Age-related mitochondrial DNA deletions: effect of dietary restriction

The signaling pathways that mediate the anti-cancer effects of caloric restriction

Caloric restriction (CR) has been shown to promote longevity and ameliorate aging-associated diseases, including cancer. Extensive research over recent decades has revealed that CR reduces IGF-1/PI3K/AKT signaling and increases sirtuin signaling. We recently found that CR also enhances ALDOA/DNA-PK/p53 signaling. In the present review, we summarize the molecular mechanisms underlying the modulation of the IGF-1/PI3K/AKT pathway, sirtuin signaling, and the ALDOA/DNA-PK/p53 pathway by CR. We also summarize the evidence concerning the roles of these signaling pathways in carcinogenesis, and discuss how they are regulated by CR. Finally, we discuss the crosstalk between these signaling pathways.

A comparison among the tissue-specific effects of aging and calorie restriction on TFAM amount and TFAM-binding activity to mtDNA in rat

BACKGROUND

Mitochondrial Transcription Factor A (TFAM) is regarded as a histone-like protein of mitochondrial DNA (mtDNA), performing multiple functions for this genome. Aging affects mitochondria in a tissue-specific manner and only calorie restriction (CR) is able to delay or prevent the onset of several age-related changes also in mitochondria.

METHODS

Samples of the frontal cortex and soleus skeletal muscle from 6- and 26-month-old ad libitum-fed and 26-month-old calorie-restricted rats and of the livers from 18- and 28-month-old ad libitum-fed and 28-month-old calorie-restricted rats were used to detect TFAM amount, TFAM-binding to mtDNA and mtDNA content.

RESULTS

We found an age-related increase in TFAM amount in the frontal cortex, not affected by CR, versus an age-related decrease in the soleus and liver, fully prevented by CR. The semi-quantitative analysis of in vivo binding of TFAM to specific mtDNA regions, by mtDNA immunoprecipitation assay and following PCR, showed a marked age-dependent decrease in TFAM-binding activity in the frontal cortex, partially prevented by CR. An age-related increase in TFAM-binding to mtDNA, fully prevented by CR, was found in the soleus and liver. MtDNA content presented a common age-related decrease, completely prevented by CR in the soleus and liver, but not in the frontal cortex.

CONCLUSIONS

The modulation of TFAM expression, TFAM-binding to mtDNA and mtDNA content with aging and CR showed a trend shared by the skeletal muscle and liver, but not by the frontal cortex counterpart.

GENERAL SIGNIFICANCE

Aging and CR appear to induce similar mitochondrial molecular mechanisms in the skeletal muscle and liver, different from those elicited in the frontal cortex.

Age- and calorie restriction-related changes in rat brain mitochondrial DNA and TFAM binding

Aging markedly affects mitochondrial biogenesis and functions particularly in tissues highly dependent on the organelle's bioenergetics capability such as the brain's frontal cortex. Calorie restriction (CR) diet is, so far, the only intervention able to delay or prevent the onset of several age-related alterations in different organisms. We determined the contents of mitochondrial transcription factor A (TFAM), mitochondrial DNA (mtDNA), and the 4.8-kb mtDNA deletion in the frontal cortex from young (6-month-old) and aged (26-month-old), ad libitum-fed (AL) and calorie-restricted (CR), rats. We found a 70 % increase in TFAM amount, a 25 % loss in mtDNA content, and a 35 % increase in the 4.8-kb deletion content in the aged AL animals with respect to the young rats. TFAM-specific binding to six mtDNA regions was analyzed by mtDNA immunoprecipitation and semiquantitative polymerase chain reaction (PCR), showing a marked age-related decrease. Quantitative real-time PCR at two subregions involved in mtDNA replication demonstrated, in aged AL rats, a remarkable decrease (60-70 %) of TFAM-bound mtDNA. The decreased TFAM binding is a novel finding that may explain the mtDNA loss in spite of the compensatory TFAM increased amount. In aged CR rats, TFAM amount increased and mtDNA content decreased with respect to young rats' values, but the extent of the changes was smaller than in aged AL rats. Attenuation of the age-related effects due to the diet in the CR animals was further evidenced by the unchanged content of the 4.8-kb deletion with respect to that of young animals and by the partial prevention of the age-related decrease in TFAM binding to mtDNA.

Aging and calorie restriction oppositely affect mitochondrial biogenesis through TFAM binding at both origins of mitochondrial DNA replication in rat liver

