Strong effects of 5-azacytidine on the in vitro lifespan of human diploid fibroblasts

Epigenetic enzymes: A role in aging and prospects for pharmacological targeting

The development of interventions aimed at improving healthspan is one of the priority tasks for the academic and public health authorities. It is also the main objective of a novel branch in biogerontological research, geroscience. According to the geroscience concept, targeting aging is an effective way to combat age-related disorders. Since aging is an exceptionally complex process, system-oriented integrated approaches seem most appropriate for such an interventional strategy. Given the high plasticity and adaptability of the epigenome, epigenome-targeted interventions appear highly promising in geroscience research. Pharmaceuticals targeted at mechanisms involved in epigenetic control of gene activity are actively developed and implemented to prevent and treat various aging-related conditions such as cardiometabolic, neurodegenerative, inflammatory disorders, and cancer. In this review, we describe the roles of epigenetic mechanisms in aging; characterize enzymes contributing to the regulation of epigenetic processes; particularly focus on epigenetic drugs, such as inhibitors of DNA methyltransferases and histone deacetylases that may potentially affect aging-associated diseases and longevity; and discuss possible caveats associated with the use of epigenetic drugs.

Epigenetic Regulation of Cellular Senescence and Aging

Aging is characterized by functional decline of diverse organs and an increased risk for several diseases. Therefore, a high interest exists in understanding the molecular mechanisms that stimulate aging at all levels, from cells and tissues to organs and organisms, in order to develop ways to promote healthy aging. While many molecular and biochemical mechanisms are already understood in some detail, the role of changes in epigenetic regulation has only begun to be considered in recent years. The age-dependent global reduction in heterochromatin, along with site-specific changes in the patterns of DNA methylation and modification of histones, have been observed in several aging model systems. However, understanding of the precise role of such changes requires further research. In this review, we will discuss the role of epigenetic regulation in aging and indicate future research directions that will help elucidate the mechanistic details of it.

PIM1 induces cellular senescence through phosphorylation of UHRF1 at Ser311

PIM1 is a proto-oncogene, encoding a serine/threonine protein kinase that regulates cell proliferation, survival, differentiation and apoptosis. Previous reports suggest that overexpression of PIM1 can induce cellular senescence. However, the molecular mechanism underlying this process is not fully understood. Here we report that UHRF1 is a novel substrate of PIM1 kinase, which could be phosphorylated at Ser311 and therefore promoted to degradation. Our data demonstrates that PIM1 destabilizes UHRF1, leading to DNA hypomethylation, which consequently results in genomic instability, increased p16 expression and subsequent induction of cellular senescence. Taken together, our results suggest that down-regulation of UHRF1 is an important mechanism of PIM1-mediated cellular senescence.

The integration of epigenetics and genetics in nutrition research for CVD risk factors

There is increasing evidence documenting gene-by-environment (G × E) interactions for CVD related traits. However, the underlying mechanisms are still unclear. DNA methylation may represent one of such potential mechanisms. The objective of this review paper is to summarise the current evidence supporting the interplay among DNA methylation, genetic variants, and environmental factors, specifically (1) the association between SNP and DNA methylation; (2) the role that DNA methylation plays in G × E interactions. The current evidence supports the notion that genotype-dependent methylation may account, in part, for the mechanisms underlying observed G × E interactions in loci such asAPOE, IL6and ATP-binding cassette A1. However, these findings should be validated using intervention studies with high level of scientific evidence. The ultimate goal is to apply the knowledge and the technology generated by this research towards genetically based strategies for the development of personalised nutrition and medicine.

Aging Epigenetics: Accumulation of Errors or Realization of a Specific Program?

Aging in mammals is known to be accompanied by a progressive loss of methylated cytosines from DNA. This loss is tissue-specific to a certain extent and affects mainly repeated sequences, transposable elements, and intergenic genome parts. Age-dependent DNA hypomethylation is correlated with and perhaps partly caused by a diminished activity of DNA methyltransferases. Along with the global DNA demethylation during aging, hypermethylation of certain genes occurs. On the whole-genome scale, an age-dependent hypermethylation is typical for genes associated with promoter CG islands, whereas hypomethylation mostly affects CG-poor genes, besides the repeated sequences, transposable elements, and intergenic genome parts mentioned above. The methylation levels of certain CG sites display strict correlation to age and thus could be used as a molecular marker to predict biological age of cells, tissues, and organisms. Epigenetic cell reprogramming, such as induced pluripotent stem cell production, leads to complete resetting of their epigenetic age.

Ethanol extract of Dalbergia odorifera protects skin keratinocytes against ultraviolet B-induced photoaging by suppressing production of reactive oxygen species

Dalbergia odorifera T. Chen (Leguminosae), an indigenous medicinal herb, has been widely used in northern and eastern Asia to treat diverse diseases. Here, we investigated the anti-senescent effects of ethanolic extracts of Dalbergia odorifera (EEDO) in ultraviolet (UV) B-irradiated skin cells. EEDO significantly inhibited UVB-induced senescence of human keratinocytes in a concentration-dependent manner, concomitant with inhibition of reactive oxygen species (ROS) generation. UVB-induced increases in the levels of p53 and p21, biomarkers of cellular senescence, were almost completely abolished in the presence of EEDO. Sativanone, a major constituent of EEDO, also attenuated UVB-induced senescence and ROS generation in keratinocytes, indicating that sativanone is an indexing (marker) molecule for the anti-senescence properties of EEDO. Finally, treatment of EEDO to mice exposed to UVB significantly reduced ROS levels and the number of senescent cells in the skin. Thus, EEDO confers resistance to UVB-induced cellular senescence by inhibiting ROS generation in skin cells.

