A cigarette component acrolein induces accelerated senescence in human diploid fibroblast IMR-90 cells

Cigarette smoking causes various diseases, including lung cancer and cardiovascular disease, and reduces life span, though the mechanisms are not well understood. We hypothesize that smoking may cause cellular mitochondrial dysfunction and oxidative stress, leading to aging acceleration. In the present study, we tested the effects of acrolein, a major representative smoking toxicant, on human lung fibroblast IMR-90 cells with regard to cellular senescence, oxidative stress, and mitochondrial function. The results showed that subacute treatment with low dose of acrolein induces the following events compared to the control cells: cell senescence demonstrated by increases in the activity of β-galactosidase, the higher expression of p53 and p21, decreases in DNA synthesis, Sirt1 expression, and telomere length; oxidative stress occurred as the increases in the production of reactive oxygen species, DNA damage, and protein oxidation; and mitochondrial dysfunction shown as decreases in the mitochondrial membrane potential, mitochondrial biogenesis regulator PGC-1 alpha and mitochondria complex I, II, III, and V. These results suggest that acrolein may accelerate aging through the mechanism of increasing oxidative stress and mitochondrial dysfunction.     

Sustained inhibition of oxidative phosphorylation impairs cell proliferation and induces premature senescence in human fibroblasts

The mitochondrial theory of aging predicts that functional alterations in mitochondria contribute to the aging process. Whereas this hypothesis implicates increased production of reactive oxygen species (ROS) as a driving force of the aging process, little is known about molecular mechanisms by which mitochondrial impairment might contribute to aging. Using cellular senescence as a model for human aging, we have recently reported partial uncoupling of the respiratory chain in senescent human fibroblasts. In the present communication, we address a potential cause-effect relationship between mitochondrial impairment and the appearance of a senescence-like phenotype in young cells. We found that treatment by antimycin A delays proliferation and induces premature senescence in a subset of the cells, associated with increased reactive oxygen species (ROS) production. Quenching of ROS by antioxidants did however not restore proliferation capacity nor prevent premature senescence. Premature senescence is also induced upon chronic exposure to oligomycin, irrespective of ROS production, and oligomycin treatment induced the up-regulation of the cdk inhibitors p16, p21 and p27, which are also up-regulated in replicative senescence. Thus, besides the well-established influence of ROS on proliferation and senescence, a reduction in the level of oxidative phosphorylation is causally related to reduced cell proliferation and the induction of premature senescence.     

Replicative senescence of human fibroblasts: the role of Ras-dependent signaling and oxidative stress

Replicative senescence of human fibroblasts is a widely used cellular model for human aging. While it is clear that telomere erosion contributes to the development of replicative senescence, it is assumed that additional factors contribute to the senescent phenotype. The free radical theory of aging suggests that oxidative damage is a major cause of aging; furthermore, the expression of activated oncogenes, such as oncogenic Ras, can induce premature senescence in primary cells. The functional relation between the various inducers of senescence is not known. The present study was guided by the hypothesis that constitutive activation of normal, unmutated Ras may contribute to senescence-induced growth arrest in senescent human fibroblasts. When various branches of Ras-dependent signaling were investigated, constitutive activation of the Ras/Raf/MEK/ERK pathway was not observed. To evaluate the role of oxidative stress for the senescent phenotype, we also investigated stress-related protein kinases. While we found no evidence for alterations in the activity of p38, we could detect an increased activity of Jun kinase in senescent fibroblasts. We also found higher levels of reactive oxygen species (ROS) in senescent fibroblasts compared to their younger counterparts. The accumulation of ROS in senescent cells may be related to the constitutive activation of Jun kinase.     

