Oxygen accelerates the accumulation of mutations during the senescence and immortalization of murine cells in culture

DNA double-strand breaks: a potential causative factor for mammalian aging?

Aging is a pleiotropic and stochastic process influenced by both genetics and environment. As a result the fundamental underlying causes of aging are controversial and likely diverse. Genome maintenance and in particular the repair of DNA damage is critical to ensure longevity needed for reproduction and as a consequence imperfections or defects in maintaining the genome may contribute to aging. There are many forms of DNA damage with double-strand breaks (DSBs) being the most toxic. Here we discuss DNA DSBs as a potential causative factor for aging including factors that generate DNA DSBs, pathways that repair DNA DSBs, consequences of faulty or failed DSB repair and how these consequences may lead to age-dependent decline in fitness. At the end we compare mouse models of premature aging that are defective for repairing either DSBs or UV light-induced lesions. Based on these comparisons we believe the basic mechanisms responsible for their aging phenotypes are fundamentally different demonstrating the complex and pleiotropic nature of this process.

Fibroblasts from naked mole-rats are resistant to multiple forms of cell injury, but sensitive to peroxide, ultraviolet light, and endoplasmic reticulum stress

Fibroblasts from long-lived mutant mice are resistant to many forms of lethal injury as well as to the metabolic effects of rotenone and low-glucose medium. Here we evaluated fibroblasts from young adult naked mole-rats (NMR; Heterocephalus glaber), a rodent species in which maximal longevity exceeds 28 years. Compared to mouse cells, NMR cells were resistant to cadmium, methyl methanesulfonate, paraquat, heat, and low-glucose medium, consistent with the idea that cellular resistance to stress may contribute to disease resistance and longevity. Surprisingly, NMR cells were more sensitive than mouse cells to H(2)O(2), ultraviolet (UV) light, and rotenone. NMR cells, like cells from Snell dwarf mice, were more sensitive to tunicamycin and thapsigargin, which interfere with the function of the endoplasmic reticulum (ER stress). The sensitivity of both Snell dwarf and NMR cells to ER stress suggests that alterations in the unfolded protein response might modulate cell survival and aging rate.

The in vivo gene expression signature of oxidative stress

How higher organisms respond to elevated oxidative stress in vivo is poorly understood. Therefore, we measured oxidative stress parameters and gene expression alterations (Affymetrix arrays) in the liver caused by elevated reactive oxygen species induced in vivo by diquat or by genetic ablation of the major antioxidant enzymes CuZn-superoxide dismutase (Sod1) and glutathione peroxidase-1 (Gpx1). Diquat (50 mg/kg) treatment resulted in a significant increase in oxidative damage within 3-6 h in wild-type mice without any lethality. In contrast, treatment of Sod1(-/-) or Gpx1(-/-) mice with a similar concentration of diquat resulted in a significant increase in oxidative damage within an hour of treatment and was lethal, i.e., these mice are extremely sensitive to the oxidative stress generated by diquat. The expression response to elevated oxidative stress in vivo does not involve an upregulation of classic antioxidant genes, although long-term oxidative stress in Sod1(-/-) mice leads to a significant upregulation of thiol antioxidants (e.g., Mt1, Srxn1, Gclc, Txnrd1), which appears to be mediated by the redox-sensitive transcription factor Nrf2. The main finding of our study is that the common response to elevated oxidative stress with diquat treatment in wild-type, Gpx1(-/-), and Sod1(-/-) mice and in untreated Sod1(-/-) mice is an upregulation of p53 target genes (p21, Gdf15, Plk3, Atf3, Trp53inp1, Ddit4, Gadd45a, Btg2, Ndrg1). A retrospective comparison with previous studies shows that induction of these p53 target genes is a conserved expression response to oxidative stress, in vivo and in vitro, in different species and different cells/organs.

Mutation frequencies and spectra in DNA polymerase eta-deficient mice

The low-fidelity polymerase eta (poleta) is required for bypass of UV-induced pyrimidine dimers inserting adenine nucleotides opposite these lesions. Mutations in the poleta gene are responsible for the genetic defect in xeroderma pigmentosum variant patients. To study if the lack of poleta significantly elevates spontaneous mutation frequency in various organs and tissues of the mouse, we crossed poleta-deficient mice with transgenic mice harboring a chromosomally integrated lacZ-plasmid reporter construct. In cultured embryonic fibroblasts from the lacZ-poleta(-/-) mice, 2.5 J/m(2) UV irradiation induced approximately 5-fold more mutations than in cells from lacZ control mice, in which an approximately 3-fold increase in mutation frequency was found compared with the normal level. Whereas untreated cells harbored mainly 1-bp deletions, UV induced both transitions and transversions, with the latter type more highly represented in the poleta-null cells than in the controls. No difference in mutation induction between the poleta-null cells and the wild-type cells was observed after treatment with N-ethyl-N-nitrosourea. Having shown the validity of the lacZ model to accurately identify poleta-associated mutagenesis, we then determined the mutant frequency at the lacZ locus in liver, spleen, and small intestine of 12-month-old animals. No differences were found between poleta-null, heterozygous, or littermate control mice. We conclude that the poleta defect is specific for UV damage and has no effect on in vivo mutagenesis in mice.

Analysis of cellular senescence in culture in vivo: the senescence-associated beta-galactosidase assay

Replicative senescence, a process first described almost 40 years ago, entails irreversible growth arrest with sustained metabolic functions, and it is also associated with increased resistance to apoptotic signals. Interest in this process has increased greatly over the last 10 years, as it has been demonstrated that senescence can function as a potent tumor suppressor mechanism. Although mounting evidence suggests that the senescent phenotype is associated with an extraordinarily complex array of gene expression patterns and interactions with the microenvironment, there is only one widely accepted marker for distinguishing such cells in vitro and in vivo. This marker is the senescence-associated expression of a pH 6 beta-galactosidase (SA-beta-gal). Here, a method for analyzing SA-beta-gal expression in cultured cells and in human and animal tissues is described, and important parameters to consider when performing such assays are also highlighted.