The role of DNA methylation in aging, rejuvenation, and age-related disease

DNA methylation is a major control program that modulates gene expression in a plethora of organisms. Gene silencing through methylation occurs through the activity of DNA methyltransferases, enzymes that transfer a methyl group from S-adenosyl-L-methionine to the carbon 5 position of cytosine. DNA methylation patterns are established by the de novo DNA methyltransferases (DNMTs) DNMT3A and DNMT3B and are subsequently maintained by DNMT1. Aging and age-related diseases include defined changes in 5-methylcytosine content and are generally characterized by genome-wide hypomethylation and promoter-specific hypermethylation. These changes in the epigenetic landscape represent potential disease biomarkers and are thought to contribute to age-related pathologies, such as cancer, osteoarthritis, and neurodegeneration. Some diseases, such as a hereditary form of sensory neuropathy accompanied by dementia, are directly caused by methylomic changes. Epigenetic modifications, however, are reversible and are therefore a prime target for therapeutic intervention. Numerous drugs that specifically target DNMTs are being tested in ongoing clinical trials for a variety of cancers, and data from finished trials demonstrate that some, such as 5-azacytidine, may even be superior to standard care. DNMTs, demethylases, and associated partners are dynamically shaping the methylome and demonstrate great promise with regard to rejuvenation.

Ligand-activated PPARδ inhibits UVB-induced senescence of human keratinocytes via PTEN-mediated inhibition of superoxide production

UV radiation-mediated photodamage to the skin has been implicated in premature aging and photoaging-related skin cancer and melanoma. Little is known about the cellular events that underlie premature senescence, or how to impede these events. In the present study we demonstrate that PPARδ (peroxisome-proliferator-activated receptor δ) regulates UVB-induced premature senescence of normal keratinocytes. Activation of PPARδ by GW501516, a specific ligand of PPARδ, significantly attenuated UVB-mediated generation of ROS (reactive oxygen species) and suppressed senescence of human keratinocytes. Ligand-activated PPARδ up-regulated the expression of PTEN (phosphatase and tensin homologue deleted on chromosome 10) and suppressed the PI3K (phosphatidylinositol 3-kinase)/Akt pathway. Concomitantly, translocation of Rac1 to the plasma membrane, which leads to the activation of NADPH oxidases and generation of ROS, was significantly attenuated. siRNA (small interfering RNA)-mediated knockdown of PTEN abrogated the effects of PPARδ on cellular senescence, on PI3K/Akt/Rac1 signalling and on generation of ROS in keratinocytes exposed to UVB. Finally, when HR-1 hairless mice were treated with GW501516 before exposure to UVB, the number of senescent cells in the skin was significantly reduced. Thus ligand-activated PPARδ confers resistance to UVB-induced cellular senescence by up-regulating PTEN and thereby modulating PI3K/Akt/Rac1 signalling to reduce ROS generation in keratinocytes.

Telomeres and telomerase: the commitment theory of cellular ageing revisited

It is not always realised that separate fibroblast populations of the same strain have very different lifespans, that is, over a million-fold range. This is best documented for human strains WI-38 and MRC-5. There is evidence that it is the molecular clock of telomere shortening which determines the growth potential of these cells. However, if a clock is set and runs its course one would expect parallel cultures to have similar lifespans. The commitment theory of fibroblast ageing proposes that commitment occurs during early cell divisions with a given probability and after that there is then a constant number of divisions until growth ceases. This constant number could be determined by the gradual loss of telomeres. The stochastic feature of the theory is the probability of the loss of the last uncommitted cells or the youngest committed cells. These cells have the longest lifespan and will give rise to the final population.

PPARδ coordinates angiotensin II-induced senescence in vascular smooth muscle cells through PTEN-mediated inhibition of superoxide generation

Cellular senescence-associated changes in blood vessels have been implicated in aging and age-related cardiovascular disorders. Here, we demonstrate that peroxisome proliferator-activated receptor (PPAR) δ coordinates angiotensin (Ang) II-induced senescence of human vascular smooth muscle cells (VSMCs). Activation of PPARδ by GW501516, a specific ligand for PPARδ, significantly attenuated Ang II-induced generation of superoxides and suppressed senescence of VSMCs. A marked increase in the levels of p53 and p21 induced by Ang II was blunted by the treatment with GW501516. Ligand-activated PPARδ up-regulated expression of phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and suppressed the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. Knockdown of PTEN with siRNA abrogated the effects of PPARδ on cellular senescence, on PI3K/Akt signaling, and on generation of ROS in VSMCs treated with Ang II. Finally, administration of GW501516 to apoE-deficient mice treated with Ang II significantly reduced the number of senescent cells in the aorta, where up-regulation of PTEN with reduced levels of phosphorylated Akt and ROS was demonstrated. Thus, ligand-activated PPARδ confers resistance to Ang II-induced senescence by up-regulation of PTEN and ensuing modulation of the PI3K/Akt signaling to reduce ROS generation in vascular cells.