Cultured senescent myoblasts derived from human vastus lateralis exhibit normal mitochondrial ATP synthesis capacities with correlating concomitant ROS production while whole cell ATP production is decreased

The free radical theory of aging says that increased oxidative stress and mitochondrial dysfunction are associated with old age. In the present study we have investigated the effects of cellular senescence on muscle energetic by comparing mitochondrial content and function in cultured muscle satellite cells at early and late passage numbers. We show that cultured muscle satellite cells undergoing senescence express a reduced mitochondrial mass, decreased whole cell ATP level, normal to increased mitochondrial ATP production under ATP utilization, increased mitochondrial membrane potential and increased superoxide/mitochondrial mass and hydrogen peroxide/mitochondrial mass ratios. Moreover, the increased ROS production correlates with the corresponding mitochondrial ATP production. Thus, myotubes differentiated from human myoblasts undergoing senescence have a reduced mitochondrial content, but the existent mitochondria express normal to increased functional capabilities. The present data suggest that the origin of aging lies outside the mitochondria and that a malfunction in the cell might be preceding and initiating the increase of mitochondrial ATP synthesis and concomitant ROS production in the single mitochondrion in response to decreased mitochondrial mass and reduced extra-mitochondrial energy supply. This then can lead to the increased damage of DNA, lipids and proteins of the mitochondria as postulated by the free radical theory of aging.     

Alcohol and mitochondria: a dysfunctional relationship

Mitochondria are intimately involved in the generation of and defense against reactive oxygen species (ROS). Mitochondria are themselves targets of oxidative stress and also contribute to mechanisms by which oxidative stress-related signals control cell fate. Ethanol promotes oxidative stress, both by increasing ROS formation and by decreasing cellular defense mechanisms. These effects of ethanol are prominent in the liver, the major site of ethanol metabolism in the body. The question remains to what extent this contributes to ethanol-dependent tissue damage or the susceptibility of cells to other stressors. In this review, we consider how mitochondrial actions of ethanol influence oxidative stress management of liver cells. Mitochondrial electron transport constitutes the major intracellular source of ROS, and ethanol treatment imposes conditions that promote ROS formation by mitochondria, the effects of which may be enhanced by a decrease in mitochondrial oxidative stress defenses. A significant target of ethanol-related increases in oxidative stress is mitochondrial DNA. Ethanol-induced damage to mitochondrial DNA, if not adequately repaired, impairs mitochondrial function, which further increases oxidative stress in the cell, leading to a vicious cycle of accumulating cell damage that is more apparent with advancing age. Uncontrolled mitochondrial formation of ROS promotes the inappropriate activation of the mitochondrial permeability transition, increasing the sensitivity of cells to other pro-apoptotic or damage signals. In combination with ethanol-induced defects in mitochondrial function, these alterations may promote both apoptotic and necrotic cell death in response to otherwise benign or beneficial challenges and contribute to the onset or progression of alcohol-induced liver diseases.     

Pleiotropic age-dependent effects of mitochondrial dysfunction on epidermal stem cells

