DNA damage and repair during cellular aging

Biogerontology: research status, challenges and opportunities

Biogerontology is the study of the biological basis of ageing and age-related diseases. The phenomenon and the process of ageing are well understood in evolutionary and biological terms; and a conceptual framework has been established within which general principles of ageing and longevity can be formulated. The phenotype of ageing in terms of progressive loss of physical function and fitness is best seen during the period of survival after the evolution-determined essential lifespan (ELS) of a species. However, the ageing phenotype is highly heterogenous and individualistic at all levels from the whole body to the molecular one. Most significantly, the process and the progression of ageing are not determined by any specific gerontogenes. Ageing is the result of imperfect maintenance and repair systems that allow a progressive shrinkage of the homeodynamic space of an individual. The challenge is to develop and apply wholistic approaches to the complex trait of ageing for maintaining and/or improving health. One such approach is that of mild stress-induced physiological hormesis by physical, mental and nutritional hormetins. Biogerontological research offers numerous opportunities for developing evidence-based novel biomedical technologies for maintaining and improving health, for preventing the onset of age-related diseases, and for extending the health-span.

The role of DNA damage and repair in aging through the prism of Koch-like criteria

Since the first publication on Somatic Mutation Theory of Aging (Szilárd, 1959), a great volume of knowledge in the field has been accumulated. Here we attempted to organize the evidence "for" and "against" the hypothesized causal role of DNA damage and mutation accumulation in aging in light of four Koch-like criteria. They are based on the assumption that some quantitative relationship between the levels of DNA damage/mutations and aging rate should exist, so that (i) the longer-lived individuals or species would have a lower rate of damage than the shorter-lived, and (ii) the interventions that modulate the level of DNA damage and repair capacity should also modulate the rate of aging and longevity and vice versa. The analysis of how the existing data meets the proposed criteria showed that many gaps should still be filled in order to reach a clear-cut conclusion. As a perspective, it seems that the main emphasis in future studies should be put on the role of DNA damage in stem cell aging.

[Biology of aging: theories, mechanisms, and perspectives]

Abstract The article reviews the major biological theories of aging, and discusses the most relevant mechanisms to explain the aging process. It begins with the evolutionary theories, explores the molecular-cellular mechanisms, and presents the perspective of the systemic theories. The complex etiology of aging is a challenge to the researchers. The knowledge on that phenomenon develops towards an integrative approach.

Does metformin prevent short-term oxidant-induced dna damage? In vitro study on lymphocytes from aged subjects

Metformin(1-(diaminomethylidene)-3,3-dimethyl-guanidine), an anti-hyperglycemic agent, also has antioxidant effects. Although the origin is not clearly understood, the antioxidant activity of metformin might result from a direct effect on reactive oxygen species (ROS) or could have an indirect action on the superoxide anions produced by hyperglycemia. The ability of metformin to modulate DNA damage produced by oxidative stress is not known. For this reason, we examined the short term effect of metformin (50 microM, 2 h) on the DNA damage of cumene hydroperoxide (CumOOH)-induced lymphocytes from aged and young control groups (n = 10 each). In this study, DNA damage elicited by CumOOH (1 mM) was detected with the Comet Assay and the ELISA technique. Our results showed a significant increase in apoptotic DNA fragmentation and DNA strand breaks (Comet assay tail factor %) that was detected before and after CumOOH induction in lymphocytes of healthy elderly subjects when compared with healthy young control. Metformin significantly decreased CumOOH-induced apoptotic DNA fragmentation and DNA strand breaks in lymphocytes from aged subjects, although it did not produce a long-term effect. The in vitro results indicate that the short-term effect of metformin can protect against prooxidant stimulus-induced DNA damage in lymphocytes from elderly subjects.

