Iron-mediated inhibition of mitochondrial manganese uptake mediates mitochondrial dysfunction in a mouse model of hemochromatosis

Previous phenotyping of glucose homeostasis and insulin secretion in a mouse model of hereditary hemochromatosis (Hfe(-/-)) and iron overload suggested mitochondrial dysfunction. Mitochondria from Hfe(-/-) mouse liver exhibited decreased respiratory capacity and increased lipid peroxidation. Although the cytosol contained excess iron, Hfe(-/-) mitochondria contained normal iron but decreased copper, manganese, and zinc, associated with reduced activities of copper-dependent cytochrome c oxidase and manganese-dependent superoxide dismutase (MnSOD). The attenuation in MnSOD activity was due to substantial levels of unmetallated apoprotein. The oxidative damage in Hfe(-/-) mitochondria is due to diminished MnSOD activity, as manganese supplementation of Hfe(-/-) mice led to enhancement of MnSOD activity and suppressed lipid peroxidation. Manganese supplementation also resulted in improved insulin secretion and glucose tolerance associated with increased MnSOD activity and decreased lipid peroxidation in islets. These data suggest a novel mechanism of iron-induced cellular dysfunction, namely altered mitochondrial uptake of other metal ions.     

Threshold Effects and Control of Oxidative Phosphorylation in Nonsynaptic Rat Brain Mitochondria

Abstract: The amount of control exerted by respiratory chain complexes in isolated nonsynaptic mitochondria prepared from rat brain on the rate of oxygen consumption was assessed using inhibitor titrations. Rotenone, myxothiazol, and KCN were used to titrate the activities of NADH:ubiquinone oxidoreductase (EC 1.6.5.3; complex I), ubiquinol:ferrocytochrome c oxidoreductase (EC 1.10.2.2; complex III), and cytochrome c oxidase (EC 1.9.3.1; complex IV), respectively. Complexes I, III, and IV shared some of the control of the rate of oxygen consumption in nonsynaptic mitochondria, having flux control coefficients of 0.14, 0.15, and 0.24, respectively. Threshold effects in the control of oxidative phosphorylation were demonstrated for complexes I, III, and IV. It was found that complex I activity could be decreased by ∼72% before major changes in mitochondrial respiration and ATP synthesis took place. Similarly, complex III and IV activities could be decreased by ∼70 and 60%, respectively, before major changes in mitochondrial respiration and ATP synthesis occurred. These results indicate that previously observed decreases in respiratory chain complex activities in some neurological disorders need to be reassessed as these decreases might not affect the overall capability of nonsynaptic mitochondria to maintain energy homeostasis unless a certain threshold of decreased complex activity has been reached. Possible implications for synaptic mitochondria and neurodegenerative disorders are also discussed.     

Effects of mitochondrial complex III disruption in the respiratory chain of Neurospora crassa

In mitochondria from most organisms, including Neurospora crassa , dimeric complex III was found associated with complex I. Additional association of complex IV with this core structure leads to the formation of a respirasome. It was recently described for bacteria and mammals that complex III is needed for the assembly/stability of complex I. To elucidate the role of complex III in the organization of the respiratory chain of N. crassa , we analysed strains devoid of either the Rieske iron-sulphur or the COREII polypeptide subunits. The mutants display reduced growth, are female sterile and lack active complex III. The supramolecular organization of the oxidative phosphorylation system was characterized by electrophoretic analyses and the efficiency of the respiratory chain analysed by oxygen consumption measurements. The results obtained indicate that absence of complex III activity is not associated with the absence of complex I or complex IV, and leads to the induction of alternative oxidase. Complex III mutant mitochondria are devoid of respirasomes but contain significant amounts of dimeric complex I (I₂) and of the supercomplex I₁IV₁. Moreso, for the first time the alternative oxidase was found associated with dimeric complex IV and with supercomplex I₁IV₁.     