Transcriptional up-regulation of antioxidant genes by PPARδ inhibits angiotensin II-induced premature senescence in vascular smooth muscle cells

This study evaluated peroxisome proliferator-activated receptor (PPAR) δ as a potential target for therapeutic intervention in Ang II-induced senescence in human vascular smooth muscle cells (hVSMCs). Activation of PPARδ by GW501516, a specific agonist of PPARδ, significantly inhibited the Ang II-induced premature senescence of hVSMCs. Agonist-activated PPARδ suppressed the generation of Ang II-triggered reactive oxygen species (ROS) with a concomitant reduction in DNA damage. Notably, GW501516 up-regulated the expression of antioxidant genes, such as glutathione peroxidase 1, thioredoxin 1, manganese superoxide dismutase and heme oxygenase 1. siRNA-mediated down-regulation of these antioxidant genes almost completely abolished the effects of GW501516 on ROS production and premature senescence in hVSMCs treated with Ang II. Taken together, the enhanced transcription of antioxidant genes is responsible for the PPARδ-mediated inhibition of premature senescence through sequestration of ROS in hVSMCs treated with Ang II.

Stable knockdown of PASG enhances DNA demethylation but does not accelerate cellular senescence in TIG-7 human fibroblasts

Demethylation of 5-methylcytosine in genomic DNA is believed to be one of the mechanisms underlying replicative life-span of mammalian cells. Both proliferation associated SNF2-like gene (PASG, also termed Lsh) and DNA methyltransferase 3B (Dnmt3b) knockout mice result in embryonic genomic hypomethylation and a replicative senescent phenotype. However, it is unclear whether gradual demethylation of DNA during somatic cell division is directly involved in senescence. In this study, we retrovirally transduced TIG-7 human fibroblasts with a shRNA against PASG and compared the rate of change in DNA methylation as well as the replicative life-span to control cells under low (3%) and ambient (20%) oxygen. Expression of PASG protein was decreased by approximately 80% compared to control cells following transduction of PASG shRNA gene. The rate of cell growth was the same in both control and PASG-suppressed cells. The rate of demethylation of DNA was significantly increased in PASG-suppressed cells as compared control cells. However, decreased PASG expression did not shorten the replicative life-span of TIG-7 cells. Culture under low oxygen extended the life-span of TIG-7 cells but did not alter the rate of DNA demethylation. While knockout of PASG during development results in genomic hypomethylation and premature senescence, our results show that while downregulation of PASG expression in a somatic cell also leads to DNA hypomethylation, there is no associated senescent phenotype. These results suggest differences in cellular consequences of hypomethylation mediated by PASG during development compared to that in somatic cells.

Epigenetic control of cellular senescence in disease: opportunities for therapeutic intervention

Understanding how senescence is established and maintained is an important area of study both for normal cell physiology and in tumourigenesis. Modifications to N-terminal tails of histone proteins, which can lead to chromatin remodelling, appear to be key to the regulation of the senescence phenotype. Epigenetic mechanisms such as modification of histone proteins have been shown to be sufficient to regulate gene expression levels and specific gene promoters can become epigenetically altered at senescence. This suggests that epigenetic mechanisms are important in senescence and further suggests epigenetic deregulation could play an important role in the bypass of senescence and the acquisition of a tumourigenic phenotype. Tumour suppressor proteins and cellular senescence are intimately linked and such proteins are now known to regulate gene expression through chromatin remodelling, again suggesting a link between chromatin modification and cellular senescence. Telomere dynamics and the expression of the telomerase genes are also both implicitly linked to senescence and tumourigenesis, and epigenetic deregulation of the telomerase gene promoters has been identified as a possible mechanism for the activation of telomere maintenance mechanisms in cancer. Recent studies have also suggested that epigenetic deregulation in stem cells could play an important role in carcinogenesis, and new models have been suggested for the attainment of tumourigenesis and bypass of senescence. Overall, proper regulation of the chromatin environment is suggested to have an important role in the senescence pathway, such that its deregulation could lead to tumourigenesis.

Dissecting the senescence-like program in tumor cells activated by Ras signaling

Activated Ras signaling can induce a permanent growth arrest in osteosarcoma cells. Here, we report that a senescence-like growth inhibition is also achieved in human carcinoma cells upon the transduction of H-Ras(V12). Ras-induced tumor senescence can be recapitulated by the transduction of activated, but not wild-type, MEK. The ability for H-Ras(V12) to suppress tumor cell growth is drastically compromised in cells that harbor endogenous activating ras mutations. Notably, growth inhibition of tumor cells containing ras mutations can be achieved through the introduction of activated MEK. Tumor senescence induced by Ras signaling can occur in the absence of p16 or Rb and is not interrupted by the inactivation of Rb, p107, or p130 via short hairpin RNA or the transduction with HPV16 E7. In contrast, inactivation of p21 via short hairpin RNA disrupts Ras-induced tumor senescence. In summary, this study uncovers a senescence-like program activated by Ras signaling to inhibit cancer cell growth. This program appears to be intact in cancer cells that do not harbor ras mutations. Moreover, cancer cells that carry ras mutations remain susceptible to tumor senescence induced by activated MEK. These novel findings can potentially lead to the development of innovative cancer intervention.

p21Waf1/Cip1 plays a critical role in modulating senescence through changes of DNA methylation