Free radicals and senescence

There is a significant body of experimental evidence that a rise in intracellular reactive oxygen species (ROS) contributes to senescence. Here we review experiments where entry into senescence has been evaluated in cells whose intracellular ROS levels have been modulated by growth in either high or low ambient oxygen concentrations, or where the cellular antioxidant status has been perturbed. In addition, we discuss the observations that senescence triggered by oncogene expression also appears to be in part mediated by a rise in ROS levels. Finally, we discuss the emerging evidence that in vivo senescence might also be triggered by a rise in cellular oxidant levels. Although these data tend to support a role for ROS in mediating senescence, significant questions remain as to whether ROS act in a random or specific fashion and what precise oxidant species acts as the potential senescence trigger.

Lack of mutagenicity of acrolein-induced DNA adducts in mouse and human cells

Acrolein is an endogenous metabolite and a ubiquitous environmental pollutant. Recently, it has been suggested that acrolein is a major etiologic agent for tobacco smoking-related lung cancer. Despite the known DNA-damaging effects of acrolein, its mutagenicity to mammalian cells remains uncertain. We have investigated acrolein-induced DNA damage in relation to mutagenesis, with special focus on DNA repair, in mouse and human cells. We mapped the formation of acrolein-induced DNA adducts and the kinetics of repair of the induced lesions in the cII transgene, the mutational target, in acrolein-treated transgenic mouse fibroblasts. Acrolein-DNA adducts were formed preferentially at specific nucleotide positions, mainly at G:C base pairs, along the cII transgene. The induced acrolein-DNA adducts were moderately resistant to DNA repair. Quantification of cII mutant frequency in acrolein-treated cells, however, revealed that acrolein was not mutagenic to these cells at doses sufficient to produce DNA adducts. Determination of supF mutant frequency in DNA repair-proficient and DNA repair-deficient human fibroblasts transfected with acrolein-treated plasmids confirmed a lack of acrolein mutagenicity. Because CpG methylation may intensify acrolein-DNA adduction, we examined whether the extent of CpG methylation in the supF gene can determine acrolein-induced mutagenesis in human cells. Enhancement of acrolein-DNA adduction by methylating CpGs in the supF sequence did not elicit a mutagenic response in human fibroblasts, however. We conclude that acrolein is not mutagenic to mouse and human fibroblasts, regardless of DNA repair capacity or methylation status of CpGs, possibly because of a highly accurate replication bypass of the induced lesions.

Intra-organ variation in age-related mutation accumulation in the mouse

Using a transgenic mouse model harboring chromosomally integrated lacZ mutational target genes, we previously demonstrated that mutations accumulate with age much more rapidly in the small intestine than in the brain. Here it is shown that in the small intestine point mutations preferentially accumulate in epithelial cells of the mucosa scraped off the underlying serosa. The mucosal cells are the differentiated villus cells that have undergone multiple cell divisions. A smaller age-related increase, also involving genome rearrangements, was observed in the serosa, which consists mainly of the remaining crypts and non-dividing smooth muscle cells. In the brain we observed an accumulation of only point mutations in no other areas than hypothalamus and hippocampus. To directly test for cell division as the determining factor in the generation of point mutations we compared mutation induction between mitotically active and quiescent embryonic fibroblasts from the same lacZ mice, treated with either UV (a point mutagen) or hydrogen peroxide (a clastogen). The results indicate that while point mutations are highly replication-dependent, genome rearrangements are as easily induced in non-dividing cells as in mitotically active ones. This strongly suggests that the point mutations found to have accumulated in the mucosal part of the small intestine are the consequence of replication errors. The same is likely true for point mutations accumulating in hippocampus and hypothalamus of the brain since neurogenesis in these two areas continues throughout life. The observed intra-organ variation in mutation susceptibility as well as the variation in replication dependency of different types of mutations indicates the need to not only extend observations made on whole organs to their sub-structures but also take the type of mutations and mitotic activity of the cells into consideration. This should help elucidating the impact of genome instability and its consequences on aging and disease.

Trends in oxidative aging theories

The early observations on the rate-of-living theory by Max Rubner and the report by Gershman that oxygen free radicals exist in vivo culminated in the seminal proposal in the 1950s by Denham Harman that reactive oxygen species are a cause of aging (free radical theory of aging). The goal of this review is to analyze recent findings relevant in evaluating Harman's theory using experimental results as grouped by model organisms (i.e., invertebrate models and mice). In this regard, we have focused primarily on recent work involving genetic manipulations. Because the free radical theory of aging is not the only theorem proposed to explain the mechanism(s) involved in aging at the molecular level, we also discuss how this theory is related to other areas of research in biogerontology, specifically, telomere/cell senescence, genomic instability, and the mitochondrial hypothesis of aging. We also discuss where we think the free radical theory is headed. It is now possible to give at least a partial answer to the question whether oxidative stress determines life span as Harman posed so long ago. Based on studies to date, we argue that a tentative case for oxidative stress as a life-span determinant can be made in Drosophila melanogaster. Studies in mice argue for a role of oxidative stress in age-related disease, especially cancer; however, with regard to aging per se, the data either do not support or remain inconclusive on whether oxidative stress determines life span.

Iodine deficiency activates antioxidant genes and causes DNA damage in the thyroid gland of rats and mice

Because thyroid nodules are frequent in areas with iodine deficiency the aim of this study was to characterise molecular events during iodine deficiency that could explain mutagenesis and nodule formation. We therefore studied gene expression of catalytic enzymes prominent for H(2)O(2) detoxification and antioxidative defence, quantified DNA oxidation and damage as well as spontaneous mutation rates (SMR) in mice and rats fed an iodine controlled diet. Antioxidative enzymes such as superoxide dismutase 3, glutathione peroxidase 4 and the peroxiredoxins 3 and 5 showed increased mRNA expression, which indicates increased radical burden that could be the cause of additional oxidized base adducts found in thyroidal genomic DNA in our experiments of iodine deficiency. Furthermore, the uracil content of thyroid DNA was significantly higher in the iodine-deficient compared to the control group. While SMR is very high in the normal thyroid gland it is not changed in experimental iodine deficiency. Our data suggest that iodine restriction causes oxidative stress and DNA modifications. A higher uracil content of the thyroid DNA could be a precondition for C-->T transitions often detected as somatic mutations in nodular thyroid tissue. However, the absence of increased SMR would argue for more efficient DNA repair in response to iodine restriction.