Identification and characterization of respirasomes in potato mitochondria

Plant mitochondria were previously shown to comprise respiratory supercomplexes containing cytochrome c reductase (complex III) and NADH dehydrogenase (complex I) of I(1)III(2) and I(2)III(4) composition. Here we report the discovery of additional supercomplexes in potato (Solanum tuberosum) mitochondria, which are of lower abundance and include cytochrome c oxidase (complex IV). Highly active mitochondria were isolated from potato tubers and stems, solubilized by digitonin, and subsequently analyzed by Blue-native (BN) polyacrylamide gel electrophoresis (PAGE). Visualization of supercomplexes by in-gel activity stains for complex IV revealed five novel supercomplexes of 850, 1,200, 1,850, 2,200, and 3,000 kD in potato tuber mitochondria. These supercomplexes have III(2)IV(1), III(2)IV(2), I(1)III(2)IV(1), I(1)III(2)IV(2), and I(1)III(2)IV(4) compositions as shown by two-dimensional BN/sodium dodecyl sulfate (SDS)-PAGE and BN/BN-PAGE in combination with activity stains for cytochrome c oxidase. Potato stem mitochondria include similar supercomplexes, but complex IV is partially present in a smaller version that lacks the Cox6b protein and possibly other subunits. However, in mitochondria from potato tubers and stems, about 90% of complex IV was present in monomeric form. It was suggested that the I(1)III(2)IV(4) supercomplex represents a basic unit for respiration in mammalian mitochondria termed respirasome. Respirasomes also occur in potato mitochondria but were of low concentrations under all conditions applied. We speculate that respirasomes are more abundant under in vivo conditions.     

The mitochondrial respiratory chain of Ustilago maydis

Ustilago maydis mitochondria contain the four classical components of the electron transport chain (complexes I, II, III, and IV), a glycerol phosphate dehydrogenase, and two alternative elements: an external rotenone-insensitive flavone-sensitive NADH dehydrogenase (NDH-2) and an alternative oxidase (AOX). The external NDH-2 contributes as much as complex I to the NADH-dependent respiratory activity, and is not modulated by Ca²⁺, a regulatory mechanism described for plant NDH-2, and presumed to be a unique characteristic of the external isozyme. The AOX accounts for the 20% residual respiratory activity after inhibition of complex IV by cyanide. This residual activity depends on growth conditions, since cells grown in the presence of cyanide or antimycin A increase its proportion to about 75% of the uninhibited rate. The effect of AMP, pyruvate and DTT on AOX was studied. The activity of AOX in U. maydis cells was sensitive to AMP but not to pyruvate, which agrees with the regulatory characteristics of a fungal AOX. Interestingly, the presence of DTT during cell permeabilisation protected the enzyme against inactivation.The pathways of quinone reduction and quinol oxidation lack an additive behavior. This is consistent with the competition of the respiratory components of each pathway for the quinol/quinone pool.     

Nitrite biotransformation by mitochondria from the earthworm Eisenia foetida (Savigny).

1. Nitrite oxidase and cytochrome-c oxidase activity catalysed by cytochrome-aa3 were assayed in earthworms and rats. 2. Cytochrome-aa3 and intact mitochondria from the two species were anaerobically incubated in the presence of nitrite; the occurrence of mitochondria-induced nitrite biotransformations was evaluated by monitoring nitrite recovery in incubation medium. Possible nitric oxide production was also tested. 3. The ratio nitrite oxidase/cytochrome-c oxidase activity was much higher in earthworms than in rats. 4. Under anaerobic conditions and in the presence of respiratory substrates, earthworm mitochondria produced a time-dependent loss of nitrite in the incubation medium. On the contrary, rat mitochondria are unable to decrease environmental nitrite concentration. 5. Results support the notion that metabolic properties of earthworm mitochondria can be considered as an adaptation to chronic nitrite exposure, this toxicant being typically present in natural habitats of these worms.     

Supercomplexes in the respiratory chains of yeast and mammalian mitochondria

Around 30-40 years after the first isolation of the five complexes of oxidative phosphorylation from mammalian mitochondria, we present data that fundamentally change the paradigm of how the yeast and mammalian system of oxidative phosphorylation is organized. The complexes are not randomly distributed within the inner mitochondrial membrane, but assemble into supramolecular structures. We show that all cytochrome c oxidase (complex IV) of Saccharomyces cerevisiae is bound to cytochrome c reductase (complex III), which exists in three forms: the free dimer, and two supercomplexes comprising an additional one or two complex IV monomers. The distribution between these forms varies with growth conditions. In mammalian mitochondria, almost all complex I is assembled into supercomplexes comprising complexes I and III and up to four copies of complex IV, which guided us to present a model for a network of respiratory chain complexes: a 'respirasome'. A fraction of total bovine ATP synthase (complex V) was isolated in dimeric form, suggesting that a dimeric state is not limited to S.cerevisiae, but also exists in mammalian mitochondria.     