Tissue homeostasis declines with age partly because stem/progenitor cells fail to self-renew or differentiate. Because mitochondrial damage can accelerate aging, we tested the hypothesis that mitochondrial dysfunction impairs stem cell renewal or function. We developed a mouse model, Tg(KRT14-cre/Esr1)^20Efu/J × Sod2^tm1Smel, that generates mitochondrial oxidative stress in keratin 14-expressing epidermal stem/progenitor cells in a temporally controlled manner owing to deletion of Sod2, a nuclear gene that encodes the mitochondrial antioxidant enzyme superoxide dismutase 2 (Sod2). Epidermal Sod2 loss induced cellular senescence, which irreversibly arrested proliferation in a fraction of keratinocytes. Surprisingly, in young mice, Sod2 deficiency accelerated wound closure, increasing epidermal differentiation and reepithelialization, despite the reduced proliferation. In contrast, at older ages, Sod2 deficiency delayed wound closure and reduced epidermal thickness, accompanied by epidermal stem cell exhaustion. In young mice, Sod2 deficiency accelerated epidermal thinning in response to the tumor promoter 12-O-tetradecanoylphorbol-13-acetate, phenocopying the reduced regeneration of older Sod2-deficient skin. Our results show a surprising beneficial effect of mitochondrial dysfunction at young ages, provide a potential mechanism for the decline in epidermal regeneration at older ages, and identify a previously unidentified age-dependent role for mitochondria in skin quality and wound closure.Stem and progenitor cells are crucial for tissue homeostasis, repair, and regeneration. In response to injury, they proliferate and differentiate to replace damaged or dysfunctional cells (1, 2). In the skin, epidermal basal cells differentiate to form distinct epidermal layers: the stratum basale (SB), stratum spinosum (SS), stratum granulosum (SG), and stratum corneum (SC) (3). In the SB layer, epidermal basal cells are identified by nuclei that stain strongly with hematoxylin and eosin (H&E). These cells differentiate to form the SS layer, composed mainly of cells with lightly stained nuclei. SS cells differentiate into the SG layer, identified by cells with prominent cytoplasmic granules, which terminally differentiate to form the SC layer containing acidophilic anucleated cells.Tissue homeostasis declines with age partly because stem/progenitor cells fail to self-renew or differentiate (4). Oxidative damage can contribute to this decline in compartments such as the hematopoietic system (5, 6). Aging is caused by intrinsic and extrinsic factors that cooperate to drive aging phenotypes (7). Mitochondrial dysfunction has been suggested to play a major role in intrinsic aging (8). Furthermore, mitochondrial damage is associated with extrinsic aging, particularly ultraviolet (UV) radiation-induced photoaging in the skin (9). Thus, mitochondrial damage may be a common link between intrinsic and extrinsic aging.Mitochondrial stress can decrease life span and health span. Superoxide dismutase 2 (SOD2) scavenges mitochondrial superoxide to protect against oxidative damage. Sod2 deficiency decreases life span in several species. Mice with constitutive Sod2 deficiency are neonatally lethal on multiple genetic backgrounds, presenting with neurodegeneration, spongiform encephalopathy, cardiomyopathy, hepatic fat accumulation, and failure to thrive; cells from these mice exhibit impaired spare respiratory capacity, genomic instability, and mitochondrial functional defects (10–17). Sod2^−/− cells also have a reduced proliferative capacity (17), consistent with the finding that constitutive Sod2 deficiency induces cellular senescence, a tumor-suppressive mechanism that irreversibly arrests cell proliferation (18), in mouse skin (19). Conversely, overexpression of mitochondrial antioxidants can partly rescue age-related pathologies (14, 20), increase organismal life span (21), and prolong stem cell replicative life span (22).Interestingly, some studies have suggested that mild mitochondrial stress can be beneficial (23). Here we show that mitochondrial stress owing to Sod2 deficiency in epidermal cells can have positive or negative effects on skin regeneration, and that these effects depend on age. We show that epidermal Sod2 deficiency induces cellular senescence, which reduces proliferative capacity in the skin but stimulates the differentiation of epidermal stem/progenitor cells. This stimulation accelerates wound closure in young mice, but the proliferative decline drains stem cell pools with aging and retards wound closure. Our findings extend the concept of antagonistic pleiotropy, which stipulates that gene action can be beneficial at young ages but deleterious at older ages, to mitochondrial function in the skin.     

Drusen, choroidal neovascularization, and retinal pigment epithelium dysfunction in SOD1-deficient mice: a model of age-related macular degeneration

Oxidative stress has long been linked to the pathogenesis of neurodegenerative diseases; however, whether it is a cause or merely a consequence of the degenerative process is still unknown. We show that mice deficient in Cu, Zn-superoxide dismutase (SOD1) have features typical of age-related macular degeneration in humans. Investigations of senescent Sod1(-/-) mice of different ages showed that the older animals had drusen, thickened Bruch's membrane, and choroidal neovascularization. The number of drusen increased with age, and exposure of young Sod1(-/-) mice to excess light induced drusen. The retinal pigment epithelial cells of Sod1(-/-) mice showed oxidative damage, and their beta-catenin-mediated cellular integrity was disrupted, suggesting that oxidative stress may affect the junctional proteins necessary for the barrier integrity of the retinal pigment epithelium. These observations strongly suggest that oxidative stress may play a causative role in age-related retinal degeneration, and our findings provide evidence for the free radical theory of aging. In addition, these results demonstrate that the Sod1(-/-) mouse is a valuable animal model to study human age-related macular degeneration.     

