Cell aging in vivo and in vitro

Mitochondrial DNA mutation analysis in human skin fibroblasts from fetal, young, and old donors

Multibase deletions in mitochondrial DNA (mtDNA) have been shown to accumulate with age in several tissues, including skin, whereas point mutations have only recently been demonstrated to increase during aging, with several specific mutations occurring at high levels (up to 50%) in skin fibroblasts obtained from old donors [Science 286(1999)774]. We have conducted a survey for a specific deletion and for point mutations in several regions of mtDNA from cultured skin fibroblasts derived from eight fetal (12-20 weeks gestational age), ten young (17-33 years of age) and 11 old (78-92 years of age) human donors. Using PCR analysis, detectable levels of the 4977 basepair (bp) 'common deletion' were present in all three age groups, with the highest deletion levels of up to 0.3% of total mtDNA found in several cell lines from old donors, although other old donor cell lines had much lower levels. Single strand conformation polymorphism (SSCP) analysis for point mutations in the non-coding D-loop region and two regions of the cytochrome oxidase 2 gene failed to reveal the presence of any single base mutations. We infer that age-related high level mutational damage in mtDNA from human skin fibroblasts may manifest both sequence and inter-individual specificity.

More than a sum of our cells

Cells in the body grow and die, cells in lab dishes grow and die, and individual organisms grow and die. The parallels seem maddeningly obvious, but scores of scientists still labor to draw the correct connections, to uncover the mechanisms that underlie aging in cell culture flasks and in whole animals. Do our cells stop growing, quit working, cease dividing, or start dying as we age? Do we die when our cells do, or are we somehow more than the sum of our cells? For decades, scientists have searched for evidence that links changes in cell growth, cell function, cell division, and cell death to the phenomenon we call aging. Although definitive proof eludes them, researchers continue to conduct experiments in tissue culture and in animal models, amassing information that points us toward a greater understanding of what aging is--and is not.

How individual age-associated changes may influence human morbidity and mortality patterns

Patterns of human mortality share common traits in different populations. They include higher mortality in early childhood, lower mortality during the reproductive period, an accelerated increase of mortality near the end of the reproductive period, and deceleration in the mortality increase at oldest old ages. The deceleration of mortality rate is one of the most intriguing recent findings in longevity research. The role of differential selection in this phenomenon has been well studied. Possible contribution of individual aging in the shape of mortality curve is also recognized. However, this contribution has not been studied in details. In this paper, we specify most common patterns of age-associated changes in an individual organism and discuss their possible influence on morbidity and mortality in population. We subdivide individual age-associated changes into three components, having different influence on morbidity and mortality: (1) basal, (2) ontogenetic, and (3) time-dependent. Basal changes are connected with the universal decrease in the rate of living during an individual life. As a result, some phenotypic effects of aging may accumulate in an organism at a slower rate with age. Basal changes are likely to contribute to a plateau of morbidity often observed at old ages, and may partially be responsible for mortality deceleration at oldest old ages. Ontogenetic component is connected with change of the stages of ontogenesis (e.g., the growth, the reproductive period and the climacteric) during an individual life. The ontogenesis-related changes contribute to wave-like patterns of morbidity in population and may partially be responsible for mortality increase at middle ages and its deceleration at old ages. Time-dependent changes are connected with long-time exposure of an organism to different harmful factors. They are most likely to contribute to morbidity and mortality acceleration. We discuss how all three components of individual age-associated changes may interact in human organism and influence patterns of morbidity and mortality in population.

Role of the intermediate filament protein vimentin in delaying senescence and in the spontaneous immortalization of mouse embryo fibroblasts