Metformin does not prevent DNA damage in lymphocytes despite its antioxidant properties against cumene hydroperoxide-induced oxidative stress

Metformin (1-(diaminomethylidene)-3,3-dimethyl-guanidine), which is the most commonly prescribed oral antihyperglycaemic drug in the world, was reported to have several antioxidant properties such as the inhibition of advanced glycation end-products. In addition to its use in the treatment of diabetes, it has been suggested that metformin may be a promising anti-aging agent. The present work was aimed at assessing the possible protective effects of metformin against DNA-damage induction by oxidative stress in vitro. The effects of metformin were compared with those of N-acetylcysteine (NAC). For this purpose, peripheral blood lymphocytes from aged (n=10) and young (n=10) individuals were pre-incubated with various concentrations of metformin (10-50microM), followed by incubation with 15microM cumene hydroperoxide (CumOOH) for 48h, under conditions of low oxidant level, which do not induce cell death. Protection against oxidative DNA damage was evaluated by use of the Comet assay and the cytokinesis-block micronucleus technique. Changes in the levels of malondialdehyde+4-hydroxy-alkenals, an index of oxidative stress, were also measured in lymphocytes. At concentrations ranging from 10microM to 50microM, metformin did not protect the lymphocytes from DNA damage, while 50microM NAC possessed an effective protective effect against CumOOH-induced DNA damage. Furthermore, NAC, but not metformin, inhibited DNA fragmentation induced by CumOOH. In contrast to the lack of protection against oxidative damage in lymphocyte cultures, metformin significantly protected the cells from lipid peroxidation in both age groups, although not as effective as NAC in preventing the peroxidative damage at the highest doses. Within the limitations of this study, the results indicate that pharmacological concentrations of metformin are unable to protect against DNA damage induced by a pro-oxidant stimulus in cultured human lymphocytes, despite its antioxidant properties.

Understanding and modulating ageing

Ageing is characterized by a progressive accumulation of molecular damage in nucleic acids, proteins and lipids. The inefficiency and failure of maintenance, repair and turnover pathways is the main cause of age-related accumulation of damage. Research in molecular gerontology is aimed at understanding the genetic and epigenetic regulation of survival and maintenance mechanisms at the levels of transcription, post-transcriptional processing, post-translational modifications, and interactions among various gene products. Concurrently, several approaches are being tried and tested to modulate ageing in a wide variety of organisms. The ultimate aim of such studies is to improve the quality of human life in old age and prolong the health-span. Various gerontomodulatory approaches include gene therapy, hormonal supplementation, nutritional modulation and intervention by free radical scavengers and other molecules. A recent approach is that of applying hormesis in ageing research and therapy, which is based on the principle of stimulation of maintenance and repair pathways by repeated exposure to mild stress. A combination of molecular, physiological and psychological modulatory approaches can realize "healthy ageing" as an achievable goal in the not-so-distant future.

Enhanced Sensitivity to Oxidant-Induced Micronucleus Frequency in Elderly Individuals Is Not Associated with Glutathione S- Transferase M1 (GSTM1) Null Genotype in Lymphocytes

BACKGROUND

A large number of studies have demonstrated that various kinds of DNA damage accumulate during aging and that oxidative stress possibly contributes to this process. Glutathione S-transferase M1 (GSTM1) can prevent their possible effects on DNA via detoxification of reactive substances that induced oxidative stress.

OBJECTIVE

To investigate the relationship between GSTM1 polymorphism and DNA sensitivity to oxidative stress with age, we used micronucleus (MN) frequency as a marker of DNA damage in lymphocytes from young and elderly subjects.

METHODS

This study was performed in 30 young (age range 20-36 years) and 30 elderly (age range 66-87 years) healthy individuals who were chosen on the basis of their GSTM1 genotype (15 GSTM1 null and 15 GSTM1 positive for each group). Lymphocytes were cultured after Ficoll isolation and treated for 48 h with a 30-muM dose of cumene hydroperoxide (CumOOH), a dose that does not decrease cell viability.

RESULTS

There was no significant difference in the MN frequency observed in control cultures from young and elderly individuals. However, the MN frequency in CumOOH-treated cultures was significantly higher in the elderly group than the young group (p < 0.001). No association was found between the GSTM1 phenotype and CumOOH-induced MN frequency.

CONCLUSIONS

The results suggest that lymphocytes of elderly individuals are more susceptible to in vitro MN induction by CumOOH. However, this difference in susceptibility is not explained by the lack of GSTM1.