It has been reported that genomic DNA methylation decreases gradually during cell culture and an organism's aging. However, less is known about the methylation changes of age-related specific genes in aging. p21(Waf1/Cip1) and p16(INK4a) are cyclin-dependent kinase (Cdk) inhibitors that are critical for the replicative senescence of normal cells. In this study, we show that p21(Waf1/Cip1) and p16(INK4a) have different methylation patterns during the aging process of normal human 2BS and WI-38 fibroblasts. p21(Waf1/Cip1) promoter is gradually methylated up into middle-aged fibroblasts but not with senescent fibroblasts, whereas p16(INK4a) is always unmethylated in the aging process. Correspondently, the protein levels of DNA methyltransferase 1 (DNMT1) and DNMT3a increase from young to middle-aged fibroblasts but decrease in the senescent fibroblasts, while DNMT3b decreases stably from young to senescent fibroblasts. p21(Waf1/Cip1) promoter methylation directly represses its expression and blocks the radiation-induced DNA damage-signaling pathway by p53 in middle-aged fibroblasts. More importantly, demethylation by 5-aza-CdR or DNMT1 RNA interference (RNAi) resulted in an increased p21(Waf1/Cip1) level and premature senescence of middle-aged fibroblasts demonstrated by cell growth arrest and high beta-Galactosidase expression. Our results suggest that p21(Waf1/Cip1) but not p16(INK4a) is involved in the DNA methylation mediated aging process. p21(Waf1/Cip1) promoter methylation may be a critical biological barrier to postpone the aging process.

Folate supplementation increases genomic DNA methylation in the liver of elder rats

The availability of folate is implicated as a determinant of DNA methylation, a functionally important feature of DNA. Nevertheless, when this phenomenon has been examined in the rodent model, the effect has not always been observed. Several reasons have been postulated for the inconsistency between studies: the rodent is less dependent on folate as a methyl source than man; juvenile animals, which most studies use, are more resistant to folate depletion than old animals; methods to measure genomic DNA methylation might not be sensitive enough to detect differences. We therefore examined the relationship between folate and genomic DNA methylation in an elder rat model with a newly developed method that can measure genomic DNA methylation sensitively and precisely. Thirty-nine 1-year-old rats were divided into three groups and fed a diet containing 0, 4.5 or 18 mumol folate/kg (folate-deplete, -replete and -supplemented groups, respectively). Rats were killed at 8 and 20 weeks. At both time points, mean liver folate concentrations increased incrementally between the folate-deplete, -replete and -supplemented rats (P for trend <0.001) and by 20 weeks hepatic DNA methylation also increased incrementally between the folate-deplete, -replete and -supplemented rats (P for trend=0.025). At both time points folate-supplemented rats had significantly increased levels of DNA methylation compared with folate-deplete rats (P<0.05). There was a strong correlation between hepatic folate concentration and genomic DNA methylation in the liver (r 0.48, P=0.004). In the liver of this animal model, dietary folate over a wide range of intakes modulates genomic DNA methylation.

Histone deacetylase inhibitors induce a senescence-like state in human cells by a p16-dependent mechanism that is independent of a mitotic clock

We show here that histone deacetylase inhibitors (HDACIs) sodium dibutyrate (SDB) and trichostatin A (TSA) induce a phenotype that has similarities to replicative senescence in human fibroblasts. There was no evidence that SDB accelerated a constitutive cell division counting mechanism as previously suggested because cells pretreated with SDB for three mean population doublings (MPDs) exhibited a similar overall proliferative life span to controls once SDB was withdrawn. SDB-treated cells upregulated the cell cycle inhibitors p21(WAF1) and p16(INK4A), but not p14(ARF), in the same sequential order as in senescence and the cells developed biochemical markers of senescence. However, the mechanism of senescence did not involve telomere dysfunction and there was no evidence for any posttranslational modification of p53. The expression of human papillomavirus (HPV) 16 E6 in human fibroblasts or targeted disruption of the p53 and p21(WAF) genes only weakly antagonized HDACI-induced senescence. However, expression of the E7 gene, which inhibits the function of pRb, cooperated with E6 to block SDB-induced senescence completely and human cells deficient in p16(INK4A) (but not p14(ARF)) were also resistant to SDB-induced senescence, suggesting that the p16(INK4A)/pRb pathway is the major mediator of HDACI-induced senescence in human cells. However, p53-/- mouse fibroblasts were resistant to HDACI-induced senescence, identifying p53 as the major pathway to senescence in this species.

Growth retardation and premature aging phenotypes in mice with disruption of the SNF2-like gene, PASG

Imperfect maintenance of genome integrity has been postulated to be an important cause of aging. Here we provide support for this hypothesis by demonstrating that the disruption of PASG (lsh), a SNF2-like factor that facilitates DNA methylation, causes global hypomethylation, developmental growth retardation and a premature aging phenotype. PASG mutant mice display signs of growth retardation and premature aging, including low birth weight, failure to thrive, graying and loss of hair, reduced skin fat deposition, osteoporosis, kyphosis, cachexia, and premature death. Fibroblasts derived from PASG mutant embryos show a replicative senescence phenotype. Both PASG mutant mice and fibroblasts demonstrate a markedly increased expression of senescence-associated tumor suppressor genes, such as p16(INK4a), that is independent of promoter methylation, but, instead, is associated with down-regulation of bmi-1, a negative regulator of p16(INK4a). These studies show that PASG is essential for properly maintaining DNA methylation and gene expression patterns that are required for normal growth and longevity. PASG mutant mice provide a useful model for the study of aging as well as the mechanisms regulating epigenetic patterning during development and postnatal life.