Because knockout of the vimentin gene in mice did not produce an immediately obvious, overt, or lethal specific phenotype, the conjecture was made that the mutation affects some subtle cellular functions whose loss manifests itself only when the mutant animals are exposed to stress. In order to substantiate this idea in a tractable in vitro system, primary embryo fibroblasts from wildtype (V(+/+)) and vimentin-knockout (V(-/-)) mice were compared with regard to their growth behavior under the pseudophysiologic conditions of conventional cell culture. Whereas in the course of serial transfer, the V(+/+) fibroblasts progressively reduced their growth potential, passed through a growth minimum around passage 12 (crisis), and, as immortalized cells, resumed faster growth, the V(-/-) fibroblasts also cut down their growth rate but much earlier, and they either did not immortalize or did so at an almost undetectable rate. Cells withdrawing from the cell cycle showed increased concentrations of reactive oxygen species and signs of oxidative damage: enlarged and flattened morphology, large nuclear volume, reinforced stress fiber system as a result of increased contents of actin and associated proteins, prominent extracellular matrix, and perinuclear masses of pathological forms of mitochondria with low membrane potential. The differences in the cell cycle behavior of the V(+/+) and V(-/-) cells in conjunction with the morphologic changes observed in mitotically arrested cells suggests a protective function of vimentin against oxidative cell damage. Because vimentin exhibits affinity for and forms crosslinkage products with recombinogenic nuclear as well as mitochondrial DNA in intact cells, it is credible to postulate that vimentin plays a role in the recombinogenic repair of oxidative damage inflicted on the nuclear and mitochondrial genome throughout the cells' replicative lifespan. Recombinational events mediated by vimentin also appear to take place when the cells pass through the genetically unstable state of crisis to attain immortality. The residual immortalization potential of V(-/-) fibroblasts might be attributable to their capacity to synthesize, in place of vimentin, the tetrameric form of a lacZ fusion protein carrying, in addition to a nuclear localization signal, the N-terminal 59 amino acids of vimentin and thus its DNA-binding site. On the basis of these results and considerations, a major biologic role of vimentin may be to protect animals during development and postnatal life against genetic damage and, because of its contribution to the plasticity of the genome, to allow them to respond to environmental challenges.

Early molecular changes in irradiated aortic endothelium

Irradiated aortic endothelial cells (EC) exhibit distinct morphological, functional, and physiological responses to ionizing radiation (IR). However, the molecular basis for these responses has not been fully characterized. Cultured bovine and rat aortic endothelial cells were exposed to single fraction doses (0-30 Gy) of gamma radiation. IR caused dose-dependent DNA strand breaks which were repaired to near baseline levels within 30 min. A dose-dependent inhibition of cell growth was noted for IR greater than 1 Gy. At doses greater than 2.5 Gy, morphologic changes consistent with apoptosis and loss of cell viability were present beginning 12-16 h after radiation, with subsequent detachment of EC from the cell monolayer. By Western blot analysis, expression of p53, gadd45, p21, and bax protein increased in a time-and dose-dependent manner; p53 expression was maximal at 3 h after IR, and gadd45, bax and p21 levels peaked at 6 h. By Reverse Transcriptase Polymerase Chain Reaction (RT-PCR), levels of p53 mRNA were not significantly increased after IR, whereas gadd45 exhibited time- and dose-dependent increase in mRNA synthesis after IR. Activation of intracellular caspases, manifest by proteolytic poly (ADP-ribose) polymerase (PARP) and lamin B cleavage, was maximal at 15 h after IR, concident with other indices of EC apoptosis, including oligonucleosomal DNA degradation, TUNEL immunostaining, and morphologic changes. The tripeptide protease inhibitor z-Val-Ala-Asp (zVAD) prevented PARP and lamin cleavage, DNA fragmentation, morphological changes, and cell detachment in irradiated EC. The combined data suggested that gamma radiation induces a dose- and time-dependent sequence of early events in cultured EC with modulate growth arrest, apoptosis, and possibly premature senescence in surviving cells.

Age-related changes in rat cerebellar basket cells: a quantitative study using unbiased stereological methods

