Genetic analysis of ageing: role of oxidative damage and environmental stresses

Nucleotide excision repair and human syndromes

DNA damage is implicated in cancer and aging, and several DNA repair mechanisms exist that safeguard the genome from these deleterious consequences. Nucleotide excision repair (NER) removes a wide diversity of lesions, the main of which include UV-induced lesions, bulky chemical adducts and some forms of oxidative damage. The NER process involves the action of at least 30 proteins in a 'cut-and-paste'-like mechanism. The consequences of a defect in one of the NER proteins are apparent from three rare recessive syndromes: xeroderma pigmentosum (XP), Cockayne syndrome (CS) and the photosensitive form of the brittle hair disorder trichothiodystrophy (TTD). Sun-sensitive skin is associated with skin cancer predisposition in the case of XP, but remarkably not in CS and TTD. Moreover, the spectrum of clinical symptoms differs considerably between the three syndromes. CS and TTD patients exhibit a spectrum of neurodevelopmental abnormalities and, in addition, TTD is associated with ichthyosis and brittle hair. These typical CS and TTD abnormalities are difficult to comprehend as a consequence of defective NER. This review briefly describes the biochemistry of the NER process, summarizes the clinical features of the NER disorders and speculates on the molecular basis underlying these pleitropic syndromes.

The influence of oxygen toxicity on yeast mother cell-specific aging

The effect of deleting both catalase genes and of increased oxygen as well as paraquat (a pro-oxidant) on the replicative life span of yeast mother cells has been investigated to test the so-called oxygen theory of aging. This is well established in higher organisms, but has not been extensively tested in the unicellular yeast model system. Life span determinations were performed in ambient air or in a controlled atmosphere (55% oxygen) and an isogenic series of strains deleted for one or both yeast catalases was used and compared with wild type. In the absence of cellular catalase, increased oxygen caused a marked decrease in life span that could be completely reversed by adding 1 mM GSH, a physiological antioxidant, to the yeast growth medium. In a second unrelated strain, the effects were similar although even the wild type showed a decrease in life span when oxygen was increased. The effect could again be compensated by addition of extracellular GSH. Our results show that manipulating the detoxification of reactive oxygen species has a profound effect on yeast aging. These findings are discussed in the light of recent results relating to oxygen toxicity in the aging process of higher organisms.

Age and organ dependent spontaneous generation of nuclear 8-hydroxydeoxyguanosine in male Fischer 344 rats

8-Hydroxydeoxyguanosine (8-OHdG) is a major oxidative DNA adduct playing roles in senescence, carcinogenesis and various disease processes. High-performance liquid chromatography with an electrochemical detection (HPLC-ECD) method has been widely used to assess organ levels of 8-OHdG, and a recently introduced immunohistochemical approach has made it possible to clarify intra-organ localization. In the present study, these methods were employed to reveal age-dependent changes in nuclear 8-OHdG within various tissues of male Fischer 344 rats between 18 fetal days and 104 weeks of age. 8-OHdG was detected in the nuclei of cerebellar small granule and small cortical cells, cerebral nerve cells, and choroid plexus epithelia of the brain and ependymal cells of the spinal cord; parenchymal cells in the anterior lobe of the pituitary and adrenal glands (mainly cortex); bronchial epithelium of the lung; intra-hepatic bile duct, pancreatic duct, glandular gastric and intestinal epithelial cells; renal tubular epithelial cells (mainly medulla); and spermatogonia and spermatocytes of the testis and seminal vesicle epithelia. The nuclear 8-OHdG levels were high (more than two lesions per 10(6) deoxyguanosines) from 7 days to 104 weeks of age in the brain, 3 to 6 weeks in the adrenal gland, 6 to 104 weeks in the lung, and 3 to 52 weeks in the testis. In the other organs, the nuclear 8-OHdG levels remained low throughout. These findings provide a basis for research dealing with oxidative stress by indicating organ-specific and age- but not aging-dependent changes in the localization of spontaneously generated nuclear 8-OHdG in intact rats. The immunohistochemical approach has advantages for assessing variation of 8-OHdG formation at the cellular level not accessible to the HPLC-ECD method.

Differential signaling pathways following oxidative stress in mutant myotonin protein kinase cDNA-transfected C2C12 cell lines

We investigated the response to oxidative stress in a model system established in C2C12 cells stably transfected with myotonin protein kinase (MtPK) cDNAs having 5, 46, 60, or 160 CTG repeats. The transformants showed CTG repeat number-dependent susceptibility to oxidative stress. Mutant MtPK cDNA transformants containing 160 CTG repeats showed apoptotic cell death by the exposure to an oxidant, a very low level of methylmercury. The addition of the antioxidant Trolox protected transformants against apoptosis. Oxidative stress activated the extracellular signal-regulated kinases (ERKs) pathway leading to cell survival in wild-type MtPK cDNA transformants, whereas mutant MtPK cDNA transformants having 160 CTG repeats were defective in the induction of the ERK pathway, although the activation of stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK) was strong and sustained. These results suggest that the susceptibility to oxidative stress in mutant MtPK cDNA transformants involves differential signaling pathways evoked following oxidative stress.