Age-dependent mitochondrial energy dynamics in the mice heart: Role of superoxide dismutase-2

The aging process alters cardiac physiology, decreases the number of cardiomyocytes and alters the energy metabolism. Mitochondrial dysfunction in aging is believed to cause these functional and phenotypic changes in the heart. Although precise understanding of alterations of mitochondrial respiration in aging is necessary to manage heart diseases in the elderly population conflicting data on the function of specific complex of electron transport chain of the heart mitochondria limits the intervention process. We have addressed these issues using the assay of mitochondrial coupling and electron flow to assess specific functional defects in mitochondria isolated from young or aged mice. Our results demonstrate that cardiac mitochondria from older mice utilize oxygen at a decreased rate via complex I, II or IV compared to younger mice. We further show that mitochondrial function decreases in young Sod2^+/− mice heart compared to young wildtype mice. However, the mitochondrial function remains unchanged in older Sod2^+/− mice heart compared to younger Sod2^+/− mice heart. Further, the oxygen consumption remains similar in old wildtype mice and old Sod2^+/− mice heart mitochondria. The expression and activity of Sod2 in young or old Sod2^+/− mice heart remain unchanged. These data demonstrate that decreased oxygen utilization in older age could have resulted in decreased mitochondrial ROS-mediated oxidative damage requiring less Sod2 for protection against mitochondrial oxidative stress in older wildtype or older Sod2^+/− mice.     

The mitochondrial theory of aging: Insight from transgenic and knockout mouse models

A substantial body of evidence has accumulated over the past 35 years in support of a role for oxidative damage to the mitochondrial respiratory chain and mitochondrial DNA in the determination of mammalian lifespan. The goal of this review is to provide a concise summary of recent studies using transgenic and knockout mouse models with altered expression of mitochondrial antioxidant enzymes (MnSOD (Sod2Tg and Sod2^+/−), thioredoxin 2 (Trx2^+/−), mitochondrial targeted catalase (mCAT) and mutant mice models that have been genetically manipulated to increase mitochondrial deletions or mutations (Polγ^D257A/D257A mutant mice) to examine the role of mitochondrial oxidative stress in aging. The majority of studies using these strategies do not support a clear role for mitochondrial oxidative stress or a vicious cycle of oxidative damage in the determination of lifespan in mice and furthermore do not support the free radical theory of aging. However, several key questions remain to be addressed and clearly more studies are required to fully understand the role of mitochondria in age-related disease and aging.     

Oxidative stress in lung epithelial cells from patients with idiopathic interstitial pneumonias.

Lung epithelial cells are a primary target for reactive oxygen species (ROS). ROS can cause oxidative deoxyribonucleic acid modification, such as 8-hydroxy-deoxyguanosine (8-OHdG). A human homologue of the MutT protein (hMTH1) prevents this modification. Mitochondria are the most important cellular source of ROS and may be susceptible to oxidative damage. The purpose of this study is to investigate oxidative stress and mitochondrial damage in lung epithelial cells from idiopathic interstitial pneumonias (IIPs). The authors analysed 8-OHdG, hMTH1, and mitochondrial proteins on lung specimens from 13 patients with IlPs consisted of eight patients with usual interstitial pneumonia and five patients with nonspecific interstitial pneumonia using Western blot analysis and immunohistochemistry. Immunoreactivity for 8-OHdG and hMTH1 was significantly increased in the lung epithelial cells from patients with IIPs compared with controls. The expression of hMTH1 was localised in the nuclear and cytoplasmic, but not the mitochondrial, fraction of lung homogenates. Immunoreactivity for mitochondrial protein and cytochrome c oxidase complex subunit IV was increased in the lung epithelial cells from patients with IIPs compared with controls. The current study concludes that oxidative stress may participate in epithelial cell damage in idiopathic interstitial pneumonia, and that increased mitochondrial mass may associate with increased reactive oxygen species production in idiopathic interstitial pneumonia.     