Aging affects mitochondria in a tissue-specific manner. Calorie restriction (CR) is, so far, the only intervention able to delay or prevent the onset of several age-related changes also in mitochondria. Using livers from middle age (18-month-old), 28-month-old and 32-month-old ad libitum-fed and 28-month-old calorie-restricted rats we found an age-related decrease in mitochondrial DNA (mtDNA) content and mitochondrial transcription factor A (TFAM) amount, fully prevented by CR. We revealed also an age-related decrease, completely prevented by CR, for the proteins PGC-1α NRF-1 and cytochrome c oxidase subunit IV, supporting the efficiency of CR to forestall the age-related decrease in mitochondrial biogenesis. Furthermore, CR counteracted the age-related increase in oxidative damage to proteins, represented by the increased amount of oxidized peroxiredoxins (PRX-SO3) in the ad libitum-fed animals. An unexpected age-related decrease in the mitochondrial proteins peroxiredoxin III (Prx III) and superoxide dismutase 2 (SOD2), usually induced by increased ROS and involved in mitochondrial biogenesis, suggested a prevailing relevance of the age-reduced mitochondrial biogenesis above the induction by ROS in the regulation of expression of these genes with aging. The partial prevention of the decrease in Prx III and SOD2 proteins by CR also supported the preservation of mitochondrial biogenesis in the anti-aging action of CR. To investigate further the age- and CR-related effects on mitochondrial biogenesis we analyzed the in vivo binding of TFAM to specific mtDNA regions and demonstrated a marked increase in the TFAM-bound amounts of mtDNA at both origins of replication with aging, fully prevented by CR. A novel, positive correlation between the paired amounts of TFAM-bound mtDNA at these sub-regions was found in the joined middle age ad libitum-fed and 28-month-old calorie-restricted groups, but not in the 28-month-old ad libitum-fed counterpart suggesting a quite different modulation of TFAM binding at both origins of replication in aging and CR.

Caloric restriction and redox state: does this diet increase or decrease oxidant production?

Calorie restriction (CR) is well established to enhance the lifespan of a wide variety of organisms, although the mechanisms are still being uncovered. Recently, some authors have suggested that CR acts through hormesis, enhancing the production of reactive oxygen species (ROS), activating stress response pathways, and increasing lifespan. Here, we review the literature on the effects of CR and redox state. We find that there is no evidence in rodent models of CR that an increase in ROS production occurs. Furthermore, results in Caenorhabditis elegans and Saccharomyces cerevisiae suggesting that CR increases intracellular ROS are questionable, and probably cannot be resolved until adequate, artifact free, tools for real-time, quantitative, and selective measurements of intracellular ROS are developed. Overall, the largest body of work indicates that CR improves redox state, although it seems improbable that a global improvement in redox state is the mechanism through which CR enhances lifespan.

Starvation, detoxification, and multidrug resistance in cancer therapy

The selection of chemotherapy drugs is based on the cytotoxicity to specific tumor cell types and the relatively low toxicity to normal cells and tissues. However, the toxicity to normal cells poses a major clinical challenge, particularly when malignant cells have acquired resistance to chemotherapy. This drug resistance of cancer cells results from multiple factors including individual variation, genetic heterogeneity within a tumor, and cellular evolution. Much progress in the understanding of tumor cell resistance has been made in the past 35 years, owing to milestone discoveries such as the identification and characterization of ABC transporters. Nonetheless, the complexity of the genetic and epigenetic rewiring of cancer cells makes drug resistance an equally complex phenomenon that is difficult to overcome. In this review, we discuss how the remarkable changes in the levels of glucose, IGF-I, IGFBP-1 and in other proteins caused by fasting have the potential to improve the efficacy of chemotherapy against tumors by protecting normal cells and tissues and possibly by diminishing multidrug resistance in malignant cells.

Age-related changes in brain mitochondrial DNA deletion and oxidative stress are differentially modulated by dietary fat type and coenzyme Q₁₀

Mitochondria-related oxidative damage is a primary event in aging and age-related neurodegenerative disorders. Some dietary treatments, such as antioxidant supplementation or the enrichment of mitochondrial membranes with less oxidizable fatty acids, reduce lipid peroxidation and lengthen life span in rodents. This study compares life-long feeding on monounsaturated fatty acids (MUFAs), such as virgin olive oil, and n-6 polyunsaturated fatty acids, such as sunflower oil, with or without coenzyme Q₁₀ supplementation, with respect to age-related molecular changes in rat brain mitochondria. The MUFA diet led to diminished age-related phenotypic changes, with lipoxidation-derived protein markers being higher among the older animals, whereas protein carbonyl compounds were lower. It is noteworthy that the MUFA diet prevented the age-related increase in levels of mitochondrial DNA deletions in the brain mitochondria from aged animals. The findings of this study suggest that age-related oxidative stress is related, at the mitochondrial level, to other age-related features such as mitochondrial electron transport and mtDNA alterations, and it can be modulated by selecting an appropriate dietary fat type and/or by suitable supplementation with low levels of the antioxidant/electron carrier molecule coenzyme Q.

Effects of calorie restriction on the age-dependent accumulation of mutations in the small intestine of lacZ-transgenic mice

To understand the effect of calorie restriction on genome maintenance systems, the age-dependent accumulation of mutations in animals maintained on high and low calorie diets was examined using lacZ-transgenic mice. Mice were fed a diet of 95 kcal/w or 65 kcal/w from 2 to 17 months of age. The mutation frequencies in the lacZ gene in epithelial tissues from the small intestine were examined at 12 and 17 months. Mutation frequencies were found to be lower in mice fed with a low calorie diet than in mice fed with a high calorie diet at the two age points. The molecular nature of the mutations was examined with DNA sequencing. It showed a predominance of transversions from G:C to T:A, and this is a typical type of mutation induced by reactive oxygen species. The fraction of this type of mutation among the different types of mutations detected was not affected by calorie restriction. The percentage of the other types of mutation was not influenced either. These results suggest that calorie restriction reduces the age-dependent accumulation of mutations by stimulating or inducing various types of DNA protection and repair systems rather than protecting cells against any specific type of DNA alteration.