Cortical cerebellar basket cells are stable postmitotic cells; hence, they are liable to endure age-related changes. Since the cerebellum is a vital organ for the postural control, equilibrium and motor coordination, we aimed to determine the quantitative morphological changes in those interneurons with the ageing process, using unbiased techniques. Material from the cerebellar cortex (Crus I and Crus II) was collected from female rats aged 2, 6, 9, 12, 15, 18, 21 and 24 mo (5 animals per each age group), fixed by intracardiac perfusion, and processed for transmission electron microscopy, using conventional techniques. Serial semithin sections were obtained (5 blocks from each rat), enabling the determination of the number-weighted mean nuclear volume (by the nucleator method). On ultrathin sections, 25 cell profiles from each animal were photographed. The volume density of the nucleus, ground substance, mitochondria, Golgi apparatus (Golgi) and dense bodies (DB), and the mean surface density of the rough endoplasmic reticulum (RER) were determined, by point counting, using a morphometric grid. The mean total volumes of the soma and organelles and the mean total surface area of the RER [SN (RER)] were then calculated. The results were analysed with 1-way ANOVA; posthoc pairwise comparisons of group means were performed using the Newman-Keuls test. The relation between age and each of the parameters was studied by regression analysis. Significant age-related changes were observed for the mean volumes of the soma, ground substance, Golgi, DB, and SN (RER). Positive linear trends were found for the mean volumes of the ground substance, Golgi, and DB; a negative linear trend was found for the SN (RER). These results indicate that rat cerebellar basket cells endure important age-related changes. The significant decrease in the SN (RER) may be responsible for a reduction in the rate of protein synthesis. Additionally, it may be implicated in a cascade of events leading to cell damage due to the excitotoxic activity of glutamate, which could interfere in the functioning of the complex cerebellar neuronal network.

Biological timing and the clock metaphor: oscillatory and hourglass mechanisms

Living organisms have developed a multitude of timing mechanisms--"biological clocks." Their mechanisms are based on either oscillations (oscillatory clocks) or unidirectional processes (hourglass clocks). Oscillatory clocks comprise circatidal, circalunidian, circadian, circalunar, and circannual oscillations--which keep time with environmental periodicities--as well as ultradian oscillations, ovarian cycles, and oscillations in development and in the brain, which keep time with biological timescales. These clocks mainly determine time points at specific phases of their oscillations. Hourglass clocks are predominantly found in development and aging and also in the brain. They determine time intervals (duration). More complex timing systems combine oscillatory and hourglass mechanisms, such as the case for cell cycle, sleep initiation, or brain clocks, whereas others combine external and internal periodicities (photoperiodism, seasonal reproduction). A definition of a biological clock may be derived from its control of functions external to its own processes and its use in determining temporal order (sequences of events) or durations. Biological and chemical oscillators are characterized by positive and negative feedback (or feedforward) mechanisms. During evolution, living organisms made use of the many existing oscillations for signal transmission, movement, and pump mechanisms, as well as for clocks. Some clocks, such as the circadian clock, that time with environmental periodicities are usually compensated (stabilized) against temperature, whereas other clocks, such as the cell cycle, that keep time with an organismic timescale are not compensated. This difference may be related to the predominance of negative feedback in the first class of clocks and a predominance of positive feedback (autocatalytic amplification) in the second class. The present knowledge of a compensated clock (the circadian oscillator) and an uncompensated clock (the cell cycle), as well as relevant models, are briefly re viewed. Hourglass clocks are based on linear or exponential unidirectional processes that trigger events mainly in the course of development and aging. An important hourglass mechanism within the aging process is the limitation of cell division capacity by the length of telomeres. The mechanism of this clock is briefly reviewed. In all clock mechanisms, thresholds at which "dependent variables" are triggered play an important role.

Putative telomere-independent mechanisms of replicative aging reflect inadequate growth conditions

Telomere shortening is the mechanism underlying replicative aging in fibroblasts. A variety of reports now claim that inactivation of the p16(INK4a)/pRB pathway is required in addition to telomere maintenance for the immortalization of cells such as skin keratinocytes and breast epithelial cells. We here show that the premature growth arrest of these cell types can be explained by an inadequate culture environment. Providing mesenchymal/epithelial interactions by cultivating the telomerase-expressing cells on feeder layers avoids the growth arrest associated with increased p16(INK4a). These results do not support a telomere-independent mechanism of replicative aging.

Genetic regulation of embryo death and senescence

The survival of the preimplantation mammalian embryo depends not only on providing the proper conditions for normal development but also on acquiring the mechanisms by which embryos cope with adversity. The ability of the early conceptus to resist stress as development proceeds may be regulated by diverse factors such as the attainment of a cell death program and protective mechanisms involving stress-induced genes and/or cell cycle modulators. This paper reviews the recent research on the genetic regulation of early embryo cell death and senescence focussing on the bovine species where possible. The different modes of cell death will be explained, clarifying the confusing cell death terminology, by advocating the recommendations set forth by the Cell Death Nomenclature Committee to extend to the embryology research field. Specific pro-death and anti-death genes will be discussed with reference to their expression patterns during early mammalian embryogenesis.