Accumulation of defective mitochondria through delayed degradation of damaged organelles and its possible role in the ageing of post-mitotic and dividing cells

The mitochondrial theory of ageing proposes that an accumulation of defective mitochondria is a major contributor to the cellular deterioration that underlies the ageing process. The plausibility of the mitochondrial theory depends critically upon the population dynamics of intact and mutant mitochondria in different cell types. Earlier work suggested that mutant mitochondria might have a replication advantage but failed to account for the fact that mutants accumulate faster in post-mitotic than in dividing cells. We describe a new mathematical model that allows for damaged mitochondria to replicate more slowly, which accommodates experimental evidence of impaired energy generation and a reduced proton gradient in defective mitochondria. However, this is compensated for by a slower degradation rate of damaged mitochondria than intact ones, as suggested by de Grey (1997), which gives damaged mitochondria a selective advantage and leads to a clonal expansion of damaged mitochondria. This theoretical result is important because it agrees with evidence that, during ageing, single muscle fibres are taken over by one or only a few types of mtDNA mutants. The model also shows that cell division can rejuvenate and stabilize the mitochondrial population, consistent with data that post-mitotic tissues accumulate mitochondrial damage faster than mitotically active tissues.

Somatic mutations and aging: a re-evaluation

Aging has been explained in terms of an accumulation of mutations in the genome of somatic cells, leading to tissue atrophy and neoplasms, as well as increased loss of function. Recent advances in transgenic mouse modeling and genomics technology have created, for the first time, the opportunity to begin testing this theory. In this paper the existing evidence for a possible role of somatic mutation accumulation in aging will be re-evaluated on the basis of the evolutionary logic of aging and recent insights in genome structure and function. New strategies for investigating the relationship between genome instability, mutation accumulation and aging will be discussed.

Metabolic mechanisms of longevity: Caloric restriction in mammals and longevity mutations in Caenorhabditis elegans; a common pathway??

Several recent studies in Caenorhabditis elegans have reported significant extension of the lifespan by probable loss of function mutations in various genes. When sequenced, many of these genes exhibited significant homology to genes in the mammalian insulin signaling cascade. For example, the daf-2 gene that has been shown to regulate lifespan in C elegans shares significant sequence homology with the insulin and IGF-1 receptor genes in mammals. Another longevity gene in the nematode, age-1, is homologous with the p110 subunit of phosphatidylinositol 3-kinase in mammals. This enzyme functions early in the mammalian insulin response cascade to influence many important cellular growth and metabolic processes. These findings and others have led to the suggestion that lifespan regulation in nematodes is controlled by a mechanism similar to that involved in lifespan extension by caloric restriction in mammals. Many intriguing similarities exist between these two model systems providing some support for this idea. However, at present there is insufficient data to conclude that similar genes or mechanisms regulate lifespan determination in nematodes and in mammals.

Cellular and molecular mechanisms of aging and age related diseases

This review on aging is focused on those cellular and molecular mechanisms which concern age related pathologies. The central question addressed is the relationship between normal aging and age-related pathologies such as osteoarthritis, cardiovascular diseases, emphysema, malignant tumors and cognitive decline, dementias. The mechanisms recognized as most important in cell and tissue aging are briefly outlined. Emphasis is laid on the importance of post-synthetic modifications of the macromolecules of the extracellular matrix and on cell matrix interactions. Loss of intercellular communication and cell-matrix interactions as a result of receptor decay and receptor uncoupling were recently recognized as key events. Unavoidable poly-pathology at advanced age may be the answer to the above question.

Mercury induces cell cytotoxicity and oxidative stress and increases beta-amyloid secretion and tau phosphorylation in SHSY5Y neuroblastoma cells

Concentrations of heavy metals, including mercury, have been shown to be altered in the brain and body fluids of Alzheimer's disease (AD) patients. To explore potential pathophysiological mechanisms we used an in vitro model system (SHSY5Y neuroblastoma cells) and investigated the effects of inorganic mercury (HgCl2) on oxidative stress, cell cytotoxicity, beta-amyloid production, and tau phosphorylation. We demonstrated that exposure of cells to 50 microg/L (180 nM) HgCl2 for 30 min induces a 30% reduction in cellular glutathione (GSH) levels (n = 13, p<0.001). Preincubation of cells for 30 min with 1 microM melatonin or premixing melatonin and HgCl2 appeared to protect cells from the mercury-induced GSH loss. Similarly, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cytotoxicity assays revealed that 50 microg/L HgCl2 for 24 h produced a 50% inhibition of MTT reduction (n = 9, p<0.001). Again, melatonin preincubation protected cells from the deleterious effects of mercury, resulting in MTT reduction equaling control levels. The release of beta-amyloid peptide (Abeta) 1-40 and 1-42 into cell culture supernatants after exposure to HgCl2 was shown to be different: Abeta 1-40 showed maximal (15.3 ng/ml) release after 4 h, whereas Abeta 1-42 showed maximal (9.3 ng/ml) release after 6 h of exposure to mercury compared with untreated controls (n = 9, p<0.001). Preincubation of cells with melatonin resulted in an attenuation of Abeta 1-40 and Abeta 1-42 release. Tau phosphorylation was significantly increased in the presence of mercury (n = 9, p<0.001), whereas melatonin preincubation reduced the phosphorylation to control values. These results indicate that mercury may play a role in pathophysiological mechanisms of AD.