Telomerase expression in normal human fibroblasts stabilizes DNA 5-methylcytosine transferase I

The finite proliferative potential of normal human fibroblasts can be overcome, a process commonly called immortalization, by the introduction of the catalytic subunit of human telomerase. In contrast to malignant transformation, the pattern of gene expression remains largely unmodified in telomerase-induced immortalization. Here we show evidence that suggests that the maintenance of a "young" pattern of gene expression by telomerization is mediated, at least in part, by a novel function of human telomerase that involves regulation of DNA methyltransferase I gene expression.

Demethylation of classical satellite 2 and 3 DNA with chromosomal instability in senescent human fibroblasts

Demethylation of genomic 5-methylcytosine is reported in aged human tissues and senesced human cells, although it is not understood to what extent this phenomenon contributes to replicative senescence. We examined methylation status of satellite 2 and 3 sequences during passages of normal human fibroblasts. These sequences are abundant in the juxtacentromeric heterochromatin of human chromosomes 1, 9 and 16, and heavily methylated in tissues of normal individuals. The decrease in DNA methylation level was two times faster in satellite 3 DNA than in satellite 2 and total DNA. Then we monitored appearance of micronuclei during the passages since they are indicative of heterochromatin decondensation or chromosome breakage. Concomitant with the DNA demethylation, micronuclei containing the heterochromatin of chromosomes 1, 9 or 16, appeared specifically. These results suggest that demethylation of heterochromatin has a role in replicative senescence through chromosome instability.

Differential inheritance modes of DNA methylation between euchromatic and heterochromatic DNA sequences in ageing fetal bovine fibroblasts

To elucidate overall changes in DNA methylation occurring by inappropriate epigenetic control during ageing, we compared fetal bovine fibroblasts and their aged neomycin-resistant versions using bisulfite-PCR technology. Reduction in DNA methylation was observed in euchromatic repeats (18S-rRNA/art2) and promoter regions of single-copy genes (the cytokeratin/beta-lactoglobulin/interleukin-13 genes). Contrastingly, a stable maintenance of DNA methylation was revealed in various heterochromatic sequences (satellite I/II/alphoid and Bov-B). The differential inheritance mode of DNA methylation was confirmed through the analysis of individual neomycin-resistant clones. These global, multi-locus analyses provide evidence on the tendency of differential epigenetic modification between genomic DNA regions during ageing.

Enhanced oxidative stress and accelerated cellular senescence in glucose-6-phosphate dehydrogenase (G6PD)-deficient human fibroblasts

Glucose-6-phosphate dehydrogenase (G6PD) is involved in the generation of reduced nicotinamide adenine dinucleotide phosphate (NADPH) and the maintenance of the cellular redox balance. The biological effects of G6PD deficiency in nucleated cells were studied using G6PD-deficient human foreskin fibroblasts (HFF). In contrast to that of normal HFF, the doubling time of G6PD-deficient cells increased readily from population doubling level (PDL) 15 to 63. This was accompanied by a significant increase in the percentage of G(1) cells. The slow-down in growth preceded an early entry of these cells into a nondividing state reminiscent of cellular senescence. These cells exhibited a significant increase in level of senescence-associated beta-galactosidase (SA-beta-gal) staining. The importance of G6PD activity in cell growth was corroborated by the finding that ectopic expression of active G6PD in the deficient cells prevented their growth retardation and early onset of senescence. Mechanistically, the enhanced fluorescence in dichlorofluorescin (H(2)DCF)-stained G6PD-deficient cells suggests the possible involvement of reactive oxygen species in senescence. Taken together, our results show that G6PD deficiency predisposes human fibroblasts to retarded growth and accelerated cellular senescence. Moreover, G6PD-deficient HFF provides a useful model system for delineating the effects of redox alterations on cellular processes.

p16/MTS1/INK4A suppresses prostate cancer by both pRb dependent and independent pathways

Tumor suppressor gene p16 is a cyclin-dependent kinase inhibitor and an important negative cell cycle regulator. The inactivation of p16 appears to be a common event in prostate cancer. Replacement of p16 inhibits prostate tumor cell growth, but the mechanism is not known. Human prostate cancer cell lines PPC-1, which has an inactivated p16, and DU145, which has a nonfunctional retinoblastoma Rb protein (pRb), were used to determine the possible mechanism of p16 mediated growth inhibition. PPC-1 cells treated with 5-aza-2'-deoxycytidine (5-aza-dC), a demethylating agent, induced p16 expression, inhibited cell growth, and induced senescence. Similarly, PPC-1 cells transduced by an adenoviral vector containing the p16 gene (AdRSVp16) produced a p16 protein that suppressed cellular proliferation and induced senescence. Co-staining of AdRSVp16-transduced PPC-1 cells by p16 immunohistochemistry and by beta-galactosidase substrate X-gal showed that the morphologically enlarged cells expressed both p16 and senescence-associated beta-galactosidase. In contrast, AdRSVp16 did not induce senescence in DU145 cells, but did inhibit its growth. However, when wild-type pRb was introduced in DU145 cells, AdRSVp16 was able to induce senescence. Thus, the mechanism by which p16 suppressed prostate cancer was dependent on the pRb functional status of cells whereby p16 caused pRb+ cells to undergo inhibition by senescence, whereas pRb- cells were also inhibited, but not by senescence.