Telomeres and telomerase: implications for cancer and aging

Maintenance of telomere stability is required for cells to escape from replicative senescence and proliferate indefinitely. Telomere length is maintained by a balance between processes that lengthen telomeres (telomerase) and processes that shorten telomeres (the end-replication problem). Telomerase is a cellular ribonucleoprotein reverse transcriptase which stabilizes telomere length by adding hexameric (TTAGGG) repeats to the telomeric ends of the chromosomes, thus compensating for the continued erosion of telomeres. Introduction of the telomerase catalytic protein component into normal telomerase-negative human cells results in restoration of telomerase activity and extension of cellular life span. Human cells with introduced telomerase maintain a normal chromosome complement and continue to grow in a normal manner. Telomerase-induced manipulations of telomere length may thus be important not only for cell and tissue engineering but also for dissecting the molecular mechanisms underlying inherited genetic diseases, as well as defining the genetic pathways leading to cancer. Because almost all human tumors express telomerase activity, inhibition of telomerase may result in gradual erosion of telomeres and eventual cessation of cell proliferation or induction of apoptosis. Thus telomerase may also be a promising target for cancer therapy.

Human keratocyte migration into collagen gels declines with in vitro ageing

Although senescence in various cell types has been shown to have detrimental effects on wound repair, the effect of this phenomenon on corneal function with increasing age has yet to be elucidated. This study investigated the effect of in vitro ageing on keratocyte migration into a collagen gel matrix. The keratocyte cell strain EK1. BR was cultured to late passage and a comparison of early passage migration with that of late passage migration was carried out. Early or late passage keratocytes were seeded onto 6 collagen gels (1.75 mg ml(-1)) for each experiment. Gels were incubated at 37 degrees C for 72 h, stained with calcein AM (0.5 mg ml(-1)) and assayed for cell migration using fluorescent microscopy. Changes in the effect of EGF on keratocyte migration with age were assessed by the addition of EGF (20 ng ml(-1)) to 3 of the 6 gels in each experiment. Proliferative lifespan was measured by immunocytochemical detection of Ki67 activity. This study shows for the first time that keratocyte migration, and migration in response to EGF stimulation, significantly declines with increasing age of keratocytes in culture (P<0.001). As keratocyte migration in response to cytokine stimulation is vital for corneal repair, the accumulation of senescent keratocytes with age may impair corneal wound healing.

Stimulation of prostaglandin E2 and interleukin-1beta production from old rat periodontal ligament cells subjected to mechanical stress

Although the severity of periodontal disease is known to be affected by the age of the host, the pathological role of aging in periodontal disease, especially that attributable to trauma from occlusion, has not been well characterized. Prostaglandin (PG)E2 and interleukin (IL)-1beta are key mediators involved in periodontal diseases, potent stimulators of bone resorption, and are produced by human periodontal ligament (PDL) cells in response to mechanical stress. To investigate age-related changes in the biosynthetic capacity of PGE2 and IL-1beta in PDL cells, we examined the effects of in vivo aging with mechanical tension on PGE2 and IL-1beta expression by rat PDL cells. PDL cells obtained from the incisors of 6-week (young) and 60-week (old) rats were cultured on flexible-bottomed culture plates. The cells were deformed by causing a 9% or 18% increase in surface area at 6 cycles per minute for 1 to 5 days. We found an approximately twofold increase in PGE2 and IL-1beta production by old PDL cells subjected to mechanical tension compared with that by young cells, although the constitutive levels were similar in both. The expression of cyclooxygenase (COX)-2 and IL-1beta mRNA (messenger ribonucleic acid) was enhanced by mechanical tension as determined by use of reverse transcription-polymerase chain reaction (RT-PCR), whereas COX-1 and IL-1beta-converting enzyme mRNA remained unchanged. It is possible that the large amount of PGE2 and IL-1beta produced by PDL cells from an aged host in response to mechanical force may be positively related to the acceleration of alveolar bone resorption.