The p66shc adaptor protein controls oxidative stress response and life span in mammals

Gene mutations in invertebrates have been identified that extend life span and enhance resistance to environmental stresses such as ultraviolet light or reactive oxygen species. In mammals, the mechanisms that regulate stress response are poorly understood and no genes are known to increase individual life span. Here we report that targeted mutation of the mouse p66shc gene induces stress resistance and prolongs life span. p66shc is a splice variant of p52shc/p46shc (ref. 2), a cytoplasmic signal transducer involved in the transmission of mitogenic signals from activated receptors to Ras. We show that: (1) p66shc is serine phosphorylated upon treatment with hydrogen peroxide (H2O2) or irradiation with ultraviolet light; (2) ablation of p66shc enhances cellular resistance to apoptosis induced by H2O2 or ultraviolet light; (3) a serine-phosphorylation defective mutant of p66shc cannot restore the normal stress response in p66shc-/- cells; (4) the p53 and p21 stress response is impaired in p66shc-/- cells; (5) p66shc-/- mice have increased resistance to paraquat and a 30% increase in life span. We propose that p66shc is part of a signal transduction pathway that regulates stress apoptotic responses and life span in mammals.

Quantitative trait loci affecting survival and fertility-related traits in Caenorhabditis elegans show genotype-environment interactions, pleiotropy and epistasis

We have identified, using composite interval mapping, quantitative trait loci (QTL) affecting a variety of life history traits (LHTs) in the nematode Caenorhabditis elegans. Using recombinant inbred strains assayed on the surface of agar plates, we found QTL for survival, early fertility, age of onset of sexual maturity, and population growth rate. There was no overall correlation between survival on solid media and previous measures of survival in liquid media. Of the four survival QTL found in these two environments, two have genotype-environment interactions (GEIs). Epistatic interactions between markers were detected for four traits. A multiple regression approach was used to determine which single markers and epistatic interactions best explained the phenotypic variance for each trait. The amount of phenotypic variance accounted for by genetic effects ranged from 13% (for internal hatching) to 46% (for population growth). Epistatic effects accounted for 9-11% of the phenotypic variance for three traits. Two regions containing QTL that affected more than one fertility-related trait were found. This study serves as an example of the power of QTL mapping for dissecting the genetic architecture of a suite of LHTs and indicates the potential importance of environment and GEIs in the evolution of this architecture.

Vitamin E enhances the immune functions of young but not old mice under restraint stress

Young and old C57BL/6 male mice were given a diet containing a high dose of vitamin E (VE treatment) and its effect on the immune system was examined before and after the exposure to restraint stress. The VE treatment per se gave rise to a slight increase of splenic T cells in percentage and a significant enhancement of Con A response of spleen cells in young, but not in old mice. The VE treatment also resulted in the enhancement of production of IL-2 and IFNgamma in young, but not in old mice. Restraint stress led to thymic involution in both young and old mice. This thymic involution was not ameliorated by the VE treatment. Percentage of splenic T cells and their mitogenic response decreased just after the stress, but soon rebounded over the control level. The VE treatment further enhanced the recovery after the stress in young mice, but on the contrary suppressed the recovery in old mice. The results in the present study suggested that the VE treatment was effective in the prevention of immunological decline of young mice before and after the exposure to the stress. On the other hand, such a preventive effect was not observed in old mice that were already in the depressed state of immunological functions.

Laboratory selection for the comparative physiologist

An increasingly popular experimental approach in comparative physiology is to study the evolution of physiological traits in the laboratory, using microbial, invertebrate and vertebrate models. Because selective conditions are well-defined, selected populations can be replicated and unselected control populations are available for direct comparison, strong conclusions regarding the adaptive value of an evolved response can be drawn. These studies have shown that physiological systems evolve rapidly in the laboratory, but not always as one would expect from comparative studies of different species. Laboratory environments are often not as simple as one thinks, so that the evolution of behavioral differences or selection acting on different life stages can lead to unanticipated results. In some cases, unexpected responses to laboratory selection may suggest new insights into physiological mechanisms, which might not be available using other experimental approaches. I outline here recent results (including success stories and caveats for the unwary investigator) and potential directions for selection experiments in comparative physiology.

Drosophila selected for extended longevity are more sensitive to heat shock

It has been demonstrated in several animal models that a brief non-lethal application of high temperature is capable of inducing an increased longevity. It is also known that an even briefer exposure to a non-lethal elevated temperature enables some organisms to subsequently survive what would normally be a lethal exposure to high temperature. Our long-lived La strain is significantly resistant to oxidative stress due to an enhanced expression of certain antioxidant defense genes and enzyme activities. We collected survival data on 12, 463 adults of normal-lived and long-lived strains of Drosophila melanogaster in order to determine if animals selected for extended longevity also had an enhanced resistance to heat shock, and whether they exhibited thermotolerance as well. We find that normal-lived animals exhibit a heat-induced longevity extension but that long-lived animals already resistant to oxidative stress exhibit a heat-induced longevity shortening. The effects of temperature stress on longevity are strain dependent and are separable from thermotolerance effects. The trait of extended longevity based on an increased resistance to oxidative stress in the adult may be purchased at the price of a decreased fitness of the adult to other important environmental parameters.