Biogerontology: the next step

After a long period of collecting empirical data describing the changes in organisms, organs, tissues, cells, and macromolecules, biogerontological research is now able to develop various possibilities for intervention. Because aging is a stochastic and nondeterministic process characterized by a progressive failure of maintenance and repair, it is reasoned that gene involved in homeodynamic repair pathways are the most likely candidate gerontogenes. A promising approach for the identification of critical gerontogenic processes is through the hormesis-like positive effects of mild stress. Stimulation of various repair pathways by mild stress has significant effects on delaying the onset of various age-associated alterations in cells, tissues, and organisms.

Gene-specific DNA repair of pyrimidine dimers does not decline during cellular aging in vitro

A large number of studies have demonstrated that various kinds of DNA damage accumulate during aging and one of the causes for this could be a decrease in DNA repair capacity. However, the level of total genomic repair has not been strongly correlated with aging. DNA repair of certain kinds of damage is known to be closely connected to the transcription process; thus, we chose to investigate the level of gene-specific repair of UV-induced damage using in vitro aging of human diploid skin fibroblasts and trabecular osteoblasts as model systems for aging. We find that the total genomic repair is not significantly affected during cellular aging of cultures of both human skin fibroblasts and trabecular osteoblasts. Gene-specific repair was analyzed during cellular aging in the dihydrofolate reductase housekeeping gene, the p53 tumor suppressor gene, and the inactive region X(754). There was no clear difference in the capacity of young and old cells to repair UV-induced pyrimidine dimers in any of the analyzed genes. Thus, in vitro senescent cells can sustain the ability to repair externally induced damage.

PCR assay of DNA damage and repair at the gene level in brain and spleen of gamma-irradiated young and old rats

The PCR amplification of fragments of transcribed (beta-actin, p53) and nontranscribed (IgE, heavy chain) genes in brain and spleen DNA from gamma-irradiated and unirradiated 2- and 28-month-old rats was studied. The amplification levels of fragments of these genes in DNA from old rats were substantially lower than those from young rats, which suggested that these gene fragments in old-rat DNA contained lesions blocking thermostable polymerase in PCR. The beta-actin and IgE gene fragments of spleen DNA from old rats exhibited a significantly higher level of lesions inhibiting Tth polymerase compared to analogous fragments of brain DNA from the same animals. DNA from the tissues of gamma-irradiated rats showed the amount of damage inhibiting amplification to be dependent on animal age and the postirradiation time before DNA isolation. As judged from the changes in the amplification level of gene fragments, there was no preferential fast repair of lesions in the actively transcribed gene beta-actin compared to the nontranscribed gene IgE (heavy chain) in the brain and spleen of gamma-irradiated young and old rats. The amplification results suggest that equal amounts of DNA lesions were repaired in the brain of both old and young rats during the first 0.5 h of the postirradiation time (fast-repair phase), whereas in the subsequent postirradiation period over 5 h (slow-repair phase), the efficiency of damage elimination in the brain DNA of old rats was markedly lower. As for the spleen tissue, the elimination of lesions blocking Tth polymerase was much lower in old gamma-irradiated animals for both of the repair phases.

Psoralen photoactivation promotes morphological and functional changes in fibroblasts in vitro reminiscent of cellular senescence

Premature aging of the skin is a prominent side effect of psoralen photoactivation, a treatment used widely for various skin disorders. The molecular mechanisms underlying premature aging upon psoralen photoactivation are as yet unknown. Here we show that treatment of fibroblasts with 8-methoxypsoralen (8-MOP) and subsequent ultraviolet A (UVA) irradiation resulted in a permanent switch of mitotic to stably postmitotic fibroblasts which acquired a high level of de novo expression of SA-beta-galactosidase, a marker for fibroblast senescence in vitro and in vivo. A single exposure of fibroblasts to 8-MOP/UVA resulted in a 5.8-fold up-regulation of two matrix-degrading enzymes, interstitial collagenase (MMP-1) and stromelysin-1 (MMP-3), over a period of &gt;120 days, while TIMP-1, the major inhibitor of MMP-1 and MMP-3, was only slightly induced. This imbalance between matrix-degrading metalloproteases and their inhibitor may lead to connective tissue damage, a hallmark of premature aging. Superoxide anion and hydrogen peroxide, but not singlet oxygen, were identified as important intermediates in the downstream signaling pathway leading to these complex fibroblast responses upon psoralen photoactivation. Collectively, the end phenotype induced upon psoralen photoactivation shares several criteria of senescent cells. In the absence of detailed molecular data on what constitutes normal aging, it is difficult to decide whether the changes reported here reflect mechanisms underlying normal cellular aging/senescence or rather produce a mimic of cellular aging/senescence by quite different pathways.