A reassessment of the telomere hypothesis of senescence

According to the telomere hypothesis of senescence, the telomeric shortening that accompanies the replication of normal somatic cells acts as the mitotic clock that eventually results in their permanent exit from the cell cycle. Although evidence consistent with the telomere hypothesis continues to accumulate, on the basis of recent findings it is suggested that instead of a single clock mechanism there are multiple inducers of senescence.

Premature senescence involving p53 and p16 is activated in response to constitutive MEK/MAPK mitogenic signaling

Oncogenic Ras transforms immortal rodent cells to a tumorigenic state, in part, by constitutively transmitting mitogenic signals through the mitogen-activated protein kinase (MAPK) cascade. In primary cells, Ras is initially mitogenic but eventually induces premature senescence involving the p53 and p16(INK4a) tumor suppressors. Constitutive activation of MEK (a component of the MAPK cascade) induces both p53 and p16, and is required for Ras-induced senescence of normal human fibroblasts. Furthermore, activated MEK permanently arrests primary murine fibroblasts but forces uncontrolled mitogenesis and transformation in cells lacking either p53 or INK4a. The precisely opposite response of normal and immortalized cells to constitutive activation of the MAPK cascade implies that premature senescence acts as a fail-safe mechanism to limit the transforming potential of excessive Ras mitogenic signaling. Consequently, constitutive MAPK signaling activates p53 and p16 as tumor suppressors.

Alterations in DNA methylation: a fundamental aspect of neoplasia

Neoplastic cells simultaneously harbor widespread genomic hypomethylation, more regional areas of hypermethylation, and increased DNA-methyltransferase (DNA-MTase) activity. Each component of this "methylation imbalance" may fundamentally contribute to tumor progression. The precise role of the hypomethylation is unclear, but this change may well be involved in the widespread chromosomal alterations in tumor cells. A main target of the regional hypermethylation are normally unmethylated CpG islands located in gene promoter regions. This hypermethylation correlates with transcriptional repression that can serve as an alternative to coding region mutations for inactivation of tumor suppressor genes, including p16, p15, VHL, and E-cad. Each gene can be partially reactivated by demethylation, and the selective advantage for loss of gene function is identical to that seen for loss by classic mutations. How abnormal methylation, in general, and hypermethylation, in particular, evolve during tumorigenesis are just beginning to be defined. Normally, unmethylated CpG islands appear protected from dense methylation affecting immediate flanking regions. In neoplastic cells, this protection is lost, possibly by chronic exposure to increased DNA-MTase activity and/or disruption of local protective mechanisms. Hypermethylation of some genes appears to occur only after onset of neoplastic evolution, whereas others, including the estrogen receptor, become hypermethylated in normal cells during aging. This latter change may predispose to neoplasia because tumors frequently are hypermethylated for these same genes. A model is proposed wherein tumor progression results from episodic clonal expansion of heterogeneous cell populations driven by continuous interaction between these methylation abnormalities and classic genetic changes.

Human T-cell clones in long-term culture as a model of immunosenescence

We have consistently observed that like other normal somatic tissue cells, human T lymphocytes manifest a finite proliferative capacity in culture in vitro. When measured in population doublings (PD), this averages about 35 PD for T-cell clones (TCC) derived from mature peripheral T cells of young adults and about 20 PD more for TCC derived from T-cell precursors in their bone marrow. We believe that alterations in surface marker phenotypes and corresponding functional changes observed in these human TCC as they progress through their finite lifespans in vitro can provide valuable information on processes of T-cell immunosenescence in vivo. They may also provide a model system for studying ways of modulating the ageing process to delay or prevent immunosenescence in the elderly and the chronically infected or possibly to accelerate immunosenescence in organ transplantation.