Impaired genome maintenance suppresses the growth hormone--insulin-like growth factor 1 axis in mice with Cockayne syndrome

Cockayne syndrome (CS) is a photosensitive, DNA repair disorder associated with progeria that is caused by a defect in the transcription-coupled repair subpathway of nucleotide excision repair (NER). Here, complete inactivation of NER in Csb^m/m/Xpa^−/− mutants causes a phenotype that reliably mimics the human progeroid CS syndrome. Newborn Csb^m/m/Xpa^−/− mice display attenuated growth, progressive neurological dysfunction, retinal degeneration, cachexia, kyphosis, and die before weaning. Mouse liver transcriptome analysis and several physiological endpoints revealed systemic suppression of the growth hormone/insulin-like growth factor 1 (GH/IGF1) somatotroph axis and oxidative metabolism, increased antioxidant responses, and hypoglycemia together with hepatic glycogen and fat accumulation. Broad genome-wide parallels between Csb^m/m/Xpa^−/− and naturally aged mouse liver transcriptomes suggested that these changes are intrinsic to natural ageing and the DNA repair–deficient mice. Importantly, wild-type mice exposed to a low dose of chronic genotoxic stress recapitulated this response, thereby pointing to a novel link between genome instability and the age-related decline of the somatotroph axis.

Studies in mice defective in two DNA repair pathways (global NER and TCR; an animal model for Cockayne syndrome) highlight a link between aging, a failure to repair DNA lesions, and metabolic alterations.

Normal metabolism routinely produces reactive oxygen species that damage DNA and other cellular components and is thought to contribute to the ageing process. Although DNA damage is typically kept in check by a variety of enzymes, several premature ageing disorders result from failure to remove damage from active genes. Patients with Cockayne syndrome (CS), a genetic mutation affecting one class of DNA repair enzymes, display severe growth retardation, neurological symptoms, and signs of premature ageing followed by an early death. Whereas mouse models for CS exhibit relatively mild deficits, we show that concomitant inactivation of a second DNA repair gene elicits severe CS pathology and ageing. Moreover, a few days after birth, these mice undergo systemic suppression of genes controlling growth, an unexpected decrease in oxidative metabolism, and an increased antioxidant response. Similar physiological changes are also triggered in normal mice by chronic exposure to DNA-damaging oxidative stress. From these findings, we conclude that DNA damage triggers a response aimed at limiting oxidative DNA damage levels (and associated tissue degeneration) to extend lifespan and promote healthy ageing. Better understanding of the ageing process will help to delineate intervention strategies to combat age-associated pathology.

Age-related accumulation of autosomal mutations in solid tissues of the mouse is gender and cell type specific

Most cancers in solid tissues increase with age and invariably contain causal mutations eliminating expression of one or more autosomal tumor suppressor genes. However, very little is known about the effect of age on autosomal mutations, often large in size, in cells of solid tissues. In this study, the frequency and spectrum of autosomal mutations were examined as a function of age for kidney epithelial cells and ear mesenchymal cells in B6D2F1 mice heterozygous for the selectable Aprt locus. Aprt mutant frequencies were found to increase with age in the kidneys of both male and female mice, but at all ages the mutant frequencies were approximately twice as high in the females, which in this strain have shorter lifespans than the males. An age-related increase in Aprt mutant frequencies was also observed for ear cells from female mice, but no significant increases in mutant frequencies were observed for the ear cells of male mice. A molecular analysis showed that the kidney and ear mutational spectra were distinct and that the age-related increases in mutant frequencies did not involve significant shifts in the mutational spectra. In total, the results demonstrate both gender and cell-type-specific patterns of autosomal mutational accumulation as a function of age in two solid tissues of the mouse.

Skin-derived fibroblasts from long-lived species are resistant to some, but not all, lethal stresses and to the mitochondrial inhibitor rotenone

Fibroblast cell lines were developed from skin biopsies of eight species of wild-trapped rodents, one species of bat, and a group of genetically heterogeneous laboratory mice. Each cell line was tested in vitro for their resistance to six varieties of lethal stress, as well as for resistance to the nonlethal metabolic effects of the mitochondrial inhibitor rotenone and of culture at very low glucose levels. Standard linear regression of species-specific lifespan against each species mean stress resistance showed that longevity was associated with resistance to death induced by cadmium and hydrogen peroxide, as well as with resistance to rotenone inhibition. A multilevel regression method supported these associations, and suggested a similar association for resistance to heat stress. Regressions for resistance to cadmium, peroxide, heat, and rotenone remained significant after various statistical adjustments for body weight. In contrast, cells from longer-lived species did not show significantly greater resistance to ultraviolet light, paraquat, or the DNA alkylating agent methylmethanesulfonate. There was a strong correlation between species longevity and resistance to the metabolic effects of low-glucose medium among the rodent cell lines, but this test did not distinguish mice and rats from the much longer-lived little brown bat. These results are consistent with the idea that evolution of long-lived species may require development of cellular resistance to several forms of lethal injury, and provide justification for evaluation of similar properties in a much wider range of mammals and bird species.

Retinal degeneration and ionizing radiation hypersensitivity in a mouse model for Cockayne syndrome

Mutations in the CSB gene cause Cockayne syndrome (CS), a DNA repair disorder characterized by UV sensitivity and severe physical and neurological impairment. CSB functions in the transcription-coupled repair subpathway of nucleotide excision repair. This function may explain the UV sensitivity but hardly clarifies the other CS symptoms. Many of these, including retinopathy, are associated with premature aging. We studied eye pathology in a mouse model for CS. Csb(m/m) mice were hypersensitive to UV light and developed epithelial hyperplasia and squamous cell carcinomas in the cornea, which underscores the importance of transcription-coupled repair of photolesions in the mouse. In addition, we observed a spontaneous loss of retinal photoreceptor cells with age in the Csb(m/m) retina, resulting in a 60% decrease in the number of rods by the age of 18 months. Importantly, when Csb(m/m) mice (as well as Csa(-/-) mice) were exposed to 10 Gy of ionizing radiation, we noticed an increase in apoptotic photoreceptor cells, which was not observed in wild-type animals. This finding, together with our observation that the expression of established oxidative stress marker genes is upregulated in the Csb(m/m) retina, suggests that (endogenous) oxidative DNA lesions play a role in this CS-specific premature-aging feature and supports the oxidative DNA damage theory of aging.

