Regular exercise reduces 8-oxodG in the nuclear and mitochondrial DNA and modulates the DNA repair activity in the liver of old rats

Exercise-induced protection of bone marrow cells following exposure to radiation

The hormetic effects of exercise training have previously been shown to enhance cellular protection against oxidative stress. Therefore, adaptations to exercise training may attenuate the harmful effects of radiation induced by oxidative stress. Flow cytometric analysis of genotoxicity (γH2AX foci and micronucleated reticulocytes (MN-RET)) and cytotoxicity (apoptosis and percentage of reticulocytes) were conducted on bone marrow cells isolated from acutely exercised (Acute EX), exercise-trained (EX), and sedentary (SED) mice following 1 and 2 Gy radiation challenges in vitro. Acute EX increased the percentage of cells with activated caspase-3 and -7 (32%, p < 0.001) and γH2AX foci formation in response to 2 Gy radiation challenge (10%, p < 0.05). Exercise training significantly attenuated γH2AX foci formation and MN-RET production in response to 1 Gy radiation challenge (18%, p < 0.05 and 22%, p < 0.05, respectively). Exercise training also significantly reduced basal percentages of cells with activated caspase-3 and -7 and in response to radiation in bone marrow cells (11%, p < 0.05). These results suggest that oxidative stress caused by acute exercise induces an adaptive response responsible for the radioprotective effects of exercise training.

Lower oxidative DNA damage despite greater ROS production in muscles from rats selectively bred for high running capacity

Artificial selection in rat has yielded high-capacity runners (HCR) and low-capacity runners (LCR) that differ in intrinsic (untrained) aerobic exercise ability and metabolic disease risk. To gain insight into how oxygen metabolism may have been affected by selection, we compared mitochondrial function, oxidative DNA damage (8-dihydroxy-guanosine; 8dOHG), and antioxidant enzyme activities in soleus muscle (Sol) and gastrocnemius muscle (Gas) of adult and aged LCR vs. HCR rats. In Sol of adult HCR rats, maximal ADP-stimulated respiration was 37% greater, whereas in Gas of adult HCR rats, there was a 23% greater complex IV-driven respiratory capacity and 54% greater leak as a fraction of electron transport capacity (suggesting looser mitochondrial coupling) vs. LCR rats. H(2)O(2) emission per gram of muscle was 24-26% greater for both muscles in adult HCR rats vs. LCR, although H(2)O(2) emission in Gas was 17% lower in HCR, after normalizing for citrate synthase activity (marker of mitochondrial content). Despite greater H(2)O(2) emission, 8dOHG levels were 62-78% lower in HCR rats due to 62-96% higher superoxide dismutase activity in both muscles and 47% higher catalase activity in Sol muscle in adult HCR rats, with no evidence for higher 8 oxoguanine glycosylase (OGG1; DNA repair enzyme) protein expression. We conclude that genetic segregation for high running capacity has generated a molecular network of cellular adaptations, facilitating a superior response to oxidative stress.

Oxygen as a friend and enemy: How to combat the mutational potential of 8-oxo-guanine

The maintenance of genetic stability is of crucial importance for any form of life. Prior to cell division in each mammalian cell, the process of DNA replication must faithfully duplicate the three billion bases with an absolute minimum of mistakes. Various environmental and endogenous agents, such as reactive oxygen species (ROS), can modify the structural properties of DNA bases and thus damage the DNA. Upon exposure of cells to oxidative stress, an often generated and highly mutagenic DNA damage is 7,8-dihydro-8-oxo-guanine (8-oxo-G). The estimated steady-state level of 8-oxo-G lesions is about 10(3) per cell/per day in normal tissues and up to 10(5) lesions per cell/per day in cancer tissues. The presence of 8-oxo-G on the replicating strand leads to frequent (10-75%) misincorporations of adenine opposite the lesion (formation of A:8-oxo-G mispairs), subsequently resulting in C:G to A:T transversion mutations. These mutations are among the most predominant somatic mutations in lung, breast, ovarian, gastric and colorectal cancers. Thus, in order to reduce the mutational burden of ROS, human cells have evolved base excision repair (BER) pathways ensuring (i) the correct and efficient repair of A:8-oxo-G mispairs and (ii) the removal of 8-oxo-G lesions from the genome. Very recently it was shown that MutY glycosylase homologue (MUTYH) and DNA polymerase lambda play a crucial role in the accurate repair of A:8-oxo-G mispairs. Here we review the importance of accurate BER of 8-oxo-G damage and its regulation in prevention of cancer.