The role of DNA damage in cellular aging: is it time for a reassessment?

There is now evidence that the immediate cause of the loss of proliferative capacity in senescent cells is mediated by a specific inhibitor. If this tentative interpretation is correct, the next hurdle will be to determine mechanism(s) that regulate this putative senescence cell inhibitor that would, in effect, be the determinant of proliferative life span. One previously proposed hypothesis predicts that the decline of replicative activity is analogous to a checkpoint response to accumulated chromosomal damage (Rosenberger et al., 1991). Advances in our basic understanding of the nature of DNA damage, DNA repair mechanisms, and the response of eukaryotic cells to accumulated DNA damage provide a solid rationale for a reassessment of the causal role of the accumulation of chromosomal damage in cell senescence in vitro.

DNA damage, mutation and fine structure DNA repair in aging

The primary focus of this review is on correlations found between DNA damage, repair, and aging. New techniques for the measurement of DNA damage and repair at the level of individual genes, in individual DNA strands and in individual nucleotides will allow us to gain information regarding the nature of these correlations. Fine structure studies of DNA damage and repair in specific regions, including active genes, telomeres, and mitochondria have begun. Considerable intragenomic DNA repair heterogeneity has been found, and there have been indications of relationships between aging and repair in specific regions. More studies are necessary, however, particularly studies of the repair of endogenous damage. It is emphasized that the information obtained must be viewed from a perspective that takes into account the total responses of the cell to damaging events and the inter-relationships that exist between DNA repair and transcription.

The initiation of senescence and its relationship to embryonic cell differentiation

Mouse embryonic stem cells have an unlimited lifespan in cultures if they are prevented from differentiating. After differentiating, they produce cells which divide only a limited number of times. These changes seen in cultures parallel events that occur in the developing embryo, where immortal embryonic cells differentiate and produce mortal somatic ones. The data strongly suggest that differentiation initiates senescence, but this view entails additional assumptions in order to explain how the highly differentiated sexual gametes manage to remain potentially immortal. Cells differentiate by blocking expression from large parts of their genome and it is suggested that losses or gains of genetic totipotency determine cellular lifespans. Cells destined to be somatic do not regain totipotency and senesce, while germ-line cells regain complete genome expression and immortality after meiosis and gamete fusions. Losses of genetic totipotency could induce senescence by lowering the levels of repair and maintenance enzymes.

The mitochondrial F1-ATPase and the aging process

A progressive dysfunction of the mitochondrion probably plays a decisive role in the aging process. In the present hypothesis it is suggested that the functional defect specifically concerns the catalytic subunit of the mitochondrial F1-ATPase. This proposal is based on observations concerning two classical models of the aging process. 1. The Werner syndrome of premature aging is autosomally recessive; meaning that this disorder--in analogy with other recessive inborn errors of metabolism--results from a single specific mutation, typically resulting in an enzyme defect. 2. The strong association between the ATPase activity of the SV40 T-antigen and the process of cellular immortalization in vitro, suggests that the putative enzyme dysfunction could concern an ATPase. The decrease with aging in the activity of the mitochondrial F1-ATPase--the main producer of ATP--could lay behind the progressive lack of homeostasis observed in senescence.