Atmospheric oxygen accelerates the induction of a post-mitotic phenotype in human dermal fibroblasts: the key protective role of glutathione

It has been proposed that ageing of human dermal fibroblasts occurs as a multi-stage process during which cells progress from a mitotic to a post-mitotic state. We describe the development of a simple and novel cell-cloning model for identifying and quantifying the different fibroblast morphotypes associated with the induction of post mitotic behaviour. We have found that under atmospheric (20%) oxygen tension a significant proportion of human dermal fibroblasts are rapidly induced to switch from a mitotic to a post-mitotic phenotype. In contrast, under more physiological (4%) oxygen conditions, the induction of a post-mitotic phenotype is largely prevented. Increasing oxidative stress by addition of hydrogen peroxide or depletion of glutathione also induced a switch from a mitotic to a post-mitotic phenotype in these cells, whereas addition of the anti-oxidant N-acetylcysteine under atmospheric (20%) oxygen tension potently inhibited this process. In addition, a statistically significant correlation was observed between the magnitude of intracellular glutathione depletion and the reduction in the population of mitotic cells in this model. We propose that the switch from a mitotic to a post-mitotic phenotype represents a process of cellular ageing and that standard atmospheric oxygen tension imposes a substantial oxidative stress on dermal fibroblasts which accelerates this process in culture. The data also suggest that intracellular glutathione levels strongly influence the induction of a post-mitotic phenotype and that, by implication, depletion of glutathione may play a significant role in the progression of cellular ageing in human skin.

Somatic cell mutations: can they provide a link between aging and cancer?

Cancers increase during aging in mammals, and an accumulating body of evidence suggests that mutational events too do likewise. Mutational events are intimately involved in the malignant process. One current view is that mutator phenotypes are required in malignant cells for a sufficient number of critical target genes to be affected. These mutator phenotypes are believed to result from underlying deficiencies in genes necessary to maintain genomic stability. This review will provide a framework for a discussion of cancer and aging by detailing with a pair of wise approach studies that address the relations between aging, cancer, and mutations. Results from these studies will be used to suggest that a mutator phenotype develops in the cells of older individuals in the absence of an underlying genetic deficiency. Instead, it is proposed that a mixture of chromosomal aberrations, DNA damage, and chronic exposure to genotoxic forces, including oxidative stress, provide the basis for this age-accelerated mutator phenotype.

Telomere dynamics in cancer progression and prevention: fundamental differences in human and mouse telomere biology

Cells from the telomerase knockout mouse immortalize with an approximately ten million-fold greater frequency than human cells. In this commentary, Wright and Shay discuss the implications of this difference between mice and men and its relationship to cancer.

In vitro replication and differentiation of normal human oral keratinocytes

The replication kinetics and cytological changes of normal human oral keratinocytes (NHOK) isolated from the basal surface of oral epithelial sheet and cultured as dispersed cells in low (0.15 mM) Ca(2+) medium without serum were analyzed. Replicating NHOK were quantitated by cell count and identified by [(3)H]thymidine uptake. Cell morphology was analyzed by phase contrast and transmission electron microscopy, and by cytochemical staining for endogenous beta-galactosidase (beta-gal) activity, involucrin, and cytokeratin types 1 and 10 (K1/K10). Primary NHOK obtained from 15 different donors whose ages ranged from 21 to 62 years consistently showed three distinct phases of replication, i.e., exponential, senescing, and senescent, which were independent of the donors' age. Initially, the cells replicated exponentially for a period of 20 days with a doubling time of 26.6 +/- 3.5 h. They then gradually entered replication arrest over a period of 18 days. The cells underwent a maximum of 22.1 +/- 2.8 population doublings. The onset of gradual replication arrest coincided with an increase in the fraction of cells, which stopped DNA synthesis within a maximum of 48 h and which stained for beta-gal. The fraction of terminally differentiated cells stained for K1/K10 did not increase until nearly all the cells had stopped replicating (senescent phase) and maximal beta-gal staining had been reached. Subsequently, the percentage of beta-gal stained cells actually decreased while the percentage of those stained for K1/K10 increased to a maximum of 80-90% within 2-3 weeks. Exposure of exponentially replicating NHOK to 5-aza-2'-deoxycytidine (5-aza CdR) inhibited DNA replication within 18-48 h and induced terminal differentiation 6 days later. In contrast, exposure of these cells to 1.5 mM Ca(2+) induced expression of involucrin and K1/K10 within 48 h without inhibiting DNA synthesis. Thus, replication arrest preceded differentiation in NHOK serially subcultured in vitro; however, differentiation could be induced without replication arrest.