Chk2 mediates stabilization of the FoxM1 transcription factor to stimulate expression of DNA repair genes

The forkhead box M1 (FoxM1) transcription factor regulates expression of cell cycle genes essential for DNA replication and mitosis during organ repair and cancer progression. Here, we demonstrate that FoxM1-deficient (-/-) mouse embryonic fibroblasts and osteosarcoma U2OS cells depleted in FoxM1 levels by small interfering RNA transfection display increased DNA breaks, as evidenced by immunofluorescence focus staining for phosphospecific histone H2AX. FoxM1-deficient cells also exhibit stimulation of p53 transcriptional activity, as evidenced by increased expression of the p21(cip1) gene. FoxM1-deficient cells display reduced expression of the base excision repair factor X-ray cross-complementing group 1 (XRCC1) and breast cancer-associated gene 2 (BRCA2), the latter of which is involved in homologous recombination repair of DNA double-strand breaks. Furthermore, FoxM1 protein is phosphorylated by checkpoint kinase 2 (Chk2) in response to DNA damage. This phosphorylation of FoxM1 on serine residue 361 caused increased stability of the FoxM1 protein with corresponding increased transcription of XRCC1 and BRCA2 genes, both of which are required for repair of DNA damage. These results identify a novel role for FoxM1 in the transcriptional response during DNA damage/checkpoint signaling and show a novel mechanism by which Chk2 protein regulates expression of DNA repair enzymes.

Induction of A.T to G.C mutations by erroneous repair of depurinated DNA following estrogen treatment of the mammary gland of ACI rats

Evidence suggests that the genotoxic mechanism of estrogens (estrone/estradiol) in breast cancer involves their oxidation to 3,4-quinones and reaction with DNA to form depurinating N3Ade and N7Gua adducts. We examined whether estrogen genotoxicity is mutagenic in the mammary gland of the female ACI rat, a model for estrogen-dependent breast cancer. Mutagenesis was studied by PCR amplification of the H-ras1 gene (exons 1-2), cloning in pUC18, transforming Escherichia coli, and sequencing the inserts in plasmids from individual colonies. Mammary glands of both estrogen-responsive (ACI and DA) and resistant (Sprague-Dawley) rats contained pre-existing mutations at frequencies of (39.8-58.8)x10(-5), the majority (62.5-100%) of which were A.T to G.C transitions. Estradiol-3,4-quinone (200 nmol) treatment of ACI rats caused rapid (6h to 1 day) mutagenesis (frequency (83.3-156.1)x10(-5); A.T to G.C 70-73.3%). The estrogen-induced A.T to G.C mutations were detected as G.T heteroduplexes, as would be expected if N3Ade depurinations caused Gua misincorporations by erroneous repair. These heteroduplexes were identified by the T.G-DNA glycosylase (TDG) assay. TDG converts G.T heteroduplexes to G.abasic sites, rendering DNA templates refractory to PCR amplification. Consequently, A.T to G.C mutations present as G.T heteroduplexes in the DNA are eliminated from the spectra. TDG treatment of mammary DNA from estradiol-3,4-quinone-treated ACI rats brought A.T to G.C mutations down to pre-existing frequencies. Our results demonstrate that treatment with estradiol-3,4-quinone, an important metabolite of estrogens, produced A.T to G.C mutations in the DNA of the mammary gland of ACI rats.

p53-Dependent and -independent functions of the Arf tumor suppressor

The Ink4a-Arf locus encodes two closely wedded tumor suppressor proteins (p16(Ink4a) and p19(Arf)) that inhibit cell proliferation by activating Rb and p53, respectively. With few exceptions, the Arf gene is repressed during mouse embryonic development, thereby helping to limit p53 expression during organogenesis. However, in adult mice, sustained hyperproliferative signals conveyed by somatically activated oncogenes can induce Arf gene expression and trigger a p53 response that eliminates incipient cancer cells. Disruption of this tumor surveillance pathway predisposes to cancer, and inactivation of INK4a- ARF by deletion, silencing, or mutation has been frequently observed in many forms of human cancer. Although it is accepted that much of Arf's tumor-suppressive activity is mediated by p53, more recent genetic evidence has pointed to additional p53- independent functions of Arf, including its ability to inhibit gene expression by a number of other transcription factors. Surprisingly, the enforced expression of Arf in mammalian cells promotes the sumoylation of several Arf-interacting proteins, implying that Arf has an associated catalytic activity. We speculate that transcriptional down-regulation in response to Arf-induced sumoylation may account for Arf's p53-independent functions.

Physiologic oxygen enhances human embryonic stem cell clonal recovery and reduces chromosomal abnormalities

Human embryonic stem cells (hESC) have great potential in regenerative medicine, provided that culture systems are established that maintain genomic integrity. Here we describe a comparison of the effects of culture in either physiologic oxygen (2%) or room oxygen (21%) on the hESC lines, H1, H9, and RH1. Physiologic oxygen enabled an average sixfold increase in clone recovery across the hESC lines tested (p < 0.001). FACS analysis showed that cells cultured in physiologic oxygen were significantly smaller and less granular. No significant changes had occurred in levels of SSEA4, SSEA1, TRA-1-60, or TRA-1-81. While karyotypic normalcy was maintained in both H1 and H9, the frequency of spontaneous chromosomal aberrations was significantly increased in room oxygen. This increase was not observed in physiologic oxygen. These results clearly demonstrate that physiologic oxygen culture conditions are indispensable for robust hES clone recovery and may enhance the isolation of novel hES lines and transgenic clones.

Age-related accumulation of a novel CD44 + CD25lowgammadelta T-cell population in hematopoietic organs of the mouse

We discovered a novel population of gammadelta T cells in the mouse that accumulates with age in hematopoietic organs, but not in epithelia. These cells are CD25low (an unusual phenotype for gammadelta T cells in the mouse); express higher levels of TCRgammadelta and CD44 than do CD25- gammadelta T cells; mainly express Vgamma2, Vgamma3, and Vgamma4 chains; and are largely quiescent. A very similar cell population appears in the late stages of fetal thymus organ cultures, suggesting that the accumulation of CD44 + CD25lowTCRgammadelta + cells is a response to stress induced by aging in vivo or by culture in vitro. The precursors of CD44 + CD25lowTCRgammadelta + cells are generated during fetal or very young adult life, as this population was undetectable in aged recipients of bone marrow from old or young donors. CD44 + CD25lowTCRgammadelta + cells may be a biomarker of aging, but could also play a role in the inflammatory changes that accompany aging.