Direct quantification of mitochondrial DNA and its 4.9-kb common deletion without DNA purification

Quantitative analysis of mitochondrial DNA (mtDNA) and its common deletion (CD) are sensitive and early markers for mitochondrial mutations and suffering. However, the use of purified DNA can lead to quantification errors because of variable DNA extraction yields due to the significant differences in size and structure between genomic DNA (gDNA) and mtDNA. We report a real-time qPCR-based protocol directly on tissue lysate, without DNA extraction. This method, which allows both absolute and relative measure, increases the measuring accuracy of the mtDNA/gDNA ratio and leads to reliable and more reproducible results when measuring the deleted/total mtDNA ratio.

Biomarker Validation for Aging: Lessons from mtDNA Heteroplasmy Analyses in Early Cancer Detection

The anticipated biological and clinical utility of biomarkers has attracted significant interest recently. Aging and early cancer detection represent areas active in the search for predictive and prognostic biomarkers. While applications differ, overlapping biological features, analytical technologies and specific biomarker analytes bear comparison. Mitochondrial DNA (mtDNA) as a biomarker in both biological models has been evaluated. However, it remains unclear whether mtDNA changes in aging and cancer represent biological relationships that are causal, incidental, or a combination of both. This article focuses on evaluation of mtDNA-based biomarkers, emerging strategies for quantitating mtDNA admixtures, and how current understanding of mtDNA in aging and cancer evolves with introduction of new technologies. Whether for cancer or aging, lessons from mtDNA based biomarker evaluations are several. Biological systems are inherently dynamic and heterogeneous. Detection limits for mtDNA sequencing technologies differ among methods for low-level DNA sequence admixtures in healthy and diseased states. Performance metrics of analytical mtDNA technology should be validated prior to application in heterogeneous biologically-based systems. Critical in evaluating biomarker performance is the ability to distinguish measurement system variance from inherent biological variance, because it is within the latter that background healthy variability as well as high-value, disease-specific information reside.

Localization of abasic sites and single-strand breaks in mitochondrial DNA from brain of aged rat, treated or not with caloric restriction diet

According to the "mitochondrial theory of aging" the lifelong accumulation of various kinds of damage to mitochondrial DNA (mtDNA) has been related to the age-dependent mitochondrial bioenergetic dysfunction. Caloric restriction (CR) diet is able to prevent or delay the onset of several age-related damages to mtDNA. The effects of aging and CR on the presence of abasic sites and single-strand breaks of the sugar-phosphate backbone in mtDNA have been analyzed by applying Ligation Mediated-PCR to a H strand region of brain mtDNA from young and old ad libitum-fed and old CR-treated rats. The region, encompassing the Direct Repeat 1 of the 4,834 bp-long deletion, is highly damaged in the old ad libitum-fed animals with respect to the young ones, whereas in the CR rats it shows a much lower extent of damage. The data confirm, at single nucleotide resolution, the protective effect of CR on the age-related mtDNA damage.

Renin-angiotensin system inhibitors protect against age-related changes in rat liver mitochondrial DNA content and gene expression

Chronic renin-angiotensin system inhibition protects against liver fibrosis, ameliorates age-associated mitochondrial dysfunction and increases rodent lifespan. We hypothesized that life-long angiotensin-II-mediated stimulation of oxidant generation might participate in mitochondrial DNA "common deletion" formation, and the resulting impairment of bioenergetic capacity. Enalapril (10 mg/kg/d) or losartan (30 mg/kg/d) administered during 16.5 months were unable to prevent the age-dependent accumulation of rat liver mitochondrial DNA "common deletion", but attenuated the decrease of mitochondrial DNA content. This evidence - together with the enhancement of NRF-1 and PGC-1 mRNA contents - seems to explain why enalapril and losartan improved mitochondrial functioning and lowered oxidant production, since both the absolute number of mtDNA molecules and increased NRF-1 and PGC-1 transcription are positively related to mitochondrial respiratory capacity, and PGC-1 protects against increases in ROS production and damage. Oxidative stress evoked by abnormal respiratory function contributes to the pathophysiology of mitochondrial disease and human aging. If the present mitochondrial actions of renin-angiotensin system inhibitors are confirmed in humans they may modify the therapeutic significance of that strategy.

Caloric restriction and genomic stability

Caloric restriction (CR) reduces the incidence and progression of spontaneous and induced tumors in laboratory rodents while increasing mean and maximum life spans. It has been suggested that CR extends longevity and reduces age-related pathologies by reducing the levels of DNA damage and mutations that accumulate with age. This hypothesis is attractive because the integrity of the genome is essential to a cell/organism and because it is supported by observations that both cancer and immunological defects, which increase significantly with age and are delayed by CR, are associated with changes in DNA damage and/or DNA repair. Over the last three decades, numerous laboratories have examined the effects of CR on the integrity of the genome and the ability of cells to repair DNA. The majority of studies performed indicate that the age-related increase in oxidative damage to DNA is significantly reduced by CR. Early studies suggest that CR reduces DNA damage by enhancing DNA repair. With the advent of genomic technology and our increased understanding of specific repair pathways, CR has been shown to have a significant effect on major DNA repair pathways, such as NER, BER and double-strand break repair.