Larger effect of aging on the perception of higher-order stimuli

Widespread deficits are known to accompany normal aging. Contrast thresholds of older and younger observers were measured for static and drifting gratings defined by luminance (first-order) or by contrast (second-order), and for a temporally segmented second-order motion stimulus. Results showed that older individuals had a larger threshold elevation for the perception of second-order stimuli than for the perception of first-order stimuli. This suggests a dissociation between the mechanisms underlying the perception of first and second-order stimuli, and demonstrates that aging may affect the more numerous processing steps required for the analysis of higher level stimuli.

Flow cytometric analysis of PKH26-labeled goldfish kidney-derived macrophages

We recently demonstrated that a goldfish macrophage cell line (GMCL) and primary in vitro-derived kidney macrophage (IVDKM) cultures contain three distinct macrophage subpopulations. Morphological, cytochemical, functional, and flow cytometric characterization of these sub-populations suggested that they may represent cells of the macrophage lineage temporally arrested at distinct differentiation junctures of fish macrophage development (putative early progenitors, monocytes, and macrophages). In this study, we examined the proliferation and differentiation events leading to the generation of mature macrophage-like cells from goldfish kidney hematopoietic tissues. The flow cytometric studies were done after labeling macrophages with PKH26 fluorescent dye and analysis of the data using the MODFIT software. Our results showed that IVDKM cultures proliferated non-synchronously, suggesting the presence of a temporal control mechanism regulating the number of cells entering the paths towards maturation. Such control is most evident during early progenitor proliferation and differentiation events. Our results showed that proliferation may not be a requirement for differentiation of early progenitors to putative monocyte and macrophage subsets. Detailed observation of the mature macrophage-like subpopulation indicated that: 1) they appear to develop from both, the differentiation of monocyte-like cells, and direct differentiation of early progenitors in the absence of a monocyte-like stage; and (2) mature macrophage-like cells appeared to be capable of self-proliferation. Our results suggest the presence of alternate pathways of fish macrophage development other than the classical hematopoietic pathway.

Age-related decline in Ras/ERK mitogen-activated protein kinase cascade is linked to a reduced association between Shc and EGF receptor

Numerous studies have demonstrated that the proliferative capacity of cells declines with age. Using rat primary hepatocytes as a model system, we recently demonstrated that this age-related decline in the proliferative response to mitogenic stimulation is associated with decreased activities of both extracellular signal-regulated kinase (ERK) and p70 S6 kinase (p70(S6k)). To unravel the molecular basis for age-related defects in the ERK pathway, we have now characterized the upstream signaling events that occur after epidermal growth factor (EGF) stimulation in young and aged hepatocytes. As previously noted for ERK, the activities of both MEK (the kinase immediately upstream of ERK) and Ras following EGF stimulation were significantly lower in aged hepatocytes. An examination of the EGF receptor (EGFR) revealed a similar amount of EGFR in the two age groups. Likewise, EGFR and Shc, an adaptor protein that plays a crucial role in linking EGFR to Ras activation, underwent tyrosine phosphorylation to a similar degree in both young and aged hepatocytes. However, in aged cells Shc was unable to form stable complexes with EGFR after EGF stimulation. Our results suggest that a decrease in the association between Shc and EGFR in aged cells underlies the age-related declines in the ERK signaling cascade and in proliferative capacity.

