metabolic risk factors for diabetes and other common diseases has begun to attract increasing attention. However, progress in this area has been limited because (1) the phenotypic effects of variation in the mitochondrial genome are difficult to isolate owing to confounding variation in the nuclear genome, imprinting phenomena, and environmental factors; and (2) few animal models have been available for directly investigating the effects of mtDNA variants on complex metabolic phenotypes in vivo. Substitution of different mitochondrial genomes on the same nuclear genetic background in conplastic strains provides a way to unambiguously isolate effects of the mitochondrial genome on complex traits. Here we show that conplastic strains of rats with identical nuclear genomes but divergent mitochondrial genomes that encode amino acid differences in proteins of oxidative phosphorylation exhibit differences in major metabolic risk factors for type 2 diabetes. These results (1) provide the first direct evidence linking naturally occurring variation in the mitochondrial genome, independent of variation in the nuclear genome and other confounding factors, to inherited variation in known risk factors for type 2 diabetes; and (2) establish that spontaneous variation in the mitochondrial genome per se can promote systemic metabolic disturbances relevant to the pathogenesis of common diseases.

Respiratory chain components involved in the glycerophosphate dehydrogenase-dependent ROS production by brown adipose tissue mitochondria

Involvement of mammalian mitochondrial glycerophosphate dehydrogenase (mGPDH, EC 1.1.99.5) in reactive oxygen species (ROS) generation was studied in brown adipose tissue mitochondria by different spectroscopic techniques. Spectrofluorometry using ROS-sensitive probes CM-H2DCFDA and Amplex Red was used to determine the glycerophosphate- or succinate-dependent ROS production in mitochondria supplemented with respiratory chain inhibitors antimycin A and myxothiazol. In case of glycerophosphate oxidation, most of the ROS originated directly from mGPDH and coenzyme Q while complex III was a typical site of ROS production in succinate oxidation. Glycerophosphate-dependent ROS production monitored by KCN-insensitive oxygen consumption was highly activated by one-electron acceptor ferricyanide, whereas succinate-dependent ROS production was unaffected. In addition, superoxide anion radical was detected as a mGPDH-related primary ROS species by fluorescent probe dihydroethidium, as well as by electron paramagnetic resonance (EPR) spectroscopy with DMPO spin trap. Altogether, the data obtained demonstrate pronounced differences in the mechanism of ROS production originating from oxidation of glycerophosphate and succinate indicating that electron transfer from mGPDH to coenzyme Q is highly prone to electron leak and superoxide generation.

Inhibition of glycerophosphate-dependent H2O2 generation in brown fat mitochondria by idebenone

The established protective effect of coenzyme Q (CoQ) analogs is dependent on the location of reactive oxygen species (ROS) generation. One of these analogs--idebenone (hydroxydecyl-ubiquinone) is used as an antioxidative therapeutic drug. We tested its scavenging effect on the glycerophosphate (GP)-dependent ROS production as this enzyme was shown as a new site in the mitochondrial respiratory chain where ROS can be generated. We observed that idebenone inhibits both GP- and succinate-dependent ROS production. Idebenone and CoQ1 were found to be more efficient in the scavenging activity (IC50: 0.052 and 0.075 microM, respectively) than CoQ3 (IC50: 45.8 microM). Idebenone also inhibited ferricyanide (FeCN)-activated, GP-dependent ROS production. Our data thus extend previous findings on the scavenging effect of idebenone and show that it can also eliminate GP-dependent ROS generation.

Molecular characterization of binding of calcium and carbohydrates by an early activation antigen of lymphocytes CD69

CD69 is the earliest leukocyte activation antigen playing a pivotal role in cellular signaling. Here, we show that a globular C-terminal domain of CD69 belonging to C-type lectins binds calcium through Asp 171, Glu 185, and Glu 187 with K(d) approximately 54 microM. Closure of the calcium-binding site results in a conformational shift of Thr 107 and Lys 172. Interestingly, structural changes in all of these amino acids lead to the formation of high-affinity binding sites for N-acetyl-D-glucosamine. Similarly, a structural change in Glu 185 and Glu 187 contributes to a high-affinity site for N-acetyl-D-galactosamine. Site-directed mutagenesis and molecular modeling allowed us to describe the structural details of binding sites for both carbohydrates. These studies explain the importance of calcium for recognition of carbohydrates by CD69 and provide an important paradigm for the role of weak interactions in the immune system.