Cell aging in vivo and in vitro

It has become a staple assumption of biology that there is an intrinsic fixed limit to the number of divisions that normal vertebrate cells can undergo before they senesce, and this limit is in some way related to aging of the organism. The notion of such a limited replicative lifespan arose from the often repeated observation that diploid fibroblasts cannot proliferate indefinitely in monolayer culture, and that the number of divisions before senescence is directly related to the in vivo lifespan of different species. The in vitro evidence is countered by estimates that the number of cell divisions in some organs of rodents and man are one or more orders of magnitude higher than the in vitro limit, with no indication of the degenerative changes seen in culture. Serial transplantation experiments in animals also exhibit many more cell divisions than the in vitro studies, with some indicating an indefinite replicative lifespan. I present evidence that vertebrate cells are severely stressed by enzymatic dispersion and sustain cumulative damage during serial subcultivations. The evidence includes large increases in cell size and its heterogeneity, reductions in replicative efficiency at low seeding densities, appearance of abnormal structures in the cytoplasm, changes in metabolism to a common cell culture type, continuous loss of methyl groups and reiterated sequences from DNA, and a constant rate of decline of growth rate with passage. This evidence is complemented by the reduction induced in the replicative life span of diploid cells by a large array of treatments which have different primary targets in the cells. The most consistent and general observation of cell behavior in aging animals, with only a few exceptions, is a reduction in the rate of cell proliferation. This reduction is perpetuated when the cells are grown in culture, indicating it is an enduring and intrinsic property of the cells rather than a systemic effect of the aging organism. A similar heritable reduction in growth rate can be induced in established cell lines by prolonged incubation at quiescence. The reduction can be exaggerated by subculturing the quiescent cells under suboptimal conditions, just as the effects of age are exaggerated under stress. The constant decline of growth rate that occurs during serial passage of diploid cells may represent a similar decay of cell function. I propose that the limit on replicative lifespan is an artifact that reflects the failure of diploid cells to adapt to the trauma of dissociation and the radically foreign environment of cell culture. It is, however, a useful artifact that has given us much information about cell behavior under stressful conditions. The overall evidence indicates cell in vivo accumulate damage over a lifetime that results in gradual loss of differentiated function and growth rate accompanied by an increased probability for the development of cancer. Such changes are normally held to a minimum by the organized state of the tissues and homeostatic regulation of the organism. The rejection of an intrinsic limit on the number of cell divisions eliminates the need for a cellular clock, such as telomere length, that counts mitoses. I offer a heuristic explanation for the gradual reduction of cell function and growth capacity with age based on a cumulative discoordination of interacting pathways within and between cells and tissues. I also make a case for the use of established cell lines as model systems for studying heritable damage to cell populations that simulates the effects of aging in vivo, and represents a relatively unexplored area of cell biology.

Oncogenic ras provokes premature cell senescence associated with accumulation of p53 and p16INK4a

Oncogenic ras can transform most immortal rodent cells to a tumorigenic state. However, transformation of primary cells by ras requires either a cooperating oncogene or the inactivation of tumor suppressors such as p53 or p16. Here we show that expression of oncogenic ras in primary human or rodent cells results in a permanent G1 arrest. The arrest induced by ras is accompanied by accumulation of p53 and p16, and is phenotypically indistinguishable from cellular senescence. Inactivation of either p53 or p16 prevents ras-induced arrest in rodent cells, and E1A achieves a similar effect in human cells. These observations suggest that the onset of cellular senescence does not simply reflect the accumulation of cell divisions, but can be prematurely activated in response to an oncogenic stimulus. Negation of ras-induced senescence may be relevant during multistep tumorigenesis.

Replicative senescence: implications for in vivo aging and tumor suppression

Normal cells have limited proliferative potential in culture, a fact that has been the basis of their use as a model for replicative senescence for many years. Recent molecular analyses have identified numerous changes in gene expression that occur as cells become senescent, and the results indicate that multiple levels of control contribute to the irreversible growth arrest. These include repression of growth stimulatory genes, overexpression of growth inhibitory genes, and interference with downstream pathways. Studies with cell types other than fibroblasts will better define the role of cell senescence in the aging process and in tumorigenesis.

Replicative senescence: considerations relating to the stability of heterochromatin domains

Replicative senescence of human diploid fibroblasts (HDF) cultured in vitro is characterized by a progressive and irreversible loss of responsiveness to mitogenic stimulation by serum. While some constraints have been placed on the nature of HDF senescence, its underlying molecular mechanism(s) remain obscure. Here, the possibility is considered that defects in cell cycle-coupled reassembly of repressive chromatin domains may contribute to HDF senescence. Features of this model are discussed in relation to established models of HDF senescence based on telomere shortening and loss of DNA methylation.

Life span prediction from the rate of age-related DNA demethylation in normal and cancer cell lines

A method has been proposed for the Hayflick Limit prediction by the analysis of the 5-methylcytosine content in DNA at earlier and later cell passages. The following facts were used as the basis of the method: (i) the rate of m5C loss from DNA remains approximately constant during cell divisions and it does not depend on the cell donor age; (ii) this rate is inversely proportional to the Hayflick Limit as well as to the life span of cell donor species; (iii) the period corresponded to loss of all m5C residues from the genome coincides with or somewhat exceeds the Hayflick Limit of normal cells. The prognosis of the Hayflick Limit has usually been found in good agreement with the experimental evidences for various human, hamster, and mouse cell lines. The method proposed may be used for early detection of precrisis and cancer cells. The age-related m5C loss may result from accumulation of the m5C-->T+C transitions occurring with DNA methylation in every cell division.

Changes of the methylation pattern of the c-myc gene during in vitro aging of IMR90 human embryonic fibroblasts

DNA modification by cytosine methylation has received considerable interest in the context of mammalian cell differentiation but is discussed controversially with respect to cellular aging. As the expression of c-myc affects strongly cellular aging and terminal differentiation, we have analysed the sequence-specific methylation pattern of the c-myc gene during proliferative aging in vitro of human embryonic fibroblasts. In this study, both, 5-methylcytidine sensitive restriction enzymes as well as genomic sequencing were used. The overall methylation pattern was found essentially stable during proliferative aging. However, specific hypermethylation of exon II during aging was observed. Furthermore, one specific cytidine located in the consensus sequence of the DNA binding factor PEBP2 was found completely methylated during most of the course of proliferative aging of the cells but became demethylated as the cells reached the end of their proliferative life span. Our results indicate the importance of establishing the sequence-specific changes of the methylation pattern of the genome during in vitro aging.