The effects of cell passages on the cell morphology and the outcome of herpes simplex virus type 1 infection

Because cell cultures are essential in biological research which involves the analysis of virus morphogenesis, this study focused on examining the significance of cell passages. Human embryonic lung fibroblasts (MRC-5) at passage (P) 27 were seeded twice a week to P 32, P 40, and P 48, when just at confluence and then infected with herpes simplex virus type 1 (HSV-1). The structure of the non-virus-infected (MOCK) and HSV-1 infected cells, the amount of cellular infectious virus particles and the capability to express HSV-1 glycoproteins C (gC-1) and D (gD-1) were investigated by phase-contrast and immunofluorescence light microscopy, immunogold cryosection EM, plaque assays, immunoblots, and total protein assays. Modified cell structure including fragmentation of tubulin fibers were visible in MOCK from P 38 onwards. The quantity of vimentin remained unchanged while actin accumulated and beta-tubulin decreased in HSV-1 infected late P cells compared to early P cultures. Cells of high P counts contained significantly fewer infectious virus particles, very likely of lower virulence, and their expression of gC-1 and gD-1 were concordantly reduced. These observations indicate that the number of cell P must be considered in order to reproduce results of cell biology and viral morphogenesis. The MRC-5 cells ought not to be passaged more than ten times beyond P 27 in the laboratory.

Relation of the slow growth phenotype to neoplastic transformation: possible significance for human cancer

Deletions are widely distributed over the genome in the most frequently occurring human cancers and are the most abundant genetic lesion found there. Deletions are highly correlated with the slow growth phenotype of mutated animal and human cells and result in chromosomal transposition when the retained ends are joined. Transpositions are only a minor source of mutation in rapidly multiplying bacteria but are a major cause of mutations in stationary bacteria. The NIH 3T3 line of mouse cells undergoes neoplastic transformation during prolonged incubation in a stationary state and expresses the slow growth phenotype on serial subculture at low density, suggesting a relation between transformation and chromosomal deletions. To further explore the relation between neoplastic transformation and the slow growth phenotype as a surrogate for deletions, two sublines of the NIH 3T3 cells with differing competence for transformation were serially subcultured in the stationary state at confluence and tested at each subculture for transformation and growth rate. Cell death in a fraction of the population and a heritable slowdown in proliferation of most of the survivors became increasingly pronounced with successive rounds of confluence. The reduction in growth rate was not proportional to the degree of transformation of the cultures, but all of the transformed cultures were slow growers at low density. All of the discrete colonies from cloning transformed cultures developed at a lower initial rate than control colonies under optimal conditions for growth, but they continued to grow at later stages, forming multilayered colonies under conditions that inhibited the further growth of the control colonies. The results suggest that prolonged incubation of NIH 3T3 cells in the stationary state results in growth-impairing deletions over a wide range of sites in the genome, but more restricted subsets of such lesions are responsible for neoplastic transformation. These findings provide dynamic, functional support in culture for the histopathological evidence that the quiescent state of cells associated with atrophy and fibrosis plays a significant role in the origin of some cancers in experimental animals and human beings.

Enhancement of LPS-stimulated plasminogen activator production in aged gingival fibroblasts

Plasminogen activator (PA) converts plasminogen to plasmin, and plasmin activates the kinin cascade and latent methalloproteases. It is known that the alteration of the PA-plasmin system affects the progression of periodontal disease. We have reported previously that LPS from Campylobacter rectus, which is associated with adult periodontitis, increased PA production in human gingival fibroblasts (hGF). The effects of in vitro- and in vivo-cellular ageing on PA production from human and rat gingival fibroblasts (rGF) were studied. In vitro cellular aged hGF were prepared by subcultivations of hGF, and in vivo aged rGF was cultured primarily from the gingival tissue of aged rats. The cells were challenged with LPS and PA released into the cultured medium was measured as PA activity. Both in vitro and in vivo cellular aged GFs produced a significantly higher PA activity by LPS compared with young GFs cell. In RT-PCR experiments, tissue type PA (tPA) mRNA levels in both aged hGF and rGF were higher than in young cells, whereas plasminogen activator inhibitor 1 (PAI-1) mRNA remained unchanged and urotype PA (uPA) mRNA was not detected. Since LPS-stimulated PA activity from gingival fibroblasts was stimulated in aged cells using both in vitro- and in vivo-experimental models, the ageing of gingival fibroblasts may have an effect on the severity of inflammation and degradation of the extracellular matrix of gingival tissues by producing a large amount of PA in response to LPS.