Tissue-Specific Effect of Age and Caloric Restriction Diet on Mitochondrial DNA Content

The effect of age and caloric-restriction (CR) diet on mitochondrial DNA (mtDNA) content in different rat tissues was investigated. A decrease of the mtDNA content occurs with aging in liver and soleus muscle, whereas there is no age-related significant change of mtDNA content in brain. CR fully reverses the age-dependent loss of mtDNA in liver and soleus, whereas it results in a significant increase of mtDNA amount above the value of aged ad libitum fed rats in brain. These results further support the tissue-specific effect of CR, likely because of the different dependence of tissues on external nutrient uptake.

Age-related mitochondrial DNA deletion in rat liver depends on dietary fat unsaturation

We fed male Wistar rats lifelong on virgin olive (rich in the monounsaturated oleic acid) or sunflower (rich in the polyunsaturated linoleic acid) oil-based diets. At 6 and 24 months, liver mitochondria were analyzed for a mitochondrial DNA (mtDNA) deletion, reactive oxygen species, antioxidants, and ultrastructural alterations. An aging-related increase in the relative amount of the deletion was observed for both dietary groups, being higher in animals fed sunflower oil. Oxidative stress was lower in virgin olive oil-fed animals. Aging led to higher superoxide dismutase, catalase, and glutathione peroxidase activities and increased alpha-tocopherol and coenzyme Q. Mitochondria from aged animals fed sunflower oil exhibited a lower number of cristae and a higher circularity. Results suggest that the age-related increase of the relative amount of deleted mtDNA depends on fat unsaturation. Moreover, the studied mtDNA deletion was correlated with mitochondrial oxidative stress and ultrastructural alterations.

Alleviation of Oxidative Damage in Multiple Tissues in Rats with Streptozotocin-Induced Diabetes by Rice Bran Oil Supplementation

The possibility of 8-hydroxy-2'-deoxyguanosine (8-OHdG) serving as a sensitive biomarker of oxidative DNA damage and oxidative stress was investigated. Reactive oxygen species (ROS) have been reported to be a cause of diabetes induced by chemicals such as streptozotocin (STZ) in experimental animals. In this study, we examined oxidative DNA damage in multiple tissues in rats with STZ-induced diabetes by measuring the levels of 8-OHdG in the liver, kidney, pancreas, brain, and heart. Levels of 8-OHdG in mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) were also determined in multiple tissues of rats treated with rice bran oil. Levels were 0.19 +/- 0.07, 0.88 +/- 0.30, 1.97 +/- 0.05, and 9.79 +/- 3.09 (1/10(5) dG) in the liver of nDNA of normal rats, nDNA of STZ-induced diabetic rats, mtDNA of normal rats, and mtDNA of STZ-induced diabetic rats, respectively. Levels of mtDNA of 8-OHdG were 10 times higher than those of nDNA in multiple tissues. Significant reductions in mtDNA 8-OHdG levels were seen in the liver, kidney, and pancreas of diabetic rats treated with rice bran oil compared with diabetic rats without intervention. Our study demonstrated that oxidative mtDNA damage may occur in multiple tissues of STZ-induced diabetics rats. Intervention with rice bran oil treatment may reverse the increase in the frequency of 8-OHdG.

Energy, quiescence and the cellular basis of animal life spans

Animals are routinely faced with harsh environmental conditions in which insufficient energy is available to grow and reproduce. Many animals adapt to this challenge by entering a dormant, or quiescent state. In some animals, such as the nematode Caenorhabditis elegans, quiescence is coincident with increased stress resistance and longevity. Here we review evidence that the rules of life span extension established in C. elegans may be generally true of most animals. That is, that the rate of animal aging correlates inversely with cellular resistance to physiological stress, particularly oxidative stress, and that stress resistance is co-regulated with the quiescence adaptation (where the latter occurs). We discuss evidence for highly conserved intracellular signalling pathways involved in energy sensing that are sensitive to aspects of mitochondrial energy transduction and can be modulated in response to energetic flux. We provide a broad overview of the current knowledge of the relationships between energy, metabolism and life span.

Chondrocyte senescence, joint loading and osteoarthritis

cellular level is not completely understood, but both aging and loading-induced stresses have been shown to undermine cell functions related to the maintenance and restoration of the cartilage matrix. Based on precedents set by studies of other age-related degenerative diseases, we have focused our laboratory work on senescence as the cause of age-dependent decline in chondrocytes and on the impact of excessive mechanical stresses in promoting senescence. We hypothesized that senescent chondrocytes accumulate with age in articular cartilage and we propose that excessive mechanical stress plays a role in this process by promoting oxidative damage in chondrocytes that ultimately causes them to senesce. To test this hypothesis, we measured cell senescence markers (beta-galactosidase expression, mitotic activity, and telomere length) in human articular cartilage chondrocytes, and determined the effects of chronic exposure to oxidative stress on chondrocyte growth and senescence. In addition, we measured the effects of abnormally high levels of mechanical shear stress on the release of oxidants in cartilage explants. We found that senescent chondrocytes accumulated with age in articular cartilage. In vitro studies showed that chronic oxidative stress caused by repeated exposure to peroxide, or by growth under superphysiologic oxygen tension caused chondrocyte populations to senesce prematurely, before extensive telomere erosion occurred. Mechanical shear stress applied to cartilage explants considerably increased the production of oxidants. These observations support the hypothesis that senescence accounts for age-related decline in chondrocyte function and indicate that mechanically induced oxidative damage plays a role in this process. This suggests that new efforts to prevent the development and progression of osteoarthritis should include strategies that slow the progression of chondrocyte senescence or replace senescent cells.