Correlation between senescence and DNA repair in cells from young and old individuals and in premature aging syndromes

Cellular aging appears to be related to and perhaps caused by diminished DNA repair. To elucidate direct correlations between DNA repair capacity and senescence various parameters of cellular aging and DNA repair were studied simultaneously. Of special interest are features of DNA repair and senescence in cultured diploid fibroblasts derived either from healthy young or elderly probands as well as from patients suffering from premature senescence syndromes (Werner syndrome, Cockayne syndrome, ataxia telangiectasia and Down syndrome). Here we demonstrate the striking parallelism between reduced maximal lifespan, elevated levels of spontaneous chromosomal breaks, higher incidence of formation of micronuclei, a significant prolongation of cell cycle duration and a diminished reactivation of in vitro injured plasmid after transfection in cells from old individuals and from patients with premature senescence syndromes, suggesting a causal relationship between senescence and DNA damage.

Activity gel and activity blotting methods for detecting DNA-modifying (repair) enzymes

Zymographical methods (activity gel, overlay gel, activity blot and activity blotting) for detecting DNA-modifying (repair) enzymes are reviewed. Emphasis is put on the novel activity blotting method in which DNA repair enzymes electrophoresed on a gel are blotted and detected on a damaged DNA-fixed nylon membrane. Its practical procedures, including a non-radioactive detection procedure, and representative results are also described.

Reduction in the amount of 8-hydroxy-2'-deoxyguanosine in the DNA of SV40-transformed human fibroblasts as compared with normal cells in culture

DNA damage due to oxidative free radicals is considered to be a major cause of ageing and age-related diseases including cancer. Of more than 20 modifications formed in DNA by the action of hydroxyl radicals, 8-hydroxy-2'-deoxyguanosine (oh8dG) is potentially highly mutagenic and is known to occur most frequently. Using HPLC combined with electrochemical (HPLC/EC) detection of oh8dG, fivefold higher levels of oh8dG are detected in the DNA of cultured normal human skin fibroblasts as compared with SV40-transformed human fibroblasts MRC-5V2. In comparison, the levels of oh8dG were similar in the growth medium of both types of cells. Applications of this method range from studies on the genomic stability and instability of normal and cancerous cells to the clinical and laboratory testing of toxic substances and drugs.

Gene expression and aging

Considerable amount of data has accumulated during the past few years showing several changes in gene expression as a function of age. However, the basic mechanism of aging still remains poorly understood. In this review, we have mainly analysed the data pertaining to the hypothesis that aging is associated with genetic instability and have attempted further to highlight the gaps that need to be bridged in order to have a clear picture of the aging phenomenon. Extensive investigations employing new and novel approaches are needed in future to elucidate the intricately interwoven patterns of molecular control that underlie the various aspects of gene expression during aging.

Protein synthesis, posttranslational modifications, and aging

Posttranslational modifications of proteins are involved in determining their activities, stability, and specificity of interaction. More than 140 major and minor modifications of proteins have been reported. Of these, only a few have been studied in relation to the aging of cells, tissues, and organisms. These include phosphorylation, methylation, ADP-ribosylation, oxidation, glycation, and deamidation. Several of these modifications occur on proteins involved in crucial cellular processes, such as DNA synthesis, protein synthesis, protein degradation, signal transduction, cytoskeletal organization, and the components of extracellular matrix. Some of the modifications are the markers of abnormal and altered proteins for rapid degradation. Others make them less susceptible to degradation by normal proteolytic enzymes, and hence these accumulate during aging.

DNA damage and repair in brain: relationship to aging

The usefulness of conducting DNA damage and repair studies in a postmitotic tissue like brain is emphasized. We review studies that use brain as a tissue to test the validity of the DNA damage and repair hypothesis of aging. As far as the accumulation of age dependent DNA damage is concerned, the data appear to overwhelmingly support the hypothesis. However, attempts to demonstrate a decline in DNA repair capacity as a function of age are conflicting and equally divided. Possible reasons for this discrepancy are discussed. It is suggested that assessment of the repair capacity of neurons with respect to a specific type of damage in a specific gene might yield more definitive answers regarding the role of DNA repair potential in the aging process and as a longevity assurance system.