The mitochondrial respiratory chain of Rhizopus stolonifer (Ehrenb.:Fr.) Vuill

Rhizopus stolonifer (Ehrenb.:Fr.) Vuill mitochondria contain the complete system for oxidative phosphorylation, formed by the classical components of the electron transport chain (complexes I, II, III, and IV) and the F₁F₀-ATP synthase (complex V). Using the native gel electrophoresis, we have shown the existence of supramolecular associations of the respiratory complexes. The composition and stoichiometry of the oxidative phosphorylation complexes were similar to those found in other organisms. Additionally, two alternative routes for the oxidation of cytosolic NADH were identified: the alternative NADH dehydrogenase and the glycerol-3-phosphate shuttles. Residual respiratory activity after inhibition of complex IV by cyanide was inhibited by low concentrations of n-octyl gallate, indicating the presence of an alternative oxidase. The K_0.5 for the respiratory substrates NADH, succinate, and glycerol-3-phosphate in permeabilized cells was higher than in isolated mitochondria, suggesting that interactions of mitochondria with other cellular elements might be important for the function of this organelle.     

Respiratory chain supercomplexes in plant mitochondria

Supercomplexes are defined associations of protein complexes, which are important for several cellular functions. This "quintenary" organization level of protein structure recently was also described for the respiratory chain of plant mitochondria. Except succinate dehydrogenase (complex II), all complexes of the oxidative phosphorylation (OXPOS) system (complexes I, III, IV and V) were found to form part of supercomplexes. Compositions of these supramolecular structures were systematically investigated using digitonin solubilizations of mitochondrial fractions and two-dimensional Blue-native (BN) polyacrylamide gel electrophoresis. The most abundant supercomplex of plant mitochondria includes complexes I and III at a 1:2 ratio (I1 + III2 supercomplex). Furthermore, some supercomplexes of lower abundance could be described, which have I2 + III4, V2, III2 + IV(1-2), and I1 + III2 + IV(1-4) compositions. Supercomplexes consisting of complexes I plus III plus IV were proposed to be called "respirasome", because they autonomously can carry out respiration in the presence of ubiquinone and cytochrome c. Plant specific alternative oxidoreductases of the respiratory chain were not associated with supercomplexes under all experimental conditions tested. However, formation of supercomplexes possibly indirectly regulates alternative respiratory pathways in plant mitochondria on the basis of electron channeling. In this review, procedures to characterize the supermolecular organization of the plant respiratory chain and results concerning supercomplex structure and function are summarized and discussed.     

Copper deficiency inhibits Ca2+-induced swelling in rat cardiac mitochondria

Cu deficiency disrupts the architecture of mitochondria, impairs respiration, and inhibits the activity of cytochrome c oxidase - the terminal, Cu-dependent respiratory complex (Complex IV) of the electron transport chain. This suggests that perturbations in the respiratory chain may contribute to the changes in mitochondrial structure caused by Cu deficiency. This study investigates the effect of Cu deficiency on Ca2+-induced mitochondrial swelling as it relates to changes in respiratory complex activities in cardiac mitochondria of rats. Male weanling rats were fed diets containing either no added Cu (Cu0), 1.5 mg Cu/kg (Cu1.5), 3 mg Cu/kg (Cu3) or 6 mg Cu/kg (Cu6). The rate of Ca2+-induced mitochondrial swelling in the presence of succinate and oligomycin was reduced, and the time to reach maximal swelling was increased only in the rats consuming Cu0 diet. Cytochrome c oxidase activity was reduced 60% and 30% in rats fed Cu0 and Cu1.5, respectively, while NADH:cytochrome c reductase (Complex I+ComplexIII) activity was reduced 30% in rats consuming both Cu0 and Cu1.5. Mitochondrial swelling is representative of mitochondrial permeability transition pore (MPTP) formation and the results suggest that Ca2+-induced MPTP formation occurs in cardiac mitochondria of Cu-deficient rats only when cytochrome c oxidase activity falls below 30% of normal. Decreased respiratory complex activities caused by severe Cu deficiency may inhibit MPTP formation by increasing matrix ADP concentration or promoting oxidative modifications that reduce the sensitivity of the calcium trigger for MPTP formation.     