Genetic and environmental conditions that increase longevity in Caenorhabditis elegans decrease metabolic rate

Mutations that increase the longevity of the soil nematode Caenorhabditis elegans could define genes involved in a process specific for aging. Alternatively, these mutations could reduce animal metabolic rate and increase longevity as a consequence. In ectotherms, longevity is often negatively correlated with metabolic rate. Consistent with these observations, environmental conditions that reduce the metabolic rate of C. elegans also extend longevity. We found that the metabolic rate of long-lived C. elegans mutants is reduced compared with that of wild-type worms and that a genetic suppressor that restored normal longevity to long-lived mutants restored normal metabolic rate. Thus, the increased longevity of some long-lived C. elegans mutants may be a consequence of a reduction in their metabolic rate, rather than an alteration of a genetic pathway that leads to enhanced longevity while maintaining normal physiology. The actual mechanism responsible for the inverse correlation between metabolic rate and longevity remains unknown.

Mutations in signal transduction proteins increase stress resistance and longevity in yeast, nematodes, fruit flies, and mammalian neuronal cells

Mutations in Ras and other signal transduction proteins increase survival and resistance to oxidative stress and starvation in stationary phase yeast, nematodes, fruit flies, and in neuronal PC12 cells. The chronological life span of yeast, based on the survival of nondividing cells in stationary phase, has allowed the identification and characterization of long-lived strains with mutations in the G-protein Ras2. This paradigm was also used to identify the in vivo sources and targets of reactive oxygen species and to examine the role of antioxidant enzymes in the longevity of yeast. I will review this model system and discuss the striking phenotypic similarities between long-lived mutants ranging from yeast to mammalian neuronal cells. Taken together, the published studies suggest that survival may be regulated by similar fundamental mechanisms in many eukaryotes and that simple model systems will contribute to our understanding of the aging process in mammals.

AlphaMUPA mice: a transgenic model for increased life span

AlphaMUPA is a line of transgenic mice that, compared with their wild type (WT) counterparts, spontaneously eat less (approximately 20%) and live longer (average approximately 20%), thus resembling dietary-restricted (DR) mice. Here, we show that body temperature was significantly reduced in alphaMUPA compared with WT throughout a wide range of ages. Plasma corticosterone was significantly higher in young alphaMUPA compared to young WT; however, it significantly declined in aged alphaMUPA, but not in aged WT. In addition, age-associated thymus involution occurred in alphaMUPA as it did in WT. Thus alphaMUPA mice appear to largely resemble, but also to somewhat differ from diet-restricted animals. We also report on four new transgenic lines that, like alphaMUPA, produced in the brain the mRNA that encodes the extracellular protease urokinase (uPA); however, transgenic uPA expression was most extensive and widespread in the alphaMUPA brain, where it also occurred in the hypothalamus. AlphaMUPA was also the only line that ate less, but also showed another characteristic, high frequency leg muscle tremor seen only at unstable body states. We hypothesize that transgenic uPA in the brain could have caused the alphaMUPA phenotypic alterations. Thus alphaMUPA offers a unique transgenic model of inherently reduced eating to investigate the homeostatic state of delayed aging at the systemic and single-cell levels.

The spe-10 mutant has longer life and increased stress resistance

We investigated the life span of spe-10 mutant nematodes. We also tested resistance of spe-10 mutants to ultraviolet (UV) light, heat, and paraquat and examined the relationship between resistance to UV light and the fertility defect of these animals. The spe-10 mutation significantly increased mean life span. Additionally, the mutation significantly increased resistance to both UV light and to heat. Resistance to paraquat was not significantly different from that of wild-type, nor were any dauers formed at 27 degrees C. No significant correlation was found between the UV resistance and the fertility defect, nor was the UV resistance attributable to a hormetic effect. These results reinforce the importance of stress resistance in specifying increased life span and indirectly suggest that this fertility defect is not a direct cause of life span extension.

Gene expression profile of aging and its retardation by caloric restriction

The gene expression profile of the aging process was analyzed in skeletal muscle of mice. Use of high-density oligonucleotide arrays representing 6347 genes revealed that aging resulted in a differential gene expression pattern indicative of a marked stress response and lower expression of metabolic and biosynthetic genes. Most alterations were either completely or partially prevented by caloric restriction, the only intervention known to retard aging in mammals. Transcriptional patterns of calorie-restricted animals suggest that caloric restriction retards the aging process by causing a metabolic shift toward increased protein turnover and decreased macromolecular damage.

Accelerated Development and Aging of the Immune System in p53-Deficient Mice

Development and aging of the immune system lead to an accumulation of memory T cells over the long term. The predominance of T cells of the memory phenotype in the T cell population induces an age-related decline in protective immune responses. We found that development and aging of the immune system were accelerated in p53-deficient (p53−/−) mice; the accumulation of memory T cells was spontaneously accelerated, and a strong T cell-dependent Ab response and Th2 cytokine expression (IL-4, IL-6, and IL-10) were induced by Ag stimulation in young p53−/− mice in the developmental stage. The high T cell proliferative response in the young mice rapidly progressed to a depressed proliferative response in adult mice. It was suggested that the loss of regulation of the cell cycle, DNA repair, and apoptosis by p53 deficiency potentially leads to immunosenescence with the accumulation of memory T cells.