Measurement of the 4,834-bp mitochondrial DNA deletion level in aging rat liver and brain subjected or not to caloric restriction diet

Several studies have demonstrated an age-related accumulation of the amount of a specific 4834-bp mitochondrial DNA (mtDNA) deletion in different tissues of rat (liver, brain, and skeletal muscle). We investigated the influence of a caloric restriction diet (CR) on a selected age-associated marker of mtDNA damage, as the 4834-bp deletion, using quantitative real-time PCR. The mtDNA deleted level has been determined with respect to the mitochondrial D-loop level, using specific primers and TaqMan probes for each target. In liver we found an age-related increase of the deletion level (twofold) that was reversed and brought back to the adult level by a CR diet. On the contrary, in the brain the age-related increase of the deletion level (eightfold) was not affected by CR at all. The different effect of the CR on the deletion level in liver and brain might be a further element supporting the tissue-specificity of the aging process.

Effects of Oxidative Damage and Telomerase Activity on Human Articular Cartilage Chondrocyte Senescence

Senescence compromises the ability of chondrocytes to maintain and repair articular cartilage. We hypothesized that oxidative stress and telomere loss contribute to chondrocyte senescence. To test this hypothesis, we compared the growth of human articular cartilage chondrocytes incubated in 5% O2 and 21% O2. Cells grown in 5% O2 reached 60 population doublings (PD) before senescing, but growth in 21% O2 induced DNA damage and premature senescence at less than 40 PD. Human telomerase reverse transcriptase (hTERT)-transduction failed to prevent chondrocyte senescence in 21% O2, but allowed 1 of 3 chondrocyte strains to exceed 90 PD in 5% O2. These results show that oxidative stress causes premature chondrocyte senescence. They may help explain the increased risk of osteoarthritis with age and after joint trauma and inflammation, and suggest that minimizing oxidative damage will help produce optimal results for chondrocyte transplantation.

Mitochondrial and nuclear DNA base excision repair are affected differently by caloric restriction

Aging is strongly correlated with the accumulation of oxidative damage in DNA, particularly in mitochondria. Oxidative damage to both mitochondrial and nuclear DNA is repaired by the base excision repair (BER) pathway. The "mitochondrial theory of aging" suggests that aging results from declining mitochondrial function, due to high loads of damage and mutation in mitochondrial DNA (mtDNA). Restriction of caloric intake is the only intervention so far proven to slow the aging rate. However, the molecular mechanisms underlying such effects are still unclear. We used caloric-restricted (CR) mice to investigate whether lifespan extension is associated with changes in mitochondrial BER activities. Mice were divided into two groups, receiving 100% (PF) or 60% (CR) of normal caloric intake, a regime that extends mean lifespan by approximately 40% in CR mice. Mitochondria isolated from CR mice had slightly higher uracil (UDG) and oxoguanine DNA glycosylase (OGG1) activities but marginally lower abasic endonuclease and polymerase gamma gap-filling activities, although these differences were tissue-specific. Uracil-initiated BER synthesis incorporation activities were significantly lower in brain and kidney from CR mice but marginally enhanced in liver. However, nuclear repair synthesis activities were increased by CR, indicating differential regulation of BER in the two compartments. The results indicate that a general up-regulation of mitochondrial BER does not occur in CR.

Development of a quantitative PCR (TaqMan) assay for relative mitochondrial DNA copy number and the common mitochondrial DNA deletion in the rat

Changes in mitochondrial DNA copy number and increases in mitochondrial DNA mutations, especially deletions, have been associated with exposure to mutagens and with aging. Common deletions that are the result of recombination between direct repeats in human and rat (4,977 and 4,834, bp, respectively) are known to increase in tissues of aged individuals. Previous studies have used long-distance PCR and Southern blot or quantitative PCR to determine the frequency of deleted mitochondrial DNA. A quantitative PCR (TaqMan) assay was developed to detect both mitochondrial DNA copy number and deletion frequency in the rat. This methodology allows not only the determination of changes in the amount of mitochondrial DNA deletion relative to total mitochondrial DNA but also to determine changes in total mitochondrial DNA relative to genomic DNA. As a validation of the assay in rat liver, the frequency of the common 4,834 bp deletion is shown to increase with age, while the relative mitochondrial DNA copy number rises at a young age (3-60 days), then decreases and holds fairly steady to 2 years of age.

The role of chondrocyte senescence in the pathogenesis of osteoarthritis and in limiting cartilage repair

BACKGROUND

With increasing age, the prevalence of osteoarthritis increases and the efficacy of articular cartilage repair decreases. As chondrocytes age, they synthesize smaller, less uniform aggrecan molecules and less functional link proteins, their mitotic and synthetic activity decline, and their responsiveness to anabolic mechanical stimuli and growth factors decreases. These observations led us to hypothesize that progressive cell senescence decreases the ability of chondrocytes to maintain and to restore articular cartilage.