Influence of aerobic fitness on age-related lymphocyte DNA damage in humans: relationship with mitochondria respiratory chain and hydrogen peroxide production

The aim of this study was to analyze the influence of aerobic fitness (AF) on age-related lymphocyte DNA damage in humans, giving special attention to the role of the mitochondrial respiratory chain and hydrogen peroxide production. Considering age and AF (as assessed by VO(2)max), 66 males (19-59 years old) were classified as high fitness (HF) or low fitness (LF) and distributed into one of the following groups: young adults (19-29 years old), adults (30-39 years old), and middle-aged adults (over 40 years old). Peripheral lymphocytes obtained at rest were used to assess DNA damage (strand breaks and formamidopyrimidine DNA glycosylase (FPG) sites through the comet assay), activity of mitochondrial complexes I and II (polarographically measured), and the hydrogen peroxide production rate (assayed by fluorescence). Results revealed a significant interaction between age groups and AF for DNA strand breaks (F = 8.415, p = .000), FPG sites (F = 11.766, p = .000), mitochondrial complex I activity (F = 7.555, p = .000), and H(2)O(2) production (F = 7.500, p = .000). Except for mitochondrial complex II activity, the age variation of the remaining parameters was significantly attenuated by HF. Considering each AF level, an increase in DNA strand breaks and FPG sites with age (r = 0.655, p = 0.000, and r = 0.738, p = 0.000, respectively) was only observed in LF. Moreover, decreased mitochondrial complex I activity with age (r = -.470, p = .009) was reported in LF. These results allow the conclusion that high AF seems to play a key role in attenuating the biological aging process.

8-Oxo-7,8-dihydroguanine: links to gene expression, aging, and defense against oxidative stress

The one-electron oxidation product of guanine, 8-oxo-7,8-dihydroguanine (8-oxoG), is an abundant lesion in genomic, mitochondrial, and telomeric DNA and RNA. It is considered to be a marker of oxidative stress that preferentially accumulates at the 5' end of guanine strings in the DNA helix, in guanine quadruplexes, and in RNA molecules. 8-OxoG has a lower oxidation potential compared to guanine; thus it is susceptible to oxidation/reduction and, along with its redox products, is traditionally considered to be a major mutagenic DNA base lesion. It does not change the architecture of the DNA double helix and it is specifically recognized and excised by 8-oxoguanine DNA glycosylase (OGG1) during the DNA base excision repair pathway. OGG1 null animals accumulate excess levels of 8-oxoG in their genome, yet they do not have shorter life span nor do they exhibit severe pathological symptoms including tumor formation. In fact they are increasingly resistant to inflammation. Here we address the rarely considered significance of 8-oxoG, such as its optimal levels in DNA and RNA under a given condition, essentiality for normal cellular physiology, evolutionary role, and ability to soften the effects of oxidative stress in DNA, and the harmful consequences of its repair, as well as its importance in transcriptional initiation and chromatin relaxation.

Redox control systems in the nucleus: mechanisms and functions

Proteins with oxidizable thiols are essential to many functions of cell nuclei, including transcription, chromatin stability, nuclear protein import and export, and DNA replication and repair. Control of the nuclear thiol-disulfide redox states involves both the elimination of oxidants to prevent oxidation and the reduction of oxidized thiols to restore function. These processes depend on the common thiol reductants, glutathione (GSH) and thioredoxin-1 (Trx1). Recent evidence shows that these systems are controlled independent of the cytoplasmic counterparts. In addition, the GSH and Trx1 couples are not in redox equilibrium, indicating that these reductants have nonredundant functions in their support of proteins involved in transcriptional regulation, nuclear protein trafficking, and DNA repair. Specific isoforms of glutathione peroxidases, glutathione S-transferases, and peroxiredoxins are enriched in nuclei, further supporting the interpretation that functions of the thiol-dependent systems in nuclei are at least quantitatively distinct, and probably also qualitatively distinct, from similar processes in the cytoplasm. Elucidation of the distinct nuclear functions and regulation of the thiol redox pathways in nuclei can be expected to improve understanding of nuclear processes and also to provide the basis for novel approaches to treat aging and disease processes associated with oxidative stress in the nuclei.