Ras proteins induce senescence by altering the intracellular levels of reactive oxygen species

Human diploid fibroblasts eventually lose the capacity to replicate in culture and enter a viable but nonproliferative state of senescence. Recently, it has been demonstrated that retroviral-mediated gene transfer into primary fibroblasts of an activated ras gene (V12ras) rapidly accelerates development of the senescent phenotype. Using this in vitro system, we have sought to define the mediators of Ras-induced senescence. We demonstrate that expression of V12Ras results in an increase in intracellular and in particular, mitochondrial reactive oxygen species. The ability of V12Ras to induce growth arrest and senescence is shown to be partially inhibited by coexpression of an activated rac1 gene. A more dramatic rescue of V12Ras-expressing cells is demonstrated when the cells are placed in a low oxygen environment, a condition in which reactive oxygen species production is inhibited. In addition, in a 1% oxygen environment, Ras is unable to trigger an increase in the level of the cyclin-dependent kinase inhibitor p21 or to activate the senescent program. Under normoxic (20% O2) conditions, the V12Ras senescent phenotype is demonstrated to be unaffected by scavengers of superoxide but rescued by scavengers of hydrogen peroxide. These results suggest that in normal diploid cells, Ras proteins regulate oxidant production and that a rise in intracellular H2O2 represents a critical signal mediating replicative senescence.

Carbonylated proteins in aging and exercise: immunoblot approaches

Protein carbonyls were studied in aging and exercise by immunoblot followed by one- or two-dimensional polyacrylamide gel electrophoresis using antibodies against 2,4-dinitrophenylhydrazones. Proteins of rat kidneys exhibited significant age-related increase in the amount of carbonyl while those of the brain and liver did not. Major carbonylated proteins in the kidney included serum albumin. In nematodes in which protein carbonyls increased with age, one of the carbonylated proteins was identified as vitellogenin, an egg-yolk protein. A possible biological significance of this protein present in abundance even after egg-laying stages is discussed in terms of protection against oxidative stress. Exhaustive exercise induced significant increase in the carbonylation of selected but unidentified proteins in the lung. This oxidative stress might be caused by xanthine oxidase in this tissue and hypoxanthine derived from ATP-depleted muscles. Exercise at high altitude caused higher carbonylation of the skeletal muscle proteins, most notably a protein likely to be actin, than that at sea level but no significant difference was observed in lipid peroxidation. These studies emphasize the value of immunoblot analysis of tissue protein carbonyls in a variety of situations where oxidative stress is likely involved.

Positive correlation between mammalian life span and cellular resistance to stress

Identifying the mechanisms determining species-specific life spans is a central challenge in understanding the biology of aging. Cellular stresses produce damage, that may accumulate and cause aging. Evolution theory predicts that long-lived species secure their longevity through investment in a more durable soma, including enhanced cellular resistance to stress. To investigate whether cells from long-lived species have better mechanisms to cope with oxidative and non-oxidative stress, we compared cellular resistance of primary skin fibroblasts from eight mammalian species with a range of life spans. Cell survival was measured by the thymidine incorporation assay following stresses induced by paraquat, hydrogen peroxide, tert-butyl hydroperoxide, sodium arsenite and alkaline pH (sodium hydroxide). Significant positive correlations between cell LD90 and maximum life span were found for all these stresses. Similar results were obtained when cell survival was measured by the MTT assay, and when lymphocytes from different species were compared. Cellular resistance to a variety of oxidative and non-oxidative stresses was positively correlated with mammalian longevity. Our results support the concept that the gene network regulating the cellular response to stress is functionally important in aging and longevity.

Mitochondria in organismal aging and degeneration

Several lines of experimentation support the view that the genetic, biochemical and bioenergetic functions of somatic mitochondria deteriorate during normal aging. Deletion mutations of the mitochondrial genome accumulate exponentially with age in nerve and muscle tissue of humans and multiple other species. In muscle, a tissue that undergoes age-related fiber loss and atrophy in humans, there is an exponential rise in the number of cytochrome-oxidase-deficient fibers, which is first detectable in the fourth decile of age. Most biochemical studies of animal mitochondrial activity indicate a decline in electron transport activity with age, as well as decreased bioenergetic capacity with age, as measured by mitochondrial membrane potential. Mitochondrial mutations may be both the result of mitochondrial oxidative stress, and cells bearing pure populations of pathogenic mitochondrial mutations are sensitized to oxidant stress. Oxidant stress to mitochondria is known to induce the mitochondrial permeability transition, which has recently been implicated in the release of cytochrome c and the initiation of apoptosis. Thus several lines of evidence support a contribution of mitochondrial dysfunction to the phenotypic changes associated with aging.

Cell cycle control, checkpoint mechanisms, and genotoxic stress

The ability of cells to maintain genomic integrity is vital for cell survival and proliferation. Lack of fidelity in DNA replication and maintenance can result in deleterious mutations leading to cell death or, in multicellular organisms, cancer. The purpose of this review is to discuss the known signal transduction pathways that regulate cell cycle progression and the mechanisms cells employ to insure DNA stability in the face of genotoxic stress. In particular, we focus on mammalian cell cycle checkpoint functions, their role in maintaining DNA stability during the cell cycle following exposure to genotoxic agents, and the gene products that act in checkpoint function signal transduction cascades. Key transitions in the cell cycle are regulated by the activities of various protein kinase complexes composed of cyclin and cyclin-dependent kinase (Cdk) molecules. Surveillance control mechanisms that check to ensure proper completion of early events and cellular integrity before initiation of subsequent events in cell cycle progression are referred to as cell cycle checkpoints and can generate a transient delay that provides the cell more time to repair damage before progressing to the next phase of the cycle. A variety of cellular responses are elicited that function in checkpoint signaling to inhibit cyclin/Cdk activities. These responses include the p53-dependent and p53-independent induction of Cdk inhibitors and the p53-independent inhibitory phosphorylation of Cdk molecules themselves. Eliciting proper G1, S, and G2 checkpoint responses to double-strand DNA breaks requires the function of the Ataxia telangiectasia mutated gene product. Several human heritable cancer-prone syndromes known to alter DNA stability have been found to have defects in checkpoint surveillance pathways. Exposures to several common sources of genotoxic stress, including oxidative stress, ionizing radiation, UV radiation, and the genotoxic compound benzo[a]pyrene, elicit cell cycle checkpoint responses that show both similarities and differences in their molecular signaling.