Increased cell-to-cell variation in gene expression in ageing mouse heart

The accumulation of somatic DNA damage has been implicated as a cause of ageing in metazoa. One possible mechanism by which increased DNA damage could lead to cellular degeneration and death is by stochastic deregulation of gene expression. Here we directly test for increased transcriptional noise in aged tissue by dissociating single cardiomyocytes from fresh heart samples of both young and old mice, followed by global mRNA amplification and quantification of mRNA levels in a panel of housekeeping and heart-specific genes. Although gene expression levels already varied among cardiomyocytes from young heart, this heterogeneity was significantly elevated at old age. We had demonstrated previously an increased load of genome rearrangements and other mutations in the heart of aged mice. To confirm that increased stochasticity of gene expression could be a result of increased genome damage, we treated mouse embryonic fibroblasts in culture with hydrogen peroxide. Such treatment resulted in a significant increase in cell-to-cell variation in gene expression, which was found to parallel the induction and persistence of genome rearrangement mutations at a lacZ reporter locus. These results underscore the stochastic nature of the ageing process, and could provide a mechanism for age-related cellular degeneration and death in tissues of multicellular organisms.

Deoxyribonucleic acid damage and spontaneous mutagenesis in the thyroid gland of rats and mice

Thyroid tumors are a frequent finding not only in iodine-deficient regions. They are predominantly characterized by somatic genetic changes (e.g. point mutations or rearrangements). Because slow thyroid proliferation is a apparent contradiction to a high frequency of tumor initiation, we characterized mutational events in thyroid. First we studied the frequency of certain base exchanges in somatic TSH receptor (TSHR) mutations and determined the spontaneous mutation rate in thyroid and liver. Then we applied different protocols of the comet assay to quantify genomic DNA damage and conducted immunohistochemistry for 8-oxoguanine as a molecular marker for oxidative stress. Among 184 somatic mutations of the human TSHR found in thyroid tumors, C-->T transitions had a unexpectedly high frequency (>32%). The mutation rate in thyroid is 8-10 times higher than in other organs. The comet assay detected increased levels of oxidized pyrimidine (2- to 3-fold) and purine (2- to 4-fold) in thyroid, compared with liver and lung, and a 1.6-fold increase of oxidized purine, compared with spleen. Immunohistochemistry revealed high levels of 8-oxoguanine in thyroid epithelial cells. We have shown a strikingly high mutation rate in the thyroid. Furthermore, results of the comet assay as well as immunohistochemistry suggest that oxidative DNA modifications are a likely cause of the higher mutation rate. It is possible that free radicals resulting from reactive oxygen species in the thyroid generate mutations more frequently. This is also supported by the spectrum of somatic mutations in the TSHR because more frequent base changes could stem from oxidized base adducts that we detected in the comet assay and with immunohistochemistry.

Sex-specific regulation of aging and apoptosis

Genetic analysis of Drosophila, mice and humans indicates that gene alleles, mutations and transgenes that affect life span tend to do so differently depending on the sex of the organism. The likely reason for this is that the sexes are different genotypes (e.g., X/X vs. X/Y) and face quite different environments: e.g., to reproduce, males have to mate with females while females have to mate with males. Genes are subject to different genetic interactions and different gene-by-environment effects in male vs. female. The consequence is that through evolution certain genes are differently selected and optimized for each sex. Both the mitochondrial genome and the X chromosome are asymmetrically inherited in Drosophila and mammals; through evolution these genes spend relatively more time under selection in females and are therefore expected to be better optimized for function in the female than in the male. Consistent with this the Drosophila X chromosome has been found to be a hotspot for sexually antagonistic fitness variation. Old Drosophila and old mammals exhibit apoptosis-an observation consistent with the idea that the mitochondria are less functional during aging due to maternal-only inheritance. One feature of aging that is common to Drosophila and mammals is that females tend to live longer than males, and this may be due in part to sub-optimal mitochondrial function in males. The data support the conclusion that a significant part of the aging phenotype is due to antagonistic pleiotropy of gene function between the sexes. Liberal application of Occam's razor yields a molecular model for the co-regulation of sex, apoptosis and life span based on the on/off status of a single gene: Sxl in Drosophila melanogaster and Xist in humans. Aging may simply represent an ancient and conserved mechanism by which genes re-assort.

Fibroblasts from long-lived Snell dwarf mice are resistant to oxygen-induced in vitro growth arrest

Snell dwarf mice live longer than controls, and show lower age-adjusted rates of lethal neoplastic diseases. Fibroblast cells from adult dwarf mice are resistant to the lethal effects of oxidative and nonoxidative stresses, including the carcinogen methyl methanesulfonate. We now report that dwarf-derived fibroblasts are slow to enter the stage of growth arrest induced by culturing normal cells under standard culture conditions at 20% O(2). Dwarf cells cultured at 20% O(2) resemble control cells cultured at 3% O(2) not only in their delayed growth arrest, but also in their rapid growth rates and resistance to both oxidative and nonoxidative forms of cytotoxic stress. Levels of the heat-shock protein HSP-70 respond to serum withdrawal and stress only in control cells, showing that intracellular signals are blunted in dwarf-derived cells. These data suggest a model in which stable epigenetic changes induced in skin fibroblasts by the hormonal milieu of the Snell dwarf lead to resistance to multiple forms of injury, including the oxidative damage that contributes to growth arrest in vitro and neoplasia in intact mice.