Decrease of cytochrome c oxidase protein in heart mitochondria of copper-deficient rats

Copper deficiency has been reported to be associated withdecreased cytochrome c oxidase activity, whichin turn may be responsible for theobserved mitochondrial impairment and cardiac failure. We isolatedmito-chondriafrom hearts of copper-deficient rats: cytochrome c oxidase activity was found to be lowerthan incopper-adequate mitochondria. The residual activity paralleled coppercontent of mitochondria and also corresponded with the heme amount associated with cytochromeaa3. In fact, lower absorption in thea-band region of cytochrome aa3 was foundfor copper-deficient rat heart mitochondria. Gel electrophoresisof protein extractedfrom mitochondrial membranes allowed measurements of protein content of thecomplexes ofoxidative phosphorylation, revealing a lower content of complex IV protein incopper-deficientrat heart mitochondria. The alterations caused by copper deficiency appear to bespecific forcytochrome c oxidase. Changes were not observed for F 0 F 1 ATP synthase activity,for heme contents ofcytochrome c and b, and for protein contents of complexes I, III and V.The present study demonstrates that the alteration of cytochrome c oxidase activityobserved in copper deficiency is due to a diminishedcontent of assembled protein and that shortnessof copper impairs heme insertion into cytochrome c oxidase.     

Control processes in oxidative phosphorylation: kinetic constraints and stoichiometry

Control processes in oxidative phosphorylation have been studied in three experimental models. (1) In isolated yeast mitochondria, external ATP is a regulatory effector of cytochrome-c oxidase activity. In phosphorylating or uncoupling states, the relationships between respiratory rate and delta mu H+, and the respiratory rate and cytochrome-c oxidase reduction level are dependent on this kinetic regulation. (2) In rat liver mitochondria, the response of the respiratory rate to uncoupler addition is age-dependent: liver mitochondria isolated from young rats maintain a greater delta mu H+ than liver mitochondria isolated from adults, with the same respiratory rate obtained with the same concentration of uncoupler. This behaviour is linked to redox proton pump properties, i.e., to the degree of intrinsic uncoupling induced by uncoupler addition. (3) The effect of almitrine, a new kind of ATPase/ATPsynthase inhibitor, was studied in mammalian mitochondria. (i) Almitrine inhibits oligomycin-sensitive ATPase - it decreases the ATPase/O value without any change in delta mu H+; (ii) almitrine increased the mechanistic H+/ATP stoichiometry of ATPase/ATPsynthase; (iii) almitrine-induced changes in H+/ATPase stoichiometry depend on the flux magnitude through ATPase. These results are discussed in terms of the following interdependent parameters; flux value, force, pump efficiency and control coefficient.     

Effect of cobalt and copper complexes with o-phenanthroline on the respiratory activity of mitochondria]

The effects of cobalt and copper complexes with o-phenantroline on the respiratory activity of mitochondria from pea sprouts and submitochondrial particles from bovine heart and on the oxidative phosphorylation in mitochondria were studied. The catalytic activity of the complexes in several components of the respiratory chain autooxidation reactions was investigated. It was shown that the bis (o-phenantroline) cobalt (II) chloride complex is more active in exidation of NADH. The tris (o-phenantroline) cobolt (III) perchlorate complex stimulates the respiratory activity of mitochondria and submitochondrial particles. Possible localization of the effect of this complex was postulated. The (o-phenantroline) copper chloride complex completely inhibits the succinate-dependent respiration of submitochondrial particles and causes disturbances in oxidative phosphorylation of mitochondria.     

Euglena gracilis rhodoquinone:ubiquinone ratio and mitochondrial proteome differ under aerobic and anaerobic conditions

Euglena gracilis cells grown under aerobic and anaerobic conditions were compared for their whole cell rhodoquinone and ubiquinone content and for major protein spots contained in isolated mitochondria as assayed by two-dimensional gel electrophoresis and mass spectrometry sequencing. Anaerobically grown cells had higher rhodoquinone levels than aerobically grown cells in agreement with earlier findings indicating the need for fumarate reductase activity in anaerobic wax ester fermentation in Euglena. Microsequencing revealed components of complex III and complex IV of the respiratory chain and the E1beta subunit of pyruvate dehydrogenase to be present in mitochondria of aerobically grown cells but lacking in mitochondria from anaerobically grown cells. No proteins were identified as specific to mitochondria from anaerobically grown cells. cDNAs for the E1alpha, E2, and E3 subunits of mitochondrial pyruvate dehydrogenase were cloned and shown to be differentially expressed under aerobic and anaerobic conditions. Their expression patterns differed from that of mitochondrial pyruvate:NADP(+) oxidoreductase, the N-terminal domain of which is pyruvate:ferredoxin oxidoreductase, an enzyme otherwise typical of hydrogenosomes, hydrogen-producing forms of mitochondria found among anaerobic protists. The Euglena mitochondrion is thus a long sought intermediate that unites biochemical properties of aerobic and anaerobic mitochondria and hydrogenosomes because it contains both pyruvate:ferredoxin oxidoreductase and rhodoquinone typical of hydrogenosomes and anaerobic mitochondria as well as pyruvate dehydrogenase and ubiquinone typical of aerobic mitochondria. Our data show that under aerobic conditions Euglena mitochondria are prepared for anaerobic function and furthermore suggest that the ancestor of mitochondria was a facultative anaerobe, segments of whose physiology have been preserved in the Euglena lineage.     