METHODS

To test this hypothesis, we measured cell senescence markers (beta-galactosidase expression, mitotic activity, and telomere length) in human articular cartilage chondrocytes from twenty-seven donors ranging in age from one to eighty-seven years. We also assessed mitochondrial DNA, membrane potential, and numerical density. To determine if chondrocyte age changes are reversible, we transfected human articular cartilage chondrocytes with the human telomerase gene (hTERT) and human papilloma virus oncogenes (E6 and E7).

RESULTS

Beta-galactosidase expression increased with age (r = 0.84, p = 0.0001), while mitotic activity and telomere length declined (r = -0.77, p = 0.001 and r = -0.71, p = 0.0004, respectively). Decreasing telomere length was closely correlated with increasing expression of beta-galactosidase and decreasing mitotic activity. As the number of population doublings increased, mitochondrial DNA was degraded, mitochondrial membrane potential was lost, and the number of mitochondria per cell declined. Transfection of human articular cartilage chondrocytes from a forty-seven-year-old donor with hTERT and human papilloma virus proto-oncogenes E6 and E7 created a cell line that has completed more than 300 population doublings as compared with an upper limit of twenty-five population doublings for normal cells. Telomere length increased in cells transduced with hTERT.

CONCLUSIONS

These findings help to explain the previously reported age-related declines in chondrocyte synthetic activity, mitotic activity, and responsiveness to anabolic cytokines and mechanical stimuli. They also suggest that in vivo chondrocyte senescence contributes to the age-related increase in the prevalence of osteoarthritis and decrease in the efficacy of cartilage repair. The creation of immortal cells with increased telomere length suggests that the progression of human chondrocytes toward senescence is not inevitable.

Influence of aging and long-term caloric restriction on oxygen radical generation and oxidative DNA damage in rat liver mitochondria

The effect of long-term caloric restriction and aging on the rates of mitochondrial H2O2 production and oxygen consumption as well as on oxidative damage to nuclear (nDNA) and mitochondrial DNA (mtDNA) was studied in rat liver tissue. Long-term caloric restriction significantly decreased H2O2 production of rat liver mitochondria (47% reduction) and significantly reduced oxidative damage to mtDNA (46% reduction) with no changes in nDNA. The decrease in ROS production was located at complex I because it only took place with complex I-linked substrates (pyruvate/malate) but not with complex II-linked substrates (succinate). The mechanism responsible for that decrease in ROS production was not a decrease in mitochondrial oxygen consumption because it did not change after long-term restriction. Instead, the caloric restricted mitochondria released less ROS per unit electron flow, due to a decrease in the reduction degree of the complex I generator. On the other hand, increased ROS production with aging in state 3 was observed in succinate-supplemented mitochondria because old control animals were unable to suppress H2O2 production during the energy transition from state 4 to state 3. The levels of 8-oxodG in mtDNA increased with age in old animals and this increase was abolished by caloric restriction. These results support the idea that caloric restriction reduces the aging rate at least in part by decreasing the rate of mitochondrial ROS production and so, the rate of oxidative attack to biological macromolecules like mtDNA.

Dietary modulation of mitochondrial DNA deletions and copy number after chemotherapy in rats

Mitochondrial DNA (mtDNA) is particularly susceptible to mutation by alkylating agents, and mitochondrial damage may contribute to the efficacy and toxicity of these agents. We found that folate supplementation decreased the frequency of the "common deletion" (4.8kb, bases 8103-12,936) in liver from untreated rats and from animals treated with cyclophosphamide but not 5-fluorouracil (5-FU). The relative abundance of mitochondrial DNA was greater after chemotherapy but there was no effect of diet. Rats fed with a purified diet had fewer mitochondrial deletions than those maintained on a cereal-based diet after chemotherapy. These results indicate that diet can modulate the extent of mitochondrial damage after cancer chemotherapy, and that folic acid supplementation may be protective against mitochondrial DNA deletions.

Effect of short-term caloric restriction on H2O2 production and oxidative DNA damage in rat liver mitochondria and location of the free radical source

Oxygen free radicals (ROS) of mitochondrial origin seem to be involved in aging. Whereas in other tissues complexes I or III of the respiratory chain contain the ROS generators, in this study we find that rat liver mitochondria generate oxygen radicals at complexes I, II, and III. Short-term (6 weeks) caloric restriction significantly decreased H2O2 production in rat liver mitochondria. This decrease in ROS production was located at complex I because it occurred with complex I-linked substrates (pyruvate/malate), but did not reach statistical significance with the complex II-linked substrate succinate. The mechanism responsible for the lowered ROS production was not a decrease in oxygen consumption. Instead, the mitochondria of caloric-restricted animals released less ROS per unit electron flow. This was due to a decrease in the degree of reduction of the complex I generator. Furthermore, oxidative damage to mitochondrial and nuclear DNA was also decreased in the liver by short-term caloric restriction. The results agree with the idea that caloric restriction delays aging, at least in part, by decreasing the rate of mitochondrial ROS generation and thus the rate of attack to molecules, like DNA, highly relevant for the accumulation of age-dependent changes.