Mutant frequencies and spectra depend on growth state and passage number in cells cultured from transgenic lacZ-plasmid reporter mice

Transgenic mice harboring the lacZ gene within a plasmid that can be recovered and amplified in Escherichia coli, to establish mutant frequencies and spectra, have provided crucial insights into the relationships between mutations, cancer and aging in vivo. Here, we use embryonic fibroblasts from transgenic lacZ-plasmid reporter mice to determine the relationship between cell proliferation in culture and mutations induced by ultraviolet (UV) light. A single dose of 2.5J/m2 of UVC to actively proliferating cells caused an approximately eight-fold increase in mutant frequency 24 h after irradiation. Identically treated quiescent cells showed a two-fold increase in mutant frequency. Thus, whereas proliferation facilitated the acquisition of mutations, it was not an absolute requirement. Characterization of the UV-induced mutations indicated that the lower mutant frequency in quiescent cells was due mainly to a reduction in point mutations; size-change mutations, indicative of translocations or deletions, were relatively unaffected by the growth state of the cells. To investigate long-term genomic stability after UVC-induced damage, we monitored the lacZ locus in irradiated cells passaged for many generations in culture. The results indicated the emergence of jackpot mutations of rapidly changing frequency, most likely reflecting the successive emergence and decline of dominant cell clones during long-term culture. These findings show that the lacZ-plasmid locus is a valid reporter for studying induced mutations in short-term cultures of both quiescent and proliferating fibroblasts. In long-term cultures, the locus is less suitable for studying induced mutations owing to the instability of the cell population.

Organ-Specific Increase in Mutation Accumulation and Apoptosis Rate in CuZn-Superoxide Dismutase–Deficient Mice

Reactive oxygen species have been implicated as a cause of cancer and aging in mammals. Mice deficient for the antioxidant enzyme CuZn-superoxide dismutase (Sod1) have a decreased life span and an elevated incidence of liver cancer. To test the hypothesis that the cancer-prone phenotype in such mice is due to accelerated spontaneous mutation accumulation, we crossed these mutants with mice harboring a neutral lacZ mutation reporter gene. At 2 months of age, the lacZ mutation frequency in the liver of the hybrid animals was already twice as high as in littermate controls of the same age. This difference in mutation frequency increased to >3-fold at 6 months of age, after which it did not increase any further. Characterization of the mutation spectra in liver of the Sod1-null mice indicated mainly GC-to-TA transversions and GC-to-AT transitions, signature mutations of oxidative stress. The accelerated mutation accumulation in liver was accompanied by an increased frequency of apoptotic cells, as indicated by an increase in both terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling- and caspase 3-stained cells at 6 and 12 months of age. In kidney, an elevated mutation frequency above controls of approximately 2.5-fold was found not earlier than at 6 months. No increased mutation accumulation was observed in brain or spleen. These results support the hypothesis, that oxidative stress is an important causal factor of cancer in mammals.

Forkhead box M1 regulates the transcriptional network of genes essential for mitotic progression and genes encoding the SCF (Skp2-Cks1) ubiquitin ligase

The Forkhead box m1 (Foxm1) gene is critical for G(1)/S transition and essential for mitotic progression. However, the transcriptional mechanisms downstream of FoxM1 that control these cell cycle events remain to be determined. Here, we show that both early-passage Foxm1(-)(/)(-) mouse embryonic fibroblasts (MEFs) and human osteosarcoma U2OS cells depleted of FoxM1 protein by small interfering RNA fail to grow in culture due to a mitotic block and accumulate nuclear levels of cyclin-dependent kinase inhibitor (CDKI) proteins p21(Cip1) and p27(Kip1). Using quantitative chromatin immunoprecipitation and expression assays, we show that FoxM1 is essential for transcription of the mitotic regulatory genes Cdc25B, Aurora B kinase, survivin, centromere protein A (CENPA), and CENPB. We also identify the mechanism by which FoxM1 deficiency causes elevated nuclear levels of the CDKI proteins p21(Cip1) and p27(Kip1). We provide evidence that FoxM1 is essential for transcription of Skp2 and Cks1, which are specificity subunits of the Skp1-Cullin 1-F-box (SCF) ubiquitin ligase complex that targets these CDKI proteins for degradation during the G(1)/S transition. Moreover, early-passage Foxm1(-)(/)(-) MEFs display premature senescence as evidenced by high expression of the senescence-associated beta-galactosidase, p19(ARF), and p16(INK4A) proteins. Taken together, these results demonstrate that FoxM1 regulates transcription of cell cycle genes critical for progression into S-phase and mitosis.

Co-mutagenic activity of arsenic and benzo[a]pyrene in mouse skin

Exposure to inorganic arsenic in drinking water is linked to skin, lung and bladder cancer in humans. The mechanism of arsenic-induced cancer is not clear, but exposure to arsenic and polycyclic arylhydrocarbons (PAH) is more carcinogenic than exposure to either type of carcinogen alone. Arsenic can also generate reactive oxygen species, suggesting that oxidation of DNA may play a role in carcinogenesis. Oxidization of guanosines in polyG tracts is known to cause frameshift mutations, and such events can be detected in situ using the G11 placental alkaline phosphatase (PLAP) transgenic mouse model, which reports frameshift mutations in a run of 11 G:C basepairs by generating cells containing heat-resistant alkaline phosphatase activity. PAH can also induce frameshift mutations. In the study described here, FVB/N mice carrying the G11 PLAP transgene were crossed to C57Bl/6 mice. Half of the hybrid mice were given drinking water with sodium arsenite (10 mg/L) for 10 weeks. Half of the arsenic treated mice were also exposed to benzo[a]pyrene (BaP) by skin painting (500 nmol/week) for 8 weeks. Another group of mice was exposed to BaP but not arsenic. The effect on frameshift mutation was assessed by staining sections of skin tissue to detect cells with PLAP activity. Arsenic alone had no significant effect. On average, mice given BaP alone had approximately three times more PLAP-positive (PLAP+) cells. By contrast, mice exposed to both arsenic and BaP exhibited 10-fold more PLAP+ cells in the skin, and these cells were often arranged in large clusters, suggesting derivation from stem cells. Whereas combined treatment produced more PLAP+ cells, stable BaP adduct levels and arsenic burdens were not higher in mice exposed to both agents compared to mice exposed to either one agent or the other.