The expression of proliferation-dependent antigens during the lifespan of normal and progeroid human fibroblasts in culture

Normal human fibroblasts display a limited lifespan in culture, which is due to a steadily decreasing fraction of cells that are able to proliferate. Using antibodies that react with antigens present in proliferating cells only, in an indirect immunofluorescence assay, we have estimated the fraction of proliferating cells in cultures of normal human fibroblasts. Furthermore, we have estimated the rate of decline in the fraction of proliferating cells during the process of cellular ageing by application of the assay to normal human fibroblasts throughout their lifespan in culture. Werner's Syndrome is an autosomal recessive disease in which individuals display symptoms of ageing prematurely. Werner's Syndrome fibroblasts display a reduced lifespan in culture compared with normal human fibroblasts. Like normal human fibroblasts, the growth of Werner's Syndrome fibroblasts is characterised by a decreasing fraction of cells reacting with the proliferation-associated antibodies throughout their lifespan in culture. However, the rate of loss of proliferating cells in Werner's Syndrome fibroblasts during the process of cellular ageing is accelerated 5- to 6-fold compared with the rate determined for normal human fibroblasts.

Induction of replicative senescence by 5-azacytidine: fundamental cell kinetic differences between human diploid fibroblasts and NIH-3T3 cells

To analyse the putative role of methylation of cytosine residues in the nuclear DNA as a regulatory step during cellular ageing, we incubated ageing human amniotic fluid derived fibroblast-like cells and non-ageing NIH-3T3 cells with 5-azacytidine. BrdUrd/Hoechst and acridine orange (AO) flow cytometry was used to compare the effects of the base analogue on cell proliferation and cell differentiation. In NIH-3T3 cultures, 96h exposures to 4 microM 5-azacytidine caused diminished cell proliferation due to cell arrest in the G1 compartments of the second and third cell cycles of serum stimulated cells. The exit from the G0/G1 compartment was not affected. The 5-azacytidine induced cell kinetic disturbances were unstable in NIH-3T3 cultures, such that pre-treated cells reverted to normal cell cycle transit within 2-3 days after termination of treatment. In contrast, 5-azacytidine pre-treated amniotic fluid derived fibroblast-like cell cultures showed persistently elevated G2 phase arrests and delayed G0/G1 phase exit kinetics, which explain the premature cessation of proliferation observed in these primary cultures. In both cell systems, 5-azacytidine exposed cultures showed elevated numbers of G1 phase cells with increased RNA content as revealed by AO flow cytometry. Again, this effect was reversible in NIH-3T3 cells but not in amniotic fluid derived fibroblast-like cells. These contrasting responses to 5-azacytidine are likely to reflect intrinsic differences in methylation patterns or de novo methylase activity between ageing cell strains and non-ageing cell lines.

Cytotoxicity of 5-aza-2'-deoxycytidine in a mammalian cell system

After the addition of 5-aza-2'-deoxycytidine, a potent inhibitor of DNA methylation and S-phase-specific cytotoxic agent, metaphase chromosomes of Chinese hamster ovary (CHO) cells exhibited a highly decondensed and extended morphology (numerous "fragile sites") at the first mitotic division. However, when a lethal dose of this drug was added in early G1 phase to cells synchronised by mitotic selection, the majority subsequently divided at the same time as an untreated control cell population with few division abnormalities and with few of the more usual types of chromosome aberrations such as gaps, breaks and exchanges. The drug-treated cells also entered and completed the second S-phase without significant delay and it was only at the second mitosis after addition of 5-azadeoxycytidine that cells showed delays in entering mitosis and significant increases in abnormal divisions concomitant with a modest increase in chromosome aberrations. If cells in a tumour behave similarly, the tumour mass would be expected to double before any reduction in tumour burden could be expected to occur.

5-Azacytidine-induced demethylation of DNA to senescent level does not block proliferation of human fibroblasts

IMR-90 human diploid fibroblasts (HDF) lose from 30-50% of their genomic 5-methyldeoxycytidine (5mdC) during the cellular aging process. In contrast, immortal SV40-transformed IMR-90 maintain a constant level of 5mdC in culture. Precrisis SV40-transformed HDF (AG3204) represent a stage in between normal cell aging and immortalization because these cells still have a finite proliferative lifespan, but it is longer than that of normal HDF and ends in cell death rather than in G1-arrest. We find that AG3204 cells continue to lose from 12-33% of their 5mdC after a population has become 99% positive for SV40 T-antigen. Both IMR-90 cells and AG3204 cells have similar levels of 5mdC (average of 2.25%) at the end of lifespan. We investigated whether this level of 5mdC is an absolute block to further proliferation by treating IMR-90 and AG3204 cells with 5-azacytidine (5azaC) to reduce their 5mdC levels below the terminal level normally achieved at end of lifespan. We find that both IMR-90 and AG3204 cells undergo extensive proliferation with subterminal levels of 5mdC and that the lifespans of both cell types are shortened by 5azaC treatment. These studies indicate that random genomic DNA demethylation to a specific level of 5mdC is not a direct cause of finite proliferative lifespan. However, the correlation between accelerated DNA demethylation and accelerated aging still suggests that these two phenomena are related. Two ways to explain this relationship are: (1) DNA demethylation during aging is not random, and/or (2) both DNA demethylation and other independent aging processes cooperate to produce finite lifespan. In both cases, accelerated random DNA demethylation could accelerate aging, but not necessarily in direct relationship to the final genomic level of 5mdC achieved during the normal aging process.