Cardiac cytochrome-c oxidase deficiency occurs during late postnatal development in progeny of copper-deficient rats

Although cytochrome-c oxidase (CCO) is a copper-dependent enzyme, the effect of maternal copper deficiency on the expression of CCO activity during postnatal development of the neonatal rat heart has not been investigated extensively. Here, we show that CCO activity in heart mitochondria isolated from neonates of copper-deficient dams did not exhibit significant reductions until postnatal days (PND) 15 and 21. In addition, immunoblot analysis indicated that the CCO subunit (Cox-1) was reduced on postnatal Days 10 and 21, and that Cox-4 was reduced on PND 21 in heart mitochondria of the neonates from copper-deficient dams. These findings indicate that the impairment of CCO activity in neonatal heart by maternal copper deficiency occurs late in the postnatal heart development. Furthermore, the concurrent reductions in Cox-1 and Cox-4 suggest that the impaired CCO activity reflects a CCO deficiency in heart mitochondria. CCO activity and Cox-1 in heart mitochondria were not fully restored by 6 weeks of postweaning copper repletion in the pups of copper-deficient dams. This indicates that prolonged maternal intake of moderately low dietary copper produces CCO deficiency in cardiac mitochondria of neonates during late postnatal heart development, after terminal differentiation of cardiomyocytes occurs. The resistance of CCO deficiency to repair by dietary copper supplementation may be related to the relatively slow turnover of the affected mitochondria in the terminally differentiated heart.     

Myxothiazol resistance in human mitochondria

We have investigated electron transfer activities of respiratory chain complexes in platelet mitochondria of a patient with intermittent ataxia and lactic acidosis who was previously reported to be deficient in the E1 (decarboxylase) component of the pyruvate dehydrogenase complex. Electron transfer from succinate to cytochrome c was normal, but the mitochondria exhibited moderately decreased (63% of control) quinol: cytochrome-c oxidoreductase activity, suggesting a defect in complex III. Consistent with some perturbation in complex III, electron flux through complex III was resistant to inhibition by myxothiazol compared to normal controls. In contrast, titration with antimycin revealed a less abnormal pattern of inhibition. The extreme specificity of myxothiazol binding at or near the quinol oxidase domain of mitochondrial cytochrome b, i.e., b-566, suggests a defect in this region of complex III which may perturb the kinetics or thermodynamics of quinol oxidation in the complex. These data suggest that the patient's illness results from a mutation in the quinol oxidase domain of mitochondrial cytochrome b (b-566).     

Identification of respiratory complexes I and III as mitochondrial sites of damage following exposure to ionizing radiation and nitric oxide.

In 32D cl 3 hematopoietic progenitor cells, the overexpression of manganese superoxide dismutase (MnSOD, SOD2), the enzyme normally found in mitochondria, protects against the damaging effects of ionizing radiation. In the presence of a nitric oxide donor, which exacerbates the damage, inhibition of mitochondrial function can be demonstrated to be associated with respiratory complexes I (NADH dehydrogenase) and III (cytochrome c reductase), but not II (succinate dehydrogenase), IV (cytochrome c oxidase), or V (ATP synthase). The same pattern of inhibition is observed in the case of isolated bovine heart mitochondria exposed to ionizing radiation and the nitric oxide donor. The addition of authentic peroxynitrite (ONO2(-)) to isolated mitochondria also results in damage to complexes I and III (but not II, IV, and V), as shown by assays of electron-transfer activities and electron paramagnetic resonance (EPR) spectroscopic measurements, suggesting ONO2(-) to be responsible for most of the observed radiation damage in both the cultured cell lines and isolated mitochondria. It is argued that, in general, production of ONO2(-) is an important contributor to radiation damage in biological systems and the implications of these findings in relation to possible mechanisms of oxidant-linked apoptosis are briefly considered.     