Early loss of proliferative potential of human peritoneal mesothelial cells in culture: the role of p16INK4a-mediated premature senescence

Much has been learned about the mechanisms underlying cellular senescence. The pathways leading to senescence appear to vary, depending on the cell type and cell culture conditions. In this respect, little is known about senescence of human peritoneal mesothelial cells (HPMC). Previous studies have significantly differed in the reported proliferative lifespan of HPMC. Therefore, in the present study, we have examined how HPMC enter state of senescence under conditions typically used for HPMC culture. HPMC were isolated from omentum and grown into senescence. The cultures were assessed for the growth rate, the presence of senescence markers, activation of cell-cycle inhibitors, and the oxidative stress. HPMC were found to reach, on average, six population doublings before senescence. The terminal growth arrest was associated with decreased expression of Ki67 antigen, increased percentage of cells in the G1 phase, reduced early population doubling level cDNA-1 mRNA expression, and the presence of senescence-associated beta-galactosidase. Compared with early-passage cells, the late-passage HPMC exhibited increased expression of p16INK4a but not of p21Cip1. In addition, these cells generated more reactive oxygen species and displayed increased presence of oxidatively modified DNA (8-hydroxy-2'-deoxyguanosine). These results demonstrate that early onset of senescence in omentum-derived HPMC may be associated with oxidative stress-induced upregulation of p16INK4a.

In vivo misregulation of genes involved in apoptosis, development and oxidative stress in mice lacking both functional Werner syndrome protein and poly(ADP-ribose) polymerase-1

Werner syndrome (WS) is a rare disorder characterized by the premature onset of a number of age-related diseases. The gene responsible for WS is believed to be involved in different aspects of transcription, replication and/or DNA repair. The poly(ADP-ribose) polymerase-1 (PARP-1) enzyme is also involved in DNA repair and is known to affect transcription of several genes. In this study, we examined the expression profile of cells lacking the normal function of either or both enzymes. All mutant cells exhibited altered expression of genes normally responding to oxidative stress. Interestingly, more than 58% of misregulated genes identified in double mutant cells were not altered in cells with either the Wrn or PARP-1 mutation alone. So, the impact on gene expression profile when both Wrn and PARP-1 are mutated was greater than a simple addition of individual mutant genotype. In addition, double mutant cultured cells showed major misregulation of genes involved in apoptosis, cell cycle control, embryonic development, metabolism and signal transduction. More importantly, in vivo analyses of double mutant mice have confirmed the increased apoptosis and the developmental defects in embryos as well as the major increase in intracellular phosphorylation and oxidative DNA damage in adult tissues. They also exhibited a progressive increase in oxidative stress with age. Thus, a major result of this study is that changes in expression of several genes and physiological functions identified in vitro were confirmed in mouse embryonic and adult tissues.

Gene expression responses to DNA damage are altered in human aging and in Werner Syndrome

The accumulation of DNA damage and mutations is considered a major cause of cancer and aging. While it is known that DNA damage can affect changes in gene expression, transcriptional regulation after DNA damage is poorly understood. We characterized the expression of 6912 genes in human primary fibroblasts after exposure to three different kinds of cellular stress that introduces DNA damage: 4-nitroquinoline-1-oxide (4NQO), gamma-irradiation, or UV-irradiation. Each type of stress elicited damage specific gene expression changes of up to 10-fold. A total of 85 genes had similar changes in expression of 3-40-fold after all three kinds of stress. We examined transcription in cells from young and old individuals and from patients with Werner syndrome (WS), a segmental progeroid condition with a high incidence of cancer, and found various age-associated transcriptional changes depending upon the type of cellular stress. Compared to young individuals, both WS and old individuals had similarly aberrant transcriptional responses to gamma- and UV-irradiation, suggesting a role for Werner protein in stress-induced gene expression. Our results suggest that aberrant DNA damage-induced gene regulation may contribute to the aging process and the premature aging in WS.

Fibroblast cell lines from young adult mice of long-lived mutant strains are resistant to multiple forms of stress

Previous studies have shown that dermal fibroblast cell lines derived from young adult mice of the long-lived Snell dwarf mutant stock are resistant, in vitro, to the cytotoxic effects of H(2)O(2), cadmium, UV light, paraquat, and heat. We show here that similar resistance profiles are seen in fibroblast cells derived from a related mutant, the Ames dwarf mouse, and that cells from growth hormone receptor-null mice are resistant to H(2)O(2), paraquat, and UV but not to cadmium. Resistance to UV light, cadmium, and H(2)O(2) are similar in cells derived from 1-wk-old Snell dwarf or normal mice, and thus the resistance of cell lines derived from young adult donors reflects developmental processes, presumably hormone dependent, that take place in the first few months of life. The resistance of cells from Snell dwarf mice to these stresses does not reflect merely antioxidant defenses: dwarf-derived cells are also resistant to the DNA-alkylating agent methyl methanesulfonate. Furthermore, inhibitor studies show that fibroblast resistance to UV light is unaffected by the antioxidants ascorbic acid and N-acetyl-L-cysteine. These data suggest that postnatal exposure to altered levels of pituitary hormones leads to development of cellular resistance to oxidative and nonoxidative stressors, which are stable through many rounds of in vitro cell division and could contribute to the remarkable disease resistance of long-lived mutant mice.

Stress defense in murine embryonic stem cells is superior to that of various differentiated murine cells

A very small number of embryonic stem (ES) cells gives rise to all tissues of the embryo proper. This means that ES cells should be equipped with highly efficient mechanisms to defend themselves against various stresses and to prevent or repair DNA damage. One of these mechanisms is a high activity of a verapamil-sensitive multidrug efflux pump. Because reactive oxygen species are a major source of DNA damage, we further tested the idea that murine ES cells might differ from their more differentiated counterparts by high levels of antioxidant defense and good DNA strand break repair capacity. This was confirmed by comparing cellular peroxide levels, total antioxidant capacity, and activity of radiation-induced strand break repair between murine ES cells and embryoid bodies or embryonic fibroblasts. Using microarrays and confirmation by reverse transcription-polymerase chain reaction, we identified several candidate antioxidant and stress-resistance genes that become downregulated during differentiation of ES cells into embryoid bodies.

Oxidative mutagenesis, mismatch repair, and aging

A PubMed search for the term "oxidative stress" yields over 29,000 articles published on the subject over the past 10 years; more than 2000 of these articles also include the term "aging" in their title or abstract. Many theories of aging predict causal roles for oxidative stress in the myriad of pathological changes that occur as a function of age, including an increasing propensity to develop cancer. A possible link between aging and cancer is the induction and accumulation of somatic mutations caused by oxidative stress. This Review focuses on small mutational events that are induced by oxidative stress and the role of mismatch repair (MMR) in preventing their formation. It also discusses a possible inhibitory effect of oxidative stress on MMR. We speculate that a synergistic interaction between oxidative damage to DNA and reduced MMR levels will, in part, account for an accumulation of small mutational events, and hence cancer, with aging.