Comparação do treinamento físico de quatro e oito semanas sobre atividade da cadeia transportadora de elétrons e marcadores de estresse oxidativo em fígado de camundongos

O presente estudo investigou o efeito de quatro e oito semanas de treinamento físico sobre a atividade dos complexos da cadeia transportadora de elétrons (CTE) e os marcadores de estresse oxidativo em fígado de camundongos. Vinte e um camundongos (CF1, 30-35g) foram distribuídos nos seguintes grupos: não treinado (NT); treinado quatro semanas (T4); treinado oito semanas (T8). Quarenta e oito horas após a última sessão de treinamento os animais foram mortos por decapitação e o fígado foi retirado e estocado em -70ºC para posterior análise. Atividade da succinato desidrogenase (SDH), dos complexos I,II,III e IV da CTE, carbonilação de proteína, conteúdo total de tióis e a atividade da superóxido dismutase foram mensurados. Os resultados demonstram que apenas oito semanas de treinamento aumentam a atividade da SDH, dos quatro complexos da CTE, da superóxido dismutase, e o conteúdo total de tióis em relação ao grupo não treinado. Houve ainda diminuição na carbonilação de proteína no respectivo grupo em relação ao NT. Em conclusão, são necessárias oito semanas de treinamento para que ocorram aumento no funcionamento mitocondrial e melhora nos marcadores de estresse oxidativo em fígado de camundongos.

Hormetic effects of reactive oxygen species by exercise: a view from animal studies for successful aging in human

Numerous anti-aging measures have been proposed to cope with age-associated decline of physiological functions and/or onset of diseases, mostly based on free radical (or oxidative stress) theory of aging, though no robust scientific data have been reported to extend human healthspan. This is due to dual (harmful as well as essential) roles of reactive oxygen species (ROS) to a body. Regular moderate exercise provides benefits upregulating defense against oxidative stress in good balance between the opposing dual roles. Sources of ROS in exercise appear to be not only mitochondria as often claimed but also enzymatic reactions catalyzed by NADPH oxidase and other oxidases. It may, therefore, be possible to mimic this aspect of exercise to promote the defense for healthspan extension by other means such as modest alcohol consumption that could upregulate activity of enzymes against oxidative stress.

Common mitochondrial DNA mutations generated through DNA-mediated charge transport

Mutation sites that arise in human mitochondrial DNA as a result of oxidation by a rhodium photooxidant have been identified. HeLa cells were incubated with [Rh(phi)(2)bpy]Cl(3) (phi is 9,10-phenanthrenequinone diimine), an intercalating photooxidant, to allow the complex to enter the cell and bind mitochondrial DNA. Photoexcitation of DNA-bound [Rh(phi)(2)bpy](3+) can promote the oxidation of guanine from a distance through DNA-mediated charge transport. After two rounds of photolysis and growth of cells incubated with the rhodium complex, DNA mutations in a portion of the mitochondrial genome were assessed via manual sequencing. The mutational pattern is consistent with dG to dT transversions in the repetitive guanine tracts. Significantly, the mutational pattern found overlaps oxidative damage hot spots seen previously. These mutations are found within conserved sequence block II, a critical regulatory element involved in DNA replication, and these have been identified as sites of low oxidation potential to which oxidative damage is funneled. On the basis of this mutational analysis and its correspondence to sites of long-range oxidative damage, we infer a critical role for DNA charge transport in generating these mutations and, thus, in regulating mitochondrial DNA replication under oxidative stress.

Exercise improves import of 8-oxoguanine DNA glycosylase into the mitochondrial matrix of skeletal muscle and enhances the relative activity

Exercise has been shown to modify the level/activity of the DNA damage repair enzyme 8-oxoguanine-DNA glycosylase (OGG1) in skeletal muscle. We have studied the impact of regular physical training (8 weeks of swimming) and detraining (8 weeks of rest after an 8-week training session) on the activity of OGG1 in the nucleus and mitochondria as well as its targeting to the mitochondrial matrix in skeletal muscle. Neither exercise training nor detraining altered the overall levels of reactive species; however, mitochondrial levels of carbonylated proteins were decreased in the trained group as assessed by electron spin resonance and biochemical approaches. Importantly, nuclear OGG1 activity was increased by daily exercise training, whereas detraining reversed the up-regulating effect of training. Interestingly, training decreased the outer-membrane-associated mitochondrial OGG1 levels, whereas detraining reversed this effect. These results suggest that exercise training improves OGG1 import into the mitochondrial matrix, thereby increasing OGG1-mediated repair of oxidized guanine bases. Taken together, our data suggest that physical inactivity could impair the mitochondrial targeting of OGG1; however, exercise training increases OGG1 levels/activity in the nucleus and specific activity of OGG1 in mitochondrial compartments, thereby augmenting the repair of oxidized nuclear and mitochondrial DNA bases.