The immunopathogenic role of reactive oxygen species in Alzheimer disease

Reactive oxygen species (ROS) are produced in many normal and abnormal processes in humans, including atheroma, asthma, joint diseases, cancer, and aging. Basal levels of ROS production in cells could be related to several physiological functions including cell proliferation, apoptosis and homeostasis. However, excessive ROS production above basal levels would impair and oxidize DNA, lipids, sugars and proteins and consequently result in dysfunction of these molecules within cells and finally cell death. A leading theory of the cause of aging indicates that free radical damage and oxidative stress play a major role in the pathogenesis of Alzheimer disease (AD). Because the brain utilizes 20% more oxygen than other tissues that also undergo mitochondrial respiration, the potential for ROS exposure increases. In fact, AD has been demonstrated to be highly associated with cellular oxidative stress, including augmentation of protein oxidation, protein nitration, glycoloxidation and lipid peroxidation as well as accumulation of Amyloid β (Aβ). The treatment with anti-oxidant compounds can provide protection against oxidative stress and Aβ toxicity. In this review, our aim was to clarify the role of ROS in pathogenesis of AD and will discuss therapeutic efficacy of some antioxidants studies in recent years in this disease.     

Somatic mtDNA mutations cause aging phenotypes without affecting reactive oxygen species production

The mitochondrial theory of aging proposes that reactive oxygen species (ROS) generated inside the cell will lead, with time, to increasing amounts of oxidative damage to various cell components. The main site for ROS production is the respiratory chain inside the mitochondria and accumulation of mtDNA mutations, and impaired respiratory chain function have been associated with degenerative diseases and aging. The theory predicts that impaired respiratory chain function will augment ROS production and thereby increase the rate of mtDNA mutation accumulation, which, in turn, will further compromise respiratory chain function. Previously, we reported that mice expressing an error-prone version of the catalytic subunit of mtDNA polymerase accumulate a substantial burden of somatic mtDNA mutations, associated with premature aging phenotypes and reduced lifespan. Here we show that these mtDNA mutator mice accumulate mtDNA mutations in an approximately linear manner. The amount of ROS produced was normal, and no increased sensitivity to oxidative stress-induced cell death was observed in mouse embryonic fibroblasts from mtDNA mutator mice, despite the presence of a severe respiratory chain dysfunction. Expression levels of antioxidant defense enzymes, protein carbonylation levels, and aconitase enzyme activity measurements indicated no or only minor oxidative stress in tissues from mtDNA mutator mice. The premature aging phenotypes in mtDNA mutator mice are thus not generated by a vicious cycle of massively increased oxidative stress accompanied by exponential accumulation of mtDNA mutations. We propose instead that respiratory chain dysfunction per se is the primary inducer of premature aging in mtDNA mutator mice.     

Testing hypotheses of aging in long-lived mice of the genus Peromyscus: association between longevity and mitochondrial stress resistance, ROS detoxification pathways, and DNA repair efficiency

