Role In Mitochondrial Oxidative Phosphorylation Research Articles

The Antioxidant Transcription Factor Nrf2 Negatively Regulates Autophagy and Growth Arrest Induced by the Anticancer Redox Agent Mitoquinone

Mitoquinone (MitoQ) is a synthetically modified, redox-active ubiquinone compound that accumulates predominantly in mitochondria. We found that MitoQ is 30-fold more cytotoxic to breast cancer cells than to healthy mammary cells. MitoQ treatment led to irreversible inhibition of clonogenic growth of breast cancer cells through a combination of autophagy and apoptotic cell death mechanisms. Relatively limited cytotoxicity was seen with the parent ubiquinone coenzyme Q(10.) Inhibition of cancer cell growth by MitoQ was associated with G(1)/S cell cycle arrest and phosphorylation of the checkpoint kinases Chk1 and Chk2. The possible role of oxidative stress in MitoQ activity was investigated by measuring the products of hydroethidine oxidation. Increases in ethidium and dihydroethidium levels, markers of one-electron oxidation of hydroethidine, were observed at cytotoxic concentrations of MitoQ. Keap1, an oxidative stress sensor protein that regulates the antioxidant transcription factor Nrf2, underwent oxidation, degradation, and dissociation from Nrf2 in MitoQ-treated cells. Nrf2 protein levels, nuclear localization, and transcriptional activity also increased following MitoQ treatment. Knockdown of Nrf2 caused a 2-fold increase in autophagy and an increase in G(1) cell cycle arrest in response to MitoQ but had no apparent effect on apoptosis. The Nrf2-regulated enzyme NQO1 is partly responsible for controlling the level of autophagy. Keap1 and Nrf2 act as redox sensors for oxidative perturbations that lead to autophagy. MitoQ and similar compounds should be further evaluated for novel anticancer activity.

Mitochondrial diseases in man and mouse.

Over the past 10 years, mitochondrial defects have been implicated in a wide variety of degenerative diseases, aging, and cancer. Studies on patients with these diseases have revealed much about the complexities of mitochondrial genetics, which involves an interplay between mutations in the mitochondrial and nuclear genomes. However, the pathophysiology of mitochondrial diseases has remained perplexing. The essential role of mitochondrial oxidative phosphorylation in cellular energy production, the generation of reactive oxygen species, and the initiation of apoptosis has suggested a number of novel mechanisms for mitochondrial pathology. The importance and interrelationship of these functions are now being studied in mouse models of mitochondrial disease.

Impairment of mitochondrial oxidative phosphorylation in the brain of aged mice

To elucidate the role of mitochondrial oxidative phosphorylation in neuronal aging, we have studied the activity of the respiratory complexes in the brain of young, adult and old mice. In synaptic mitochondria, we found a significant decrease in complexes IV (29%, P < 0.001) and V (21%, P < 0.01) in old as compared with adult mice. Nonsynaptic mitochondria also showed a senescent decrease in complexes I (15%, P < 0.01), II + III (34%, P < 0.01) and IV (17%, P < 0.01) activities. These findings suggest a dysfunction in mitochondrial oxidative phosphorylation in brain aging.

On the Role of Mitochondrial Oxidative Phosphorylation in Photosynthesis Metabolism as Studied by the Effect of Oligomycin on Photosynthesis in Protoplasts and Leaves of Barley (Hordeum vulgare)