Effect of dietary restriction on age-related increase of liver susceptibility to peroxidation in rats

Dietary restriction (DR) increases life span and decreases age-related diseases in experimental animals. It has received a great deal of attention in connection with the relationship between aging, nutrition, and oxidative stress because oxidative injury in several organ systems is a prominent feature in aging. We investigated the possibility that DR can protect vulnerable liver lipids against age-related increases of peroxidation. Male Fischer 344 rats fed ad libitum (AL) or dietarily restricted (maintained on 60% of AL food intake) were killed by decapitation at 4 (young) or 12 mon (adult) of age. Phosphatidylcholine hydroperoxide (PCOOH) concentration of liver was determined using a chemiluminescent high-performance liquid chromatographic method. Liver PCOOH increased with age in adult rats, but less of an increase of PCOOH was seen in DR rats, which is consistent with results on production of thiobarbituric acid-reactive substances and oxygen-derived free radicals. No significant differences were found in liver superoxide dismutase and catalase activity between AL and DR groups of young and adult rats. Liver triglyceride and cholesterol contents were lower in DR than AL rats at 12 mon. Fatty acid compositions of phosphatidylcholine and phosphatidylethanolamine indicated that the ratio of (20:3n-6 + 20:4n-6)/18:2n-6, an index of linoleic acid (18:2n-6) desaturation, was lower in DR than in AL rats. We concluded that DR suppresses age-related oxidative damage in liver by modulating the amount of lipid as well as fatty acid composition.

Mitochondrial DNA in aging and degenerative disease

The mitochondrial DNA encodes only a few gene products compared to the nuclear DNA. These products, however, play a decisive role in determining cell function. Should this DNA mutate spontaneously or be damaged by free radicals the functionality of the gene products will be compromised. A number of mitochondrial genetic diseases have been identified. Some of these are quite serious and involve the central nervous system as well as muscle, heart, liver and kidney. Aging has been characterized by a gradual increase in base deletions in this DNA. This increase in deletion mutation has been suggested to be the cumulative result of exposure to free radicals.

Calorie restriction and age-related oxidative stress

Calorie restriction (CR) in mammals has been recognized as the best characterized and most reproducible strategy for extending maximum survival, retarding physiological aging, and delaying the onset of age-related pathologic conditions in mammals. The overwhelming majority of studies using CR have used short-lived rodent species, although current work using rhesus and squirrel monkeys will determine whether this paradigm is also relevant to manipulating the rate of primate aging. The mechanism by which restricted calorie intake modifies the rate of aging and pathology has been the subject of much controversy, although an attenuation in the lifetime accumulation of oxidative damage appears to be a central feature. Although the majority of studies have focused on the ability of cells from calorie-restricted animals to scavenge free radicals to explain the slower accrual of oxidative damage with age, it is not established that CR has a consistent effect to upregulate the activity of these enzymes in all tissues. A major effect of calorie-restricted feeding now appears to be on the rate of production or leak of free radicals from the mitochondria. The details of the adaptation and the signaling pathway that induces this effect are currently unknown.

Calorie restriction modulates age-dependent changes in the retinas of Brown Norway rats

The present study examined the effect of a 40% reduction in caloric intake (CR) versus ad libitum (AL) feeding on retinal aging. CR- and AL-fed Brown Norway (BN) rats were obtained at 12, 24 and 30 months of age from the National Center for Toxicological Research (NCTR). Age-dependent declines in outer nuclear layer (ONL=photoreceptor) cell densities, ONL height, inner nuclear layer (INL) cell densities, and thicknesses of the inner retina and whole retina were quantified in thick sections at six loci across the circumference of the sensory retina (four peripheral, two central). Data were analyzed by repeated measures, general linear models. Aging in both diet groups was associated with declines in ONL cell density, ONL height, peripheral INL cell density and total retinal thickness (P< or =0.05). However, ONL cell densities, ONL height and retinal thickness were significantly greater in the CR versus AL diet group at all three ages (P< or =0.005). CR was also associated with a trend for greater peripheral INL cell density (P=0.06) and with greater INL thickness at 30 months (Bonferroni P=0.03). Elevated ONL cell densities in the CR-12 cohort relative to the AL-12 cohort could be explained by diet-associated differences in retinal length, i.e. delayed retinal growth in response to CR. Enhanced ONL cell density, ONL height, INL cell density, INL thickness and total retinal thickness in the CR-30 cohort appear to be as a result of reduced rates of retinal cell loss between 24 and 30 months. However, the protective effect of CR in retinas of older animals may also reflect the initial growth-associated enhancements which were observed in 12 month-old animals. The rat retina may provide a useful model for elucidating the neuroprotective mechanism(s) of CR.

Aging and high concentrations of glucose potentiate injury to mitochondrial DNA

Deletions of mitochondrial DNA (mtDNA) are associated with aging and several chronic diseases. We have reported heterogeneous mutations between base pair 8468 and 13446 in mtDNA, the region known as the "common" deletion, in muscle of older humans with impaired glucose tolerance or diabetes mellitus. To further characterize potential effects of age and glycemia on mtDNA integrity, we studied corpulent JCR:LA-cp rats that are characterized by insulin resistance, hyperinsulinemia, and hyperlipidemia, factors strongly associated with both aging and cardiovascular disease. In addition to skeletal muscle, we isolated vascular smooth muscle cells (VSMC) from aortas of 6-, 12-, and 17-month-old rats and exposed them to 5-, 25-, 62-, and 100-mM glucose or a combination of hypoxanthine (100 microM) and xanthine oxidase (0.025 U/ml) to generate reactive oxygen species in separate cultures. Long- and short-fragment and nested polymerase chain reaction was used to detect mutations in the common deletion region. The data demonstrate that aging and the cp genotype confer susceptibility to mtDNA deletions in vivo and that high glucose concentrations can induce mtDNA mutations in vitro. Accordingly, aging and glucose-related oxidative stress and possibly hyperinsulinemia may contribute to alterations in mitochondrial gene integrity and the cp genotype appears to increase the susceptibility of muscle to the age-related accumulation of mtDNA mutations.