Demethylation of satellite I DNA during senescence of bovine adrenocortical cells in culture

Over the finite proliferative life span of cultured bovine adrenocortical cells, satellite I DNA shows a progressive and extensive loss of methylation at CCGG sites. This was shown by Southern blotting after digestion with the methylation-sensitive enzyme HpaII alone, which provides a sensitive indicator of methylation loss, or digestion with the combination of EcoRI and HpaII, which provides a quantitative indication of loss of methylation. Bovine tissues, including adrenal cortex, all showed a much higher level of satellite methylation than cultured adrenocortical cells. After adrenocortical cells are placed in culture, some demethylation of satellite I is seen as early as 10 population doublings. By 80 population doublings, loss of satellite DNA methylation is extensive. The loss does not appear to prevent continued cell division, since an extended life span clone of bovine adrenocortical cells transfected with SV40 T antigen showed a similar pattern of extensive demethylation. Satellite demethylation has been reported in aging in vivo and the present cell culture system may provide an in vitro model for this form of genetic instability.

Altered cellular responsiveness during ageing

The capacity of cells and organisms to respond to external stimuli and to maintain stability in order to survive decreases progressively during ageing. The mitogenic and stimulatory effects of growth factors, hormones and other agents are reduced significantly during cellular ageing. The sensitivity of ageing cells to toxic agents including antibiotics, phorbol esters, radiations and heat shock increases. This failure of homeostasis during cellular ageing does not appear to be due to any quantitative and qualitative defects in the receptor systems. Instead, metabolic defects in the pathways of macromolecular synthesis may be the basis of altered cellular responsiveness during ageing.

Changes in gene expression and DNA methylation in adrenocortical cells senescing in culture

Recent experiments in cultured bovine adrenocortical cells show that the previously observed phenotypic switching of CYP17 (steroid 17 alpha-hydroxylase) expression is preceded at a much earlier time by changes in methylation in the CYP17 5' flanking region. Two CpG sites that are methylated in the adrenal cortex in vivo were observed to undergo rapid demethylation when adrenocortical cells were placed in culture. Two adjacent CpG sites that are also methylated in vivo did not demethylate; these two sites are completely nonmethylated in fibroblasts. All CpG sites downstream, in the promoter or coding region, are always methylated in all tissues and in bovine adrenocortical cells even after many population doublings in culture. In contrast to the specific and rapid demethylation of sites in CYP17, satellite I shows a slower and apparently random loss of methylation that extends over the entire replicative life span. These changes in methylation provide examples of genetic instability in cells that undergo senescence in culture. Future experiments will focus on the relationship of these events to the phenotypic switching process.

Oxidants and antioxidants in proliferative senescence

In terms of the amount of experimental research it has generated the free radical theory of ageing is one of the most popular hypotheses to explain this ubiquitous phenomenon. From the theory two postulates were derived: either cellular defence mechanisms against free radical-dependent oxidants deteriorate during ageing of cells, or essential, unrepairable damages are imparted to the cell by oxidants regardless of the activity of antioxidant defence systems. The many reports dealing with a putative breakdown in antioxidant defence systems failed to positively support this postulate. However, a minor depletion in cellular glutathione by exposure to a model lipophilic peroxide led to a significant decrement in DNA and protein synthesis. In other words, the glutathione redox cycle is intrinsically fallible with respect to defending the cellular DNA replication system against this model lipophilic peroxide. Interestingly, after ageing in culture cells a partial uncoupling of the NADPH-producing and -consuming systems tends to take place. Experiments involving the addition of antioxidants to the culture medium have failed to significantly extend the lifespan of cultured diploid somatic cells. The level of antioxidants appears to be a modulator rather than a primary determinant of cellular ageing in culture. Several lines of evidence suggest that DNA damages accumulate during ageing of the organism, but no oxidant-related DNA damage has been pinpointed in the cultured cell system. Human mutants with defects in antioxidant enzymes have not shown conclusive signs of accelerated ageing. Cells from patients with Werner's syndrome (progeria of the adult), on the other hand, do not suffer from a defect in their antioxidant defence system, nor do they accumulate more than normal amounts of autofluorescent products resulting from lipid peroxidation. The recent finding that Werner's syndrome constitutes a mutator phenotype may prompt the comparison of oxidant- and ageing-related mutation spectra in order to investigate a mutational theory of ageing as a new derivative from the free radical hypothesis.