In the present review we discuss the potential use of two long-lived mice of the genus Peromyscus—the white-footed mouse (P. leucopus) and the deer mouse (P. maniculatus) maximum lifespan potential ～8 years for both—to test predictions of theories about aging from the oxidative stress theory, mitochondrial theory and inflammatory theory. Previous studies have shown that P. leucopus cells exhibit superior antioxidant defense mechanisms and lower cellular production of reactive oxygen species (ROS) than do cells of the house mouse, Mus musculus (maximum lifespan ～3.5 years). We present new data showing that mitochondria in P. leucopus cells produce substantially less ROS than mitochondria in M. musculus cells, and that P. leucopus mitochondria exhibit superior stress resistance to those of M. musculus. We also provide evidence that components of the DNA repair system (e.g., pathways involved in repair of DNA damage induced by γ-irradiation) are likely to be more efficient in P. leucopus than in M. musculus. We propose that mitochondrial stress resistance, ROS detoxification pathways and more efficient DNA repair contribute to the previously documented resistance of P. leucopus cells toward oxidative stress-induced apoptosis. The link between these three pathways and species longevity is discussed.     

Mitochondrial alterations, cellular response to oxidative stress and defective degradation of proteins in aging

Respiratory function decline and increase ofoxidative stress in mitochondria have beenproposed as important contributors to humanaging. A wide spectrum of alterations in agedindividuals and senescent cells are similar andare correlated to cellular response tosublethal dose of oxidative stress. Thesealterations and responses include: (1) declinein mitochondrial respiratory function; (2)increase in the rate of production of reactiveoxygen species (ROS); (3) accumulation ofmitochondrial DNA (mtDNA) mutations; (4)increase in the levels of oxidative damage toDNA, protein, and lipids; and (5) decrease inthe capacities of degradation of oxidativelydamaged proteins and other macromolecules. Responses to oxidative stress and theirsubsequent interactions in tissues result inthe deleterious effect of ROS on the cellularfunction, which culminate in aging anddegenerative diseases. In this review, wefocus on the roles that ROS play in age-relatedoxidative damage to mtDNA and proteins andoxidative stress responses at the molecular andcellular levels. The alterations of geneexpression profiles elicited by oxidativestress in aging animals are discussed. Wesuggest that the increase in mitochondrialproduction of ROS and decline in the cellularcapacity to cope with oxidative stress andsubsequent accumulation of mtDNA mutations andoxidized proteins play an important role in theaging process.     

The role of the electron transport gene SDHC on lifespan and cancer

Much attention has been focused on the hypothesis that oxidative damage contributes to cellular and organismal aging. A mev-1 mutation in the cytochrome b large subunit (SDHC) of complex II results in superoxide anion (O(2)(-)) overproduction and therefore leads to apoptosis and precocious aging in the nematode Caenorhabditis elegans. To extend these data, a transgenic mouse cell line was constructed with a homologous mutation to mev-1. Many of the mutant nematode phenotypes (e.g., increased superoxide anion production, apoptosis) were recapitulated in the mouse. In addition, a significant fraction of the cells that survived apoptosis were transformed. These data support the notion that oxidative stress from mitochondria play an important role of both apoptosis, which leads to precocious aging, and cancer.     

Accumulation of oxidative DNA damage restricts the self-renewal capacity of human hematopoietic stem cells

Stem cells of highly regenerative organs including blood are susceptible to endogenous DNA damage caused by both intrinsic and extrinsic stress. Response mechanisms to such stress equipped in hematopoietic stem cells (HSCs) are crucial in sustaining hematopoietic homeostasis but remain largely unknown. In this study, we demonstrate that serial transplantation of human HSCs into immunodeficient mice triggers replication stress that induces incremental elevation of intracellular reactive oxygen species (ROS) levels and the accumulation of persistent DNA damage within the human HSCs. This accumulation of DNA damage is also detected in HSCs of clinical HSC transplant patients and elderly individuals. A forced increase of intracellular levels of ROS by treatment with a glutathione synthetase inhibitor aggravates the extent of DNA damage, resulting in the functional impairment of HSCs in vivo. The oxidative DNA damage activates the expression of cell-cycle inhibitors in a HSC specific manner, leading to the premature senescence among HSCs, and ultimately to the loss of stem cell function. Importantly, treatment with an antioxidant can antagonize the oxidative DNA damage and eventual HSC dysfunction. The study reveals that ROS play a causative role for DNA damage and the regulation of ROS have a major influence on human HSC aging.     