Enhanced glutathione levels and oxidoresistance mediated by increased glucose-6-phosphate dehydrogenase expression

Glucose-6-phosphate dehydrogenase (G6PD) is the key enzyme of the pentose phosphate pathway that is responsible for the generation of NADPH, which is required in many detoxifying reactions. We have recently demonstrated that G6PD expression is induced by a variety of chemical agents acting at different steps in the biochemical pathway controlling the intracellular redox status. Although we obtained evidence that the oxidative stress-mediated enhancement of G6PD expression is a general phenomenon, the functional significance of such G6PD induction after oxidant insult is still poorly understood. In this report, we used a GSH-depleting drug that determines a marked decrease in the intracellular pool of reduced glutathione and a gradual but notable increase in G6PD expression. Both effects are seen soon after drug addition. Once G6PD activity has reached the maximum, the GSH pool is restored. We suggest and also provide the first direct evidence that G6PD induction serves to maintain and regenerate the intracellular GSH pool. We used HeLa cell clones stably transfected with the human G6PD gene that display higher G6PD activity than the parent HeLa cells. Although the activities of glutathione peroxidase, glutathione reductase, and catalase were comparable in all strains, the concentrations of GSH were significantly higher in G6PD-overexpressing clones. A direct consequence of GSH increase in these cells is a decreased reactive oxygen species production, which makes these cells less sensitive to the oxidative burst produced by external stimuli. Indeed, all clones that constitutively overexpress G6PD exhibited strong protection against oxidants-mediated cell killing. We also observe that NF-kappaB activation, in response to tumor necrosis factor-alpha treatment, is strongly reduced in human HeLa cells overexpressing G6PD.

Cancer from the outside, aging from the inside: mouse models to study the consequences of defective nucleotide excision repair

In recent years, mouse models have been generated to study the syndromes associated with a defect in nucleotide excision repair (NER). Thus, via conventional knockout gene targeting or by mimicking patient-specific alleles, mouse models for xeroderma pigmentosum (XP), Cockayne syndrome (CS) and photosensitive trichothiodystrophy (TTD) have been obtained. The generation of this series of mouse mutants allows in vivo investigation of some intriguing questions that have puzzled the field, such as the paradoxical absence of cancer development in TTD and CS despite their NER deficiencies, and the role of the ERCC1 gene in mitotic recombination and cross-link repair. Other interesting issues include the pathophysiology of the non-NER related clinical symptoms in TTD and CS patients and the proposed involvement of NER and transcription in the process of aging. This review will focus on data obtained thus far and discuss further utilization of the mouse mutants for unraveling some of the fascinating and medically relevant aspects associated with defects in NER and related processes.

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?

Causes of aging

A broad biological approach makes it possible to understand why aging exists and also why different mammalian species have very different maximum longevities. The adult organism is maintained in a functional state by at least ten major mechanisms, which together constitute a substantial proportion of all biological processes. These maintenance mechanisms eventually fail, because the evolved physiological and anatomical design of higher animals is incompatible with continual survival. The life span of each mammalian species depends on the efficiency of maintenance of their cells, tissues, and organs, and there is much evidence that such maintenance is more effective in long-lived species, such as humans, than in short-lived small mammals. It is also evident that there is an inverse relationship between reproductive potential and longevity, which would be expected if available metabolic resources are shared between investment in reproduction and investment in the preservation of the adult body. It is proposed that the eventual failure of maintenance leads to the pathological changes seen in age-associated disease. Although we now have a biological understanding of the aging process, much future research will be needed to uncover the cellular and molecular changes that give rise to age-associated diseases. The major aim of such research is to devise procedures to delay or prevent the onset of these diseases.

Genomic structure and sequence of a human homologue (NTHL1/NTH1) of Escherichia coli endonuclease III with those of the adjacent parts of TSC2 and SLC9A3R2 genes

Genomic cloning and sequencing of a human homologue (the gene name, endonuclease III-like 1; gene symbol, NTHL1 or NTH1) for Escherichia coli endonuclease III, that is involved in pyrimidine base excision repair, were performed. The sequence covered the entire NTHL1 gene consisting of six exons and five introns spanning 8kb with 5' flanking (8kb) and 3' flanking (3.8kb) regions. Southern blot analysis suggested that the NTHL1 gene exists as a single copy in a haploid genome. The sequenced 5' flanking region lacks typical TATA and CAAT boxes, but contains a CpG island having putative binding sites for several transcription factors such as Ets1 and Sp1. The NTHL1 gene lies immediately adjacent to the tuberous sclerosis 2 (TSC2) gene on chromosome 16p13.3 in a 5'-to-5' orientation. Transcription initiation sites of both NTHL1 and TSC2 genes were suggested to be multiple by 5' RACE experiments. The northern hybridization experiment suggested that both genes are expressed in all tissues, but at different levels. Downstream of the NTHL1 gene, the gene for the regulatory factor 2 (SLC9A3R2/E3KARP; also called OCTS2, TKA-1 and SIP-1) of the solute carrier family 9 (sodium/hydrogen exchanger), isoform A3, lies in a 3'-to-3' orientation. This paper demonstrates for the first time the spatial relationship of these three genes (TSC2, NTHL1 and SLC9A3R2) at the nucleotide level, and the presence of multiple transcription initiation sites of the NTHL1 and TSC2 genes.