Comparison of the effect of mitochondrial inhibitors on mitochondrial membrane potential in two different cell lines using flow cytometry and spectrofluorometry

BACKGROUND

Determination of mitochondrial membrane potential (DeltaPsim) is widely used to characterize cellular metabolism, viability, and apoptosis. Changes of DeltaPsim induced by inhibitors of oxidative phosphorylation characterize respective contributions of mitochondria and glycolysis to adenosine triphosphate (ATP) synthesis.

METHODS

DeltaPsim in BSC-40 and HeLa G cell lines was determined by flow cytometry and spectrofluorometry. Its changes induced by specific mitochondrial inhibitors were evaluated using 3,3'-dihexyloxacarbocyanine iodide (DiOC6(3)), tetramethylrhodamine ethyl ester, and MitoTracker Red. Mitochondrial function was further characterized by oxygen consumption.

RESULTS

Inhibition of respiration by antimycin A or uncoupling of mitochondria by FCCP decreased DeltaPsim in both cell lines. Inhibition of ATP production by oligomycin or atractyloside induced a moderate decrease of DeltaPsim in HeLa G cells and an increase of DeltaPsim in BSC-40 cells. Statistically significant differences in DeltaPsim between the two cell lines were found with both flow cytometry and spectrofluorometry. Respirometry showed higher basal and FCCP-stimulated respiration in BSC-40 cells.

CONCLUSION

Changes of DeltaPsim and oxygen consumption showed that BSC-40 cells are more sensitive than HeLa G cells to inhibitors of mitochondrial function, suggesting that BSC-40 cells are more dependent than HeLa G cells on aerobic ATP production. Determination of DeltaPsim changes by flow cytometry exhibited greater sensitivity than the ones by spectrofluorometry.

Inhibitory effects of Bcl-2 on mitochondrial respiration

In contrast to the well-established anti-apoptotic effect of Bcl-2 protein, we have recently demonstrated that Bcl-2 overexpression by vaccinia virus causes apoptosis in BSC-40 cells, while it prevents apoptosis in HeLa G cells. Given the key role of mitochondria in the process of apoptosis, we focused on effects of Bcl-2 expression on mitochondrial energetics of these two cell lines. In this study we present data indicating that BSC-40 cells derive their ATP mainly from oxidative phosphorylation whereas HeLa G cells from glycolysis. More importantly, we show that in both cell lines, Bcl-2 inhibits mitochondrial respiration and causes a decrease of the ATP/ADP ratio. However, it appears that BSC-40 cells cannot sustain this decrease and die, while HeLa G cells survive, being adapted to the low ratio of ATP/ADP maintained by glycolysis. Based on this observation, we propose that the outcome of Bcl-2 expression is determined by the type of cellular ATP synthesis, namely that Bcl-2 causes apoptosis in cells relying on oxidative phosphorylation.

Protooncogene Bcl-2 induces apoptosis in several cell lines

Using a recombinant vaccinia virus expressing protooncogene Bcl-2, we demonstrate opposite effects of the expressed Bcl-2 in two cell lines: apoptosis induction in BSC-40 cells and apoptosis prevention in HeLa G cells. The apparent molecular weight of the expressed Bcl-2, its amounts and its effects on the mitochondrial membrane potential are comparable in both cell lines, suggesting that the consequences of Bcl-2 expression depend on the cellular environment. To further support these findings we demonstrate the pro-apoptotic effect of the expressed Bcl-2 in several other cell lines.

Inhibition of mitochondrial cytochrome C oxidase by dicarbanonaborates

Dicarbanonaborates inhibit the mitochondrial cytochrome c oxidase activity. In contrast to mitochondrial ATPase or glycerol phosphate dehydrogenase, inhibition of cytochrome c oxidase was not competitive and the residual, drug-insensitive activity was higher. These results indicate that dicarbanonaborates inhibit various mitochondrial membrane-bound enzymes through different mechanisms.