Homoeostatic imbalance during cellular ageing: altered responsiveness

The inability of normal cells to maintain themselves for ever is a reflection of homoeostatic imbalance and a progressive failure of maintenance. Ageing cells respond less to growth stimulants whereas they show increased sensitivity to toxic agents including antibiotics, phorbol esters, radiation and other physical stresses. No major quantitative and qualitative defects in the receptor systems have been detected that could explain the reasons for altered responsiveness during ageing. Random metabolic defects in the processes involved in maintaining homoeostasis may be critical for causing homoeostatic imbalance, cellular ageing and death.

Protein markers for cellular mortality and immortality

Fixed mortality of normal somatic cells is a well-established fact though the mechanism underlying this universal phenomenon remains unknown. Use of immortal cells in conjunction with their normal mortal counterparts has delineated the dominant genetic nature of the senescent phenotype over immortalization. Although the involvement of proteins in determining the entry/exit/arrest of cells in the cell cycle is evident from the literature, none of them has been confirmed for its role in senescence-associated irreversible cell cycle exit/arrest. The identification of true mortality markers might be possible by selecting a system of natural and conditional aging achieved by the fusion of mortal and spontaneously immortalized cells of the same origin. We report here a few such protein markers which might serve as useful handles to tease out the molecular events determining mortality/immortality of cultured cells.

A re-examination of the effects of ionizing radiation on lifespan and transformation of human diploid fibroblasts

Human diploid fibroblasts, strain MRC-5, were sequentially irradiated with 60Co gamma rays at intervals during their in vitro lifespan. The results indicate that 3 or 6 doses of 1 Gy can increase lifespan, and the same was true for cells treated with 3 doses of 3 Gy. Higher doses (5 x 3 Gy) did reduce growth potential, suggesting either that mid-late passage cells become more sensitive to radiation, or that doses beyond a given threshold reduce population lifespan by multiple cellular hits. The life extension induced by gamma rays might be due to an induced hypermethylation of DNA. Alternatively, oxygen radicals produced by irradiation might trigger an adaptive stress response which would remove damaged macromolecules and thereby increase the cells' growth potential. Whichever explanation is correct, the results show that the human fibroblast system is not appropriate for the study of the well known effect of ionizing radiation in shortening the lifespan of experimental animals. Contrary to earlier published results, populations of cells treated with cumulative doses of 15 Gy or 18 Gy and held for nearly 3 months after they had reached senescence (Phase III), produced no foci of transformed cells.

Protein synthesis and the components of protein synthetic machinery during cellular aging

The slowing down of protein synthesis is a change widely observed during the aging of organisms. It has also been claimed that a decline in the rate of protein synthesis occurs during cellular aging. However, the evidence in favour of this view is not clear-cut, and reliable estimates of rates of protein synthesis during cellular aging have yet to be made. Studies on various components of the protein synthetic machinery during cellular aging have revealed a decline in the efficiency and accuracy of ribosomes, an increase in the levels of rRNA and tRNA, and a decrease in the amounts and activities of elongation factors. Detailed studies on the structure and function of ribosomes, tRNA isoacceptor profiles, activities of aminoacyl-tRNA synthetases, levels and activities of initiation factors, rates of protein elongation, and the accuracy of protein synthesis will be needed before the molecular mechanisms of the regulation of protein synthesis during cellular aging can be understood.

ADP-ribosylation of nuclear proteins in rat ventral prostate during ageing

Poly(ADPR)polymerase activity and poly(ADP-ribosyl)ation of nuclear proteins have been investigated in ventral prostate nuclei of different aged rats (14, 28, 60, 180, 360 day old animals), by reverse-phase HPLC and acetic acid-urea polyacrylamide gel electrophoresis. The major ADP-ribose acceptor proteins were identified as histone H1 and H2b. It is concluded that concomitant with major changes to chromatin organization, poly(ADP-ribosyl)ation reaction is progressively inhibited during aging of rat ventral prostate. These results support the hypothesis that prostatic dysfunction in senescent animals is related to a failure of DNA repair mechanisms and deregulated template activity.