Oxidative stress as a mediator of life history trade-offs: mechanisms, measurements and interpretation

The concept of trade-offs is central to our understanding of life-history evolution. The underlying mechanisms, however, have been little studied. Oxidative stress results from a mismatch between the production of damaging reactive oxygen species (ROS) and the organism's capacity to mitigate their damaging effects. Managing oxidative stress is likely to be a major determinant of life histories, as virtually all activities generate ROS. There is a recent burgeoning of interest in how oxidative stress is related to different components of animal performance. The emphasis to date has been on immediate or short-term effects, but there is an increasing realization that oxidative stress will influence life histories over longer time scales. The concept of oxidative stress is currently used somewhat loosely by many ecologists, and the erroneous assumption often made that dietary antioxidants are necessarily the major line of defence against ROS-induced damage. We summarize current knowledge on how oxidative stress occurs and the different methods for measuring it, and highlight where ecologists can be too simplistic in their approach. We critically review the potential role of oxidative stress in mediating life-history trade-offs, and present a framework for formulating appropriate hypotheses and guiding experimental design. We indicate throughout potentially fruitful areas for further research.

Muscle-derived ROS and thiol regulation in muscle fatigue

Muscles produce oxidants, including reactive oxygen species (ROS) and reactive nitrogen species (RNS), from a variety of intracellular sources. Oxidants are detectable in muscle at low levels during rest and at higher levels during contractions. RNS depress force production but do not appear to cause fatigue of healthy muscle. In contrast, muscle-derived ROS contribute to fatigue because loss of function can be delayed by ROS-specific antioxidants. Thiol regulation appears to be important in this biology. Fatigue causes oxidation of glutathione, a thiol antioxidant in muscle fibers, and is reversed by thiol-specific reducing agents. N-acetylcysteine (NAC), a drug that supports glutathione synthesis, has been shown to lessen oxidation of cellular constituents and delay muscle fatigue. In humans, NAC pretreatment improves performance of limb and respiratory muscles during fatigue protocols and extends time to task failure during volitional exercise. These findings highlight the importance of ROS and thiol chemistry in fatigue, show the feasibility of thiol-based countermeasures, and identify new directions for mechanistic and translational research.

Hormetic effects of regular exercise in aging: correlation with oxidative stress

To explore mechanisms of the beneficial consequences of regular exercise, we studied the effects of regular swimming and treadmill exercise on oxidative stress in the brain and liver of rats. Protein carbonyl was significantly reduced and the activity of proteasome was upregulated in the brain extracts of young and middle-aged animals after 9 weeks of swimming training. Furthermore, their cognitive functions were significantly improved. In separate experiments, the activation of transcription nuclear factor kappaB was attenuated in the liver of old rats after 8 weeks of regular treadmill exercise and the DNA binding activity of glucocorticoid receptor reduced with age was restored, suggesting that inflammatory reactions are alleviated by the regimen. This was accompanied by upregulation of the glutathione level and reduced reactive oxygen species generation. Similar training reduced the 8-oxodeoxyguanosine content in the nuclear and mitochondrial DNA of the liver of old rats. Thus, these findings, together with reports of other investigators, suggest that moderate regular exercise attenuates oxidative stress. The mild oxidative stress possibly elicited by regular exercise appears to manifest a hormesis-like effect in nonmuscular tissues, constituting beneficial mechanisms of exercise by adaptively upregulating various antioxidant mechanisms, including antioxidative and repair-degradation enzymes for damaged molecules. Importantly, the adaptation induced by regular exercise was effective even if initiated late in life.

The basal levels of 8-oxoG and other oxidative modifications in intact mitochondrial DNA are low even in repair-deficient (Ogg1(-/-)/Csb(-/-)) mice

Mitochondrial DNA (mtDNA) is assumed to be highly prone to damage by reactive oxygen species (ROS) because of its location in close proximity to the mitochondrial electron transport chain. Accordingly, mitochondrial oxidative DNA damage has been hypothesized to be responsible for various neurological diseases, ageing and cancer. Since 7,8-dihydro-8-oxoguanine (8-oxoG), one of the most frequent oxidative base modifications, is removed from the mitochondrial genome by the glycosylase OGG1, the basal levels of this lesion are expected to be highly elevated in Ogg1(-/-) mice. To investigate this hypothesis, we have used a mtDNA relaxation assay in combination with various repair enzymes (Fpg, MutY, endonuclease III, endonuclease IV) to determine the average steady-state number of oxidative DNA modifications within intact (supercoiled) mtDNA from the livers of wild-type mice and those deficient in OGG1 and/or the Cockayne syndrome B (CSB) protein for mice aged up to 23 months. The levels of all types of oxidative modifications were found to be less than 12 per million base pairs, and the difference between wild-type and repair-deficient (Ogg1(-/-)/Csb(-/-)) mice was not significant. Thus, the increase of 8-oxoG caused by the repair deficiency in intact mtDNA is not much higher than in the nuclear DNA, i.e., not more than a few modifications per million base pairs. Based on these data, it is hypothesized that the load of oxidative base modifications in mtDNA is efficiently reduced during replication even in the absence of excision repair.