Low concentrations of oligomycin, which strongly inhibit mitochondrial oxidative phosphorylation but do not affect chloroplast photophosphorylation, caused an inhibition of photosynthesis by 30 to 40% in barley (Hordeum vulgare L.) leaf protoplasts. This inhibition is reversed and the full rate of photosynthesis is regained when the protoplasts are ruptured so as to leave the chloroplasts intact. Oligomycin fed into barley leaves by the transpiration stream inhibited photosynthesis in these leaves by up to 60%. The measurement of metabolites in protoplast and leaf extracts showed that oligomycin caused a decrease in the ATP/ADP ratio and an increase in the content of glucose- and fructose 6-phosphate. Subcellular analysis of protoplasts revealed that the decrease in ATP/ADP ratio in the cytosol was larger than in the stroma and that the increase in hexose monophosphates was restricted to the cytosol, whereas the stromal hexosemonophosphates decreased upon the addition of oligomycin. Moreover, oligomycin caused an increase in the triosephosphate-3-phosphoglycerate ratio. It is concluded from these results that during photosynthesis of a plant leaf cell mitochondrial oxidative phosphorylation contributes to the ATP supply of the cell and prevents overreduction of the chloroplast redox carriers by oxidizing reductive equivalents generated by photosynthetic electron transport.

Role of mitochondrial oxidative phosphorylation in regulation of coronary blood flow.

Regulation of coronary blood flow was studied in isolated rat hearts perfused under various metabolic conditions. Alterations in coronary flow were induced by hypoxia, amobarbital (Amytal) infusion, increase in work load of the heart, and adenosine infusion. Hypoxia induced, on the average, a 92.5% rise in coronary flow; 0.88 mM Amytal, a 85.7% increase; 12 microM adenosine, a 49.5% rise; and increased work load (elevation of the perfusion pressure from 6.9 kPa to 12.8 kPa), a 53.4% increase. In normoxia, adenosine, inosine, and hypoxanthine were present in the effluent in very low concentrations, and these greatly increased in response to hypoxia. In contrast, increased coronary flow caused by Amytal infusion or by elevated perfusion pressure was not accompanied by elevation in the effluent concentration of adenosine and its catabolites. Infusion of Amytal was followed by decrease in oxygen consumption of the heart and increase in oxygen tension in the effluent. This indicates that tissue oxygen tension per se can not be responsible for the regulation of coronary blood flow. Analysis of the data showed that under conditions in which there was a decrease in the tissue [ATP]free/[ADP]free[Pi] an increase in coronary flow was observed irrespective of the nature of the vasodilatory stimulus. It is concluded that mitochondrial oxidative phosphorylation provides a link between tissue oxygen metabolism and coronary blood flow. Mechanisms are discussed whereby changes in the cellular energy state ([ATP]free/[ADP]free[Pi]) are coupled to vasodilation, including possible direct effects on the vascular smooth muscle and/or generation of "second messengers."

A hypothesis for a role for unsaturated fatty acids in electron transport and its potential application to understanding the mitochondrial respiratory chain

Unsaturated fatty acids are required for mitochondrial oxidative phosphorylation and electron transport, though their role has not been determined. We have considered the possibility that unsaturated fatty acids might facilitate electron transfer from non-heme iron to heme iron. Unsaturated fatty acids markedly enhanced the reduction of ferric cytochrome c by ferrous iron. Neither stearic acid nor methyl arachidonic acid were effective suggesting that both a double bond and the carboxylic acid group were essential. Thus unsaturated fatty acids can directly facilitate electron transfer from non-heme iron to heme iron. We hypothesize that unsaturated fatty acids may play a similar critical role in mitochondrial oxidative phosphorylation and other biological processes where rapid electron transfer occurs.

Role of mitochondrial oxidative phosphorylation in the maintenance of intracellular pH

The oxidative phosphorylation of isolated rabbit heart mitochondria and the accompanying pH changes were simultaneously measured in the same sample. The nearly constant extramitochondrial proton concentration during State 4 respiration decreased considerably under conditions of oxidative phosphorylation. On the other hand, the proton concentration increased when ATP was hydrolysed by the mitochondrial ATPase. The lysis of mitochondria by Triton X-100 equilibrating the pH difference across the inner membrane did not cause significant change in the proton concentration of the sample neither before nor after State 3 respiration. The accumulation of protons during the extramitochondrial hexokinase reaction was reduced by the mitochondrial oxidative phosphorylation. It is suggested that in the normoxic myocardial cell, the protons generated by the extramitochondrial hydrolysis of ATP are utilized during mitochondrial oxidative phosphorylation.