Neurons from senescence-accelerated SAMP8 mice are protected against frailty by the sirtuin 1 promoting agents melatonin and resveratrol

The senescence-accelerated prone 8 (SAMP8) mouse strain shows early cognitive loss that mimics the deterioration of learning and memory in the elderly and is widely used as an animal model of aging. SAMP8 mouse brain suffers oxidative stress, as well as tau- and amyloid-related pathology. Mitochondrial dysfunction and the subsequent increase in cellular oxidative stress are central to the aging processes of the organism. Here, we examined the mitochondrial status of neocortical neurons cultured from SAMP8 and senescence-accelerated-resistant (SAMR1) mice. SAMP8 mouse mitochondria showed a reduced membrane potential and higher vulnerability to inhibitors and uncouplers than SAMR1 mitochondria. DL-buthionine-[S,R]-sulfoximine (BSO) caused greater oxidative damage in neurons from SAMP8 mice than in those from SAMR1 mice. This increased vulnerability, indicative of frailty-associated senescence, was protected by the anti-aging agents melatonin and resveratrol. The sirtuin 1 inhibitor, sirtinol, demonstrated that the neuroprotection against BSO was partially mediated by increased sirtuin 1 expression. Melatonin, like resveratrol, enhanced sirtuin 1 expression in neuron cultures of SAMR1 and SAMP8 mice. Therefore, a deficiency in the neuroprotection and longevity of the sirtuin 1 pathway in SAMP8 neurons may contribute to the early age-related brain damage in these mice. This supports the therapeutic use of sirtuin 1-enhancing agents against age-related nerve cell dysfunction and brain frailty.     

Mitochondria, oxidative DNA damage, and aging

Protection from reactive oxygen species (ROS) and from mitochondrial oxidative damage is well known to be necessary to longevity. The relevance of mitochondrial DNA (mtDNA) to aging is suggested by the fact that the two most commonly measured forms of mtDNA damage, deletions and the oxidatively induced lesion 8-oxo-dG, increase with age. The rate of increase is species-specific and correlates with maximum lifespan. It is less clear that failure or inadequacies in the protection from reactive oxygen species (ROS) and from mitochondrial oxidative damage are sufficient to explain senescence. DNA containing 8-oxo-dG is repaired by mitochondria, and the high ratio of mitochondrial to nuclear levels of 8-oxo-dG previously reported are now suspected to be due to methodological difficulties. Furthermore, MnSOD −/+ mice incur higher than wild type levels of oxidative damage, but do not display an aging phenotype. Together, these findings suggest that oxidative damage to mitochondria is lower than previously thought, and that higher levels can be tolerated without physiological consequence. A great deal of work remains before it will be known whether mitochondrial oxidative damage is a “clock” which controls the rate of aging. The increased level of 8-oxo-dG seen with age in isolated mitochondria needs explanation. It could be that a subset of cells lose the ability to protect or repair mitochondria, resulting in their incurring disproportionate levels of damage. Such an uneven distribution could exceed the reserve capacity of these cells and have serious physiological consequences. Measurements of damage need to focus more on distribution, both within tissues and within cells. In addition, study must be given to the incidence and repair of other DNA lesions, and to the possibility that repair varies from species to species, tissue to tissue, and young to old.     

Evolving insight into the role of mitochondrial DNA mutations in aging

Mitochondria have occupied a central place in theories on the underlying cellular mechanisms of eukaryotic aging for several decades and much debate has ensued regarding the role of oxidative stress and mitochondrial genomic damage in these processes. Mouse models with greatly enhanced mitochondrial mutagenesis have produced dramatic aging-like phenotypes but recent results have led some to reassess whether such models are relevant to naturally occurring aging mechanisms. Here, we discuss the evolving insight that may be gained from these models regarding the contribution of mitochondrial DNA mutations to aging.