Apolipoprotein E 4 Allele as a Genetic Risk Factor for Left Ventricular Failure in Homozygous β-Thalassemia

In homozygous β-thalassemia, the organ damage is mainly attributed to excessive iron deposition through the formation of oxygen free radicals. Despite appropriate transfusion and chelation therapy and low ferritin levels, patients still develop organ failure, heart failure being the main cause of death. This study was designed to determine whether the decreased antioxidant activity of the apolipoprotein E (APOE) 4 allele could represent a genetic risk factor for the development of left ventricular failure (LVF) in β-thalassemia homozygotes. A total of 251 Greek β-thalassemia homozygotes were studied. Patients were divided in three groups: group A (n = 151) with no cardiac impairment, group C (n = 47) with LVF, and 53 patients with LV dilatation and normal LV systolic function constituted the group B. DNA was obtained from all patients, and the polymerase chain reaction was used to analyze the polymorphism at the APOE locus. The APOE allele frequencies were compared with those of a Greek control sample of 216 healthy blood donors. Patients with no cardiac impairment had an APOE 4 allele frequency (7.9%) not different from population controls (6.5%, P &gt; .05), while patients with LVF had a significantly higher frequency of APOE 4 (12.8%) than the controls (P &lt; .05, odds ratio = 2.11, 95% confidence interval 1.03 to 4.32). The APOE 4 allele may represent an important genetic risk factor for the development of organ damage in homozygous β-thalassemia.

© 1998 by The American Society of Hematology.

Apolipoprotein E 4 Allele as a Genetic Risk Factor for Left Ventricular Failure in Homozygous β-Thalassemia

In homozygous β-thalassemia, the organ damage is mainly attributed to excessive iron deposition through the formation of oxygen free radicals. Despite appropriate transfusion and chelation therapy and low ferritin levels, patients still develop organ failure, heart failure being the main cause of death. This study was designed to determine whether the decreased antioxidant activity of the apolipoprotein E (APOE) 4 allele could represent a genetic risk factor for the development of left ventricular failure (LVF) in β-thalassemia homozygotes. A total of 251 Greek β-thalassemia homozygotes were studied. Patients were divided in three groups: group A (n = 151) with no cardiac impairment, group C (n = 47) with LVF, and 53 patients with LV dilatation and normal LV systolic function constituted the group B. DNA was obtained from all patients, and the polymerase chain reaction was used to analyze the polymorphism at the APOE locus. The APOE allele frequencies were compared with those of a Greek control sample of 216 healthy blood donors. Patients with no cardiac impairment had an APOE 4 allele frequency (7.9%) not different from population controls (6.5%, P > .05), while patients with LVF had a significantly higher frequency of APOE 4 (12.8%) than the controls (P < .05, odds ratio = 2.11, 95% confidence interval 1.03 to 4.32). The APOE 4 allele may represent an important genetic risk factor for the development of organ damage in homozygous β-thalassemia.

© 1998 by The American Society of Hematology.

Ovarian ageing and the general biology of senescence

Ovarian ageing is not only of major importance in its own right but is also of interest for its relationship with the general biology of senescence. A key feature of ageing is the distinction in higher animals between the immortality of the germ-line and the mortality of somatic cells and tissues. The ovary contains the female germ cells, and it is through these cells that the female contribution to germ-line immortality is effected. It is abundantly clear that individual oocytes can and do age and that the ageing of the ovary plays a major role in initiating or accelerating a series of other senescent changes. To understand how ovarian ageing fits within the general biology of senescence, it is necessary to explain why ageing occurs at all, to examine the likely mechanisms of general ageing, and to ask whether there is anything special about ovarian ageing and its relationship with the human menopause. Research on ovarian ageing interacts with the our emerging understanding of the general biology of senescence at many levels, ranging from the evolution of the human life history to the biochemical and cellular mechanisms of ageing and longevity.

Life extension and stress resistance in Caenorhabditis elegans modulated by the tkr-1 gene

The nematode Caenorhabditis elegans is widely used to study aging, development, behavior and other basic metazoan processes [1-3]. The only mutants directly identified on the basis of their extended longevity in any metazoan have been isolated in C. elegans [4,5]. All life-extension mutants (Age mutants) previously identified in C. elegans result from hypo-morphic or nullo-morphic mutations. We have identified a new class of gerontogene (a gene whose alteration causes life extension) that increases life span when overexpressed. The first gene in this class has been designated tyrosine kinase receptor-1 (tkr-1); it encodes a putative receptor tyrosine kinase. Overexpression of tkr-1 in transgenics increases longevity 40-100% (average 65%), confers increased resistance to heat and ultraviolet (UV) irradiation in transgenic nematodes, and does not alter development or fertility. Unlike previously identified gerontogenes, tkr-1 positively modulates stress resistance and longevity. These results further support the positive relationship between increased stress resistance and increased longevity seen in all previously studied longevity mutants. This transgenic system is an effective means for identifying overexpression gerontogenes.