Increased mutation in mice genetically predisposed to oxidative damage in the brain

Harlequin (Hq) mice develop ataxia due to an X-linked recessive mutation in the gene encoding apoptosis-inducing factor (Aif). Brain cells in Hq mice contain the modified base 8-hydroxydeoxyguanosine (8-OHdG), suggesting that the defect in Aif causes increased DNA oxidation in these cells. Because oxidative damage is mutagenic, Hq mice might suffer increased mutation in the brain. To examine this possibility, mutation in the brain was assessed using the Tg(betaA-G11PLAP) mouse model, which allows mutant cells to be visualized in tissue sections in situ. Hq mice exhibited more and larger patches of PLAP positive tissue in the brain. PLAP+ cells were observed in all areas of the brain. No increase in the number of PLAP+ cells was seen in three other tissues, suggesting that the effect of Aif deficiency on mutation was specific to brain.

Telomere-independent cellular senescence in human fetal cardiomyocytes

Fetal cardiomyocytes have been proposed as a potential source of cell-based therapy for heart failure. This study examined cellular senescence in cultured human fetal ventricular cardiomyocytes (HFCs). HFCs were isolated and identified by immunocytochemistry and RT-PCR. Cells were found to senesce after 20-25 population doublings, as determined by growth arrest, morphological changes and senescence-associated beta-galactosidase activity. Using the telomeric repeat amplification protocol assay, telomerase activity was undetectable in primary HFCs. Cells were transduced to express the human reverse transcriptase subunit (hTERT) of telomerase. This resulted in greatly increased telomerase activity, but no significant lifespan extension. Analysis of telomere length in primary HFCs revealed that the senescent phenotype was not accompanied by telomere shortening. Telomeres in hTERT-positive cells were elongated in comparison with primary cells, and elongation was retained in senescent cells. Levels of the tumor suppressor protein p16INK4A increased in all senescent cells whether telomerase-positive or -negative. Senescence was accompanied by a decline in transcript levels of the polycomb gene Bmi-1, Ets1 and Ets2 transcription factors, and Id1, Id2 and Id3 helix-loop-helix proteins, suggesting roles for these genes in maintenance of cardiomyocyte proliferative capacity. In addition to offering novel insights into the behavior of human fetal cardiomyocytes in culture, these findings have implications for the development of a cell-based therapy for cardiac injury using primary fetal heart tissue.

The impact of DNA damage, genetic mutation and cellular responses on cancer prevention, longevity and aging: observations in humans and mice

Over the past 5 years, data collected from the mouse suggest that pathways important for either preventing or resolving DNA damage are longevity assurance mechanisms whose critical overall function is somatic cell maintenance, a necessary part of cancer prevention. These pathways include those that reduce DNA damage levels caused by exogenous sources, replication errors and by-products of cellular respiration. Unresolved DNA damage leads to permanent mutations in the genetic code that may be oncogenic. Therefore, pathways that resolve DNA damage are important anti-cancer mechanisms. As an important line of defense, there are a variety of pathways that repair DNA damage. In addition, there are anti-cancer pathways that respond to DNA damage by either preventing cellular replication or inducing cell death. Genes in these pathways, termed longevity assurance genes (LAG), code for proteins that reduce cancer incidence and as a result assures a sufficiently long health span needed for reproduction. Data from mouse models, many that were originally designed to study cancer, are showing that a potential consequence of DNA damage and responses to DNA damage is aging; these models support the hypothesis that at least some aspects of normal aging are the consequence of anticancer mechanisms designed to deal with damaged DNA.

Senescence and immortalization: role of telomeres and telomerase

Telomere dynamics are a critical component of both aging and cancer. Telomeres progressively shorten in almost all dividing cells and most human cells do not express or maintain sufficient telomerase activity to fully maintain telomeres. There is accumulating evidence that when only a few telomeres are short, they form end-associations, leading to a DNA damage signal resulting in replicative senescence (a cellular growth arrest, also called the M1 stage). In the absence of cell-cycle checkpoint pathways (e.g. p53 and or p16/Rb), cells bypass M1 senescence and telomeres continue to shorten eventually resulting in crisis (also called the M2 stage). M2 is characterized by many 'uncapped' chromosome ends, end-fusions, chromosome breakage fusion-bridge cycles, mitotic catastrophe and a high fraction of apoptotic cells. In a rare M2 cell, telomerase (a cellular reverse transcriptase) can be reactivated or up-regulated, resulting in indefinite cell proliferation. This cellular immortalization is a potentially rate-limiting step in carcinogenesis that is important for the continuing evolution of most advanced cancers. In this perspective we will present our views on the evidence for telomere dysfunction in aging and in cancer progression. We will argue that telomere shortening in the absence of other alterations may be a potent tumor suppressor mechanism and we will discuss the evidence for and against the major molecular mechanisms proposed to initiate replicative senescence.

Replicative senescence: a critical review

Human cells in culture have a limited proliferative capacity. After a period of vigorous proliferation, the rate of cell division declines and a number of changes occur in the cells including increases in size, in secondary lysosomes and residual bodies, nuclear changes and a number of changes in gene expression which provide biomarkers for senescence. Although human cells in culture have been used for over 40 years as models for understanding the cellular basis of aging, the relationship of replicative senescence to aging of the organism is still not clear. In this review, we discuss replicative senescence in the light of current information on signal transduction and mitogenesis, cell stress, apoptosis, telomere changes and finally we discuss replicative senescence as a model of aging in vivo.

Genomic instability, aging, and cellular senescence

Aging can be defined in practical terms as a series of time-related processes that ultimately bring life to a close. Genomic instability has been implicated as a major causal factor in aging. Here, we describe the use of a transgenic mouse model, harboring lacZ reporter genes as part of a plasmid construct integrated at one or more chromosomal locations, to study genomic instability during aging of different mouse organs and tissues as well as in mouse embryonic fibroblasts during primary culture.