Endogenous oxidative damage of mtDNA

Almost a decade ago, based on analytical measurements of the oxidative DNA adduct 8-oxo-deoxyguanosine (oxo8dG), it was reported that mitochondrial DNA suffers greater endogenous oxidative damage than nuclear DNA. The subsequent discovery that somatic deletions of mitochondrial DNA occur in humans, and that they do so to the greatest extent in metabolically active tissues, strengthened the hypothesis that mitochondrial DNA is particularly susceptible to endogenous oxidative attack. This hypothesis was (and is) appealing for a number of reasons. Nevertheless, solid direct support for the hypothesis is lacking. Since the initial measurements, attempts to repeat the observation of greater oxidation of mitochondrial DNA have resulted in a range of measurements that spans over four orders of magnitude. Moreover, this range includes values that are as low as published values for nuclear DNA. In the last 2 years or so, it has become apparent that the quantification of oxidative DNA adducts is prone to artifactual oxidation. We have reported that the analysis of small quantities of DNA may be particularly susceptible to such interference. Because yields of mitochondrial DNA are generally low, a systematic artifact associated with low quantities of DNA may have elevated the apparent level of adduct oxo8dG in mitochondrial DNA relative to nuclear DNA in some studies. Whatever the cause for the experimental variation, the huge disparity between published measurements of oxidative damage makes it impossible to conclude that mitochondrial DNA suffers greater oxidation than nuclear DNA. Despite the present confusion, however, there are reasons to hypothesize that this is indeed the case. We briefly describe methods being developed by a number of workers that are likely to surmount current obstacles and allow the hypothesis to be tested definitively.

A low degree of fatty acid unsaturation leads to lower lipid peroxidation and lipoxidation-derived protein modification in heart mitochondria of the longevous pigeon than in the short-lived rat

Birds have a maximum longevity (MLSP) much greater than mammals of similar metabolic rate and body size. Thus, they are ideal models to identify longevity characteristics not linked to low metabolic rates. In this investigation, we show that the fatty acid double bond content of total lipids and phosphatidylcholine, phosphatidylethanolamine and cardiolipin fractions of heart mitochondria is intrinsically lower in pigeons (MLSP = 35 years) than in rats (MLSP = 4 years). This is mainly due to a lower content of the most highly unsaturated docosahexaenoic acid (22:6n-3) and in some fractions arachidonic acid (20:4n-6). The lower double bond content leads to a lower sensitivity to in vitro lipid peroxidation, and is associated with a lower concentration of lipid peroxidation products in vivo, and a lower level of malondialdehyde-lysine protein adducts in heart mitochondria of pigeons than rats. These results, together with those previously obtained in other species or tissues, suggest that a low degree of fatty acid unsaturation is a general characteristic of longevous homeothermic vertebrate animals both when they have low metabolic rates (mammals of large body size) or high metabolic rates (small sized birds). This constitutive trait helps to protect their tissues and mitochondria against lipid peroxidation and oxidative protein modification and can be a factor contributing to their slow rate of aging. The results also show, for the first time in a physiological model, that lipid peroxidizability is related to lipoxidative protein damage.

Mitochondrial aging: open questions

Interest in the role of mitochondria in aging has intensified in recent years. This focus on mitochondria originated in part from the free radical theory of aging, which argues that oxidative damage plays a key role in degenerative senescence. Among the numerous mechanisms known to generate oxidants, leakage of the superoxide anion and hydrogen peroxide from the mitochondrial electron transport chain are of particular interest, due to the correlation between species-specific metabolic rate ("rate of living") and life span. Phenomenological studies of mitochondrial function long ago noted a decline in mitochondrial function with age, and on-going research continues to add to this body of knowledge. The extranuclear somatic mutation theory of aging proposes that the accumulation of mutations in the mitochondrial genome may be responsible in part for the mitochondrial phenomenology of aging. Recent studies of mitochondrial DNA (mtDNA) deletions have shown that they increase with age in humans and other mammals. Currently, there exist numerous important and fundamental questions surrounding mitochondria and aging. Among these are (1) How important are mitochondrial oxidants in determining overall cellular oxidative stress? (2) What are the mechanisms of mitochondrial oxidant generation? (3) How are lesions and mutations in mtDNA formed? (4) How important are mtDNA lesions and mutations in causing mitochondrial dysfunction? (5) How are mitochondria regulated, and how does this regulation change during aging? (6) What are the dynamics of mitochondrial turnover? (7) What is the relationship between mitochondrial damage and lipofuscinogenesis? (8) What are the relationships among mitochondria, apopotosis, and aging? and (9) How can mitochondrial function (ATP generation and the establishment of a membrane potential) and dysfunction (oxidant generation) be modulated and degenerative senescence thereby treated?