Endogenous beta-galactosidase activity in continuously nonproliferating cells

Cytochemically detectable activity of endogenous beta-galactosidase was found at pH 6.0 in Swiss 3T3 cells after long-term incubation in low serum or in the presence of heparin concentrations known to reversibly inhibit cell proliferation. A high percentage of beta-galactosidase-positive cells were detected in U937 and HL60 cultures at the late stage of macrophage-like differentiation induced by TPA. Interestingly, a small number of beta-galactosidase-positive cells were found even in the growing Swiss 3T3 cultures. These positive cells expressed morphological features similar to those of senescent cells. Thus, the activity of beta-galactosidase at pH 6.0 cannot be considered an exclusive marker of senescent cells since it is expressed in other types of nonproliferating cells.

A mutation in succinate dehydrogenase cytochrome b causes oxidative stress and ageing in nematodes

Much attention has focused on the aetiology of oxidative damage in cellular and organismal ageing. Especially toxic are the reactive oxygen byproducts of respiration and other biological processes. A mev-1(kn1) mutant of Caenorhabditis elegans has been found to be hypersensitive to raised oxygen concentrations. Unlike the wild type, its lifespan decreases dramatically as oxygen concentrations are increased from 1 to 60%. Strains bearing this mutation accumulate markers of ageing (such as fluorescent materials and protein carbonyls) faster than the wild type. We show here that mev-1 encodes a subunit of the enzyme succinate dehydrogenase cytochrome b, which is a component of complex II of the mitochondrial electron transport chain. We found that the ability of complex II to catalyse electron transport from succinate to ubiquinone is compromised in mev-1 animals. This may cause an indirect increase in superoxide levels, which in turn leads to oxygen hypersensitivity and premature ageing. Our results indicate that mev-1 governs the rate of ageing by modulating the cellular response to oxidative stress.

Expanded CTG repeats in myotonin protein kinase increase susceptibility to oxidative stress

The effect of oxidative stress on myogenic cells with expanded CTG repeats in the myotonin protein kinase (MtPK) gene was investigated using MtPK cDNA-transformants in order to investigate the disease process underlying myotonic dystrophy. We employed methylmercury as a model for reagents that produce reactive oxygen species (ROS). Mutant MtPK cDNA transformants containing 46 CTG repeats treated with 1 microM methylmercury for 24 h underwent cell death showing the characteristics of apoptosis. In contrast, methylmercury-induced cytotoxicity was weaker in wild type MtPK cDNA transformants. Antioxidants such as N-acetyl-L-cysteine and trolox suppressed methylmercury-induced apoptosis, indicating that the intracellular generation of ROS plays an important role. These studies suggest that expanded CTG repeats in MtPK increase the susceptibility of cells to oxidative stress.

Disruption of redox homeostasis in the transforming growth factor-alpha/c-myc transgenic mouse model of accelerated hepatocarcinogenesis

In previous studies we have demonstrated that transforming growth factor (TGF)-alpha/c-myc double transgenic mice exhibit an enhanced rate of cell proliferation, accumulate extensive DNA damage, and develop multiple liver tumors between 4 and 8 months of age. To clarify the biochemical events that may be responsible for the genotoxic and carcinogenic effects observed in this transgenic model, several parameters of redox homeostasis in the liver were examined prior to development of hepatic tumors. By 2 months of age, production of reactive oxygen species, determined by the peroxidation-sensitive fluorescent dye, 2',7'-dichlorofluorescin diacetate, was significantly elevated in TGF-alpha/c-myc transgenic hepatocytes versus either wild type or c-myc single transgenic cells, and occurred in parallel with an increase in lipid peroxidation. Concomitantly with a rise in oxidant levels, antioxidant defenses were decreased, including total glutathione content and the activity of glutathione peroxidase, whereas thioredoxin reductase activity was not changed. However, hepatic tumors which developed in TGF-alpha/c-myc mice exhibited an increase in thioredoxin reductase activity and a very low activity of glutathione peroxidase. Furthermore, specific deletions were detected in mtDNA as early as 5 weeks of age in the transgenic mice. These data provide experimental evidence that co-expression of TGF-alpha and c-myc transgenes in mouse liver promotes overproduction of reactive oxygen species and thus creates an oxidative stress environment. This phenomenon may account for the massive DNA damage and acceleration of hepatocarcinogenesis observed in the TGF-alpha/c-myc mouse model.

Pineal peptide preparation epithalamin increases the lifespan of fruit flies, mice and rats

Treatment with pineal peptide preparation epithalamin was followed by the increase of the mean lifespan of female D. melanogaster, SHR mice, C3H/Sn mice and LIO rats by 11-31% (P < 0.05). Ninety percent mortality as well as maximum lifespan were increased in fruit flies, C3H/Sn mice and rats. Mortality rate was decreased by 52% in D. melanogaster, by 52% in rats, by 27% in C3H/Sn mice. It did not change in SHR mice exposed to epithalamin. Treatment with the pineal peptide increased MRDT in flies, C3H/Sn mice and rats. It has been shown that epithalamin increased synthesis and secretion of melatonin in rats and inhibits free radical processes in rats and in D. melanogaster. It is suggested that antioxidative properties of epithalamin lead to increased lifespan of three different animal species.