Deficiency of complex III of the mitochondrial respiratory chain in a patient with facioscapulohumeral disease.

Facioscapulohumeral disease (FSHD), an inherited neuromuscular disorder, is characterized by progressive wasting of specific muscle groups, particularly the proximal musculature of the upper limbs; the primary defect in this disorder is unknown. We studied a patient with FSHD to determine whether the mitochondrial respiratory chain was functionally abnormal. Muscle biopsy revealed fiber atrophy with patchy staining for oxidative enzymes. Electron microscopy of a liver section showed many enlarged mitochondria with paracrystalline inclusions. Decreased oxidation of the respiratory substrates-alanine and succinate-in skin fibroblasts suggested a deficiency of complex III of the electron-transport chain; cytochrome c oxidase activity (complex IV) was in the normal range. Biochemical analysis of liver supported the fibroblast data, since succinate oxidase activity (electron-transport activity through complexes II-IV) was reduced, whereas complex IV activity was normal. Furthermore, analysis of the cytochrome spectrum in liver revealed typical peaks for cytochromes cc1 and aa3, whereas cytochrome b (a component of complex III) was undetectable. Southern blot analysis of fibroblast mtDNA revealed no major deletions or rearrangements. Our study provides the first documentation of a specific enzyme-complex deficiency associated with FSHD.     

Decreased activities of ubiquinol:ferricytochrome c oxidoreductase (complex III) and ferrocytochrome c:oxygen oxidoreductase (complex IV) in liver mitochondria from rats with hydroxycobalamin[c-lactam]-induced methylmalonic aciduria.

Rats treated with hydroxycobalamin[c-lactam] (HCCL), a cobalamin analogue that induces methylmalonic aciduria, have increased hepatic mitochondrial content and increased oxidative metabolism of pyruvate and palmitate per hepatocyte. The present studies were undertaken to characterize oxidative metabolism in isolated liver mitochondria from rats treated with HCCL. After 5-6 weeks, state 3 oxidation rates for diverse substrates are reduced in mitochondria from HCCL-treated rats. Similar reductions of mitochondrial oxidation rates are obtained with dinitrophenol-uncoupled mitochondria excluding defective phosphorylation as a cause for the observed decrease in mitochondrial oxidation. The activities of mitochondrial oxidases are reduced in HCCL-treated rats and demonstrate a defect in complex IV. Investigation of the complexes of the respiratory chain reveals a 32% decrease of ubiquinol:ferricytochrome c oxidoreductase (complex III) activity and a 72% decrease of ferrocytochrome c:oxygen oxidoreductase (complex IV) activity in mitochondria from 5-6-week HCCL-treated rats as compared with controls. Liver mitochondria from HCCL-treated rats also demonstrate decreased cytochrome content per mg of mitochondrial protein (25% decrease of cytochrome b and 52% decrease of cytochrome a + a3 as compared with control rats). The HCCL-treated rat represents an animal model for the study of the consequences of respiratory chain defects in liver mitochondria.     

Oxidized low-density lipoprotein and 15-deoxy-delta 12,14-PGJ2 increase mitochondrial complex I activity in endothelial cells

Oxidized lipids are capable of initiating diverse cellular responses through both receptor-mediated mechanisms and direct posttranslational modification of proteins. Typically, exposure of cells to low concentrations of oxidized lipids induces cytoprotective pathways, whereas high concentrations result in apoptosis. Interestingly, mitochondria can contribute to processes that result in either cytoprotection or cell death. The role of antioxidant defenses such as glutathione in adaptation to stress has been established, but the potential interaction with mitochondrial function is unknown and is examined in this article. Human umbilical vein endothelial cells (HUVEC) were exposed to oxidized LDL (oxLDL) or the electrophilic cyclopentenone 15-deoxy-Delta 12,14-PGJ2 (15d-PGJ2). We demonstrate that complex I activity, but not citrate synthase or cytochrome-c oxidase, is significantly induced by oxLDL and 15d-PGJ2. The mechanism is not clear at present but is independent of the induction of GSH, peroxisome proliferator-activated receptor (PPAR)-gamma, and PPAR-alpha. This response is dependent on the induction of oxidative stress in the cells because it can be prevented by nitric oxide, probucol, and the SOD mimetic manganese(III) tetrakis(4-benzoic acid) porphyrin chloride. This increased complex I activity appears to contribute to protection against apoptosis induced by 4-hydroxynonenal.