Enhancement of lipopolysaccharide-stimulated PGE2 and IL-1beta production in gingival fibroblast cells from old rats

The effect of aging on gingival fibroblasts in response to bacterial infection was studied. Rat gingival fibroblast (rGF) cells were cultured from gingival tissue removed from young (6 weeks old) and old (20 months old) rats. Both types of rGF cells were challenged with lipopolysaccharide (LPS) from the periodontal pathogen Campylobacter rectus. The levels of prostaglandin E2 (PGE2) and interleukin 1beta (IL-1beta) released into the cultured medium were measured by a specific radioimmunoassay. LPS stimulated PGE2 and IL-1beta production in a dose-and time-dependent manner in rGF cells from both young and old rats was seen. Production of PGE2 and IL-1beta by rGF cells from the old rats was higher than those from the young in response to LPS. This greater ability from the older rGF cells to produce PGE2 and IL-1beta was due to higher mRNA levels of cyclooxygenase 2 and IL-1beta, respectively. In contrast, cyclooxygenase-1 and IL-1beta converting enzyme gene mRNA levels remained unchanged. Because LPS-stimulated PGE2 and IL-1beta production was enhanced by in vivo cellular aging, aging of GF may affect the severity of inflammation and bone resorption by producing a large amount of PGE2 and IL-1beta in response to bacterial infection.

Dynamic Changes in Mouse Hematopoietic Stem Cell Numbers During Aging

To address the fundamental question of whether or not stem cell populations age, we performed quantitative measurements of the cycling status and frequency of hematopoietic stem cells in long-lived C57BL/6 (B6) and short-lived DBA/2 (DBA) mice at different developmental and aging stages. The frequency of cobblestone area-forming cells (CAFC) day-35 in DBA fetal liver was twofold to threefold higher than in B6 mice, and by late gestation, the total stem cell number was nearly as large as that of young DBA adults. Following a further ≈50% increase in stem cells between 6 weeks and 1 year of age, numbers in old DBA mice dropped precipitously between 12 and 20 months of age. In marked contrast, this stem cell population in B6 mice increased at a constant rate from late gestation to 20 months of age with no signs of abatement. Throughout development an inverse correlation was observed between stem cell numbers and the percentage of cells in S-phase. Because a strong genetic component contributed to the changes in stem cell numbers during aging, we quantified stem cells of 20-month old BXD recombinant inbred (RI) mice, derived from B6 and DBA progenitor strains, thus permitting detailed interstrain genetic analysis. For each BXD strain we calculated the stem cell increase or decrease as mice aged from 2 to 20 months. Net changes in CAFC-day 35 numbers among BXD strains ranged from an ≈10-fold decrease to an ≈10-fold increase. A genome-wide search for loci associated with this quantitative trait was performed. Several loci contribute to the trait—putative loci map to chromosomes X, 2, and 14. We conclude that stem cell numbers fluctuate widely during aging and that this has a strong genetic basis.

Genetic analysis of hemopoietic cell cycling in mice suggests its involvement in organismal life span

Normal somatic cells undergo replicative senescence in vitro but the significance of this process in organismic aging remains controversial. We have shown previously that hemopoietic stem cells of common inbred strains of mice vary widely in cycling activity and that this parameter is inversely correlated with strain-dependent mean life span. To assess whether cell cycling and life span are causally related, we searched for quantitative trait loci (QTLs) that contributed to variation of these traits in BXH and BXD recombinant inbred mice. Two QTLs, mapping to exactly the same intervals on chromosomes 7 and 11, were identified that were associated with variation of both cell cycling and life span. The locus on chromosome 11 mapped to the cytokine cluster, a segment that shows synteny with human chromosome 5q, in which deletions are strongly associated with myelodysplastic syndrome. These data indicate that steady-state cell turn-over, here measured in hemopoietic progenitor cells, may have a significant effect on the mean life span of mammals.

Premature induction of aging in sublethally H2O2-treated young MRC5 fibroblasts correlates with increased glutathione peroxidase levels and resistance to DNA breakage

Human MRC5 fibroblasts, at different passages in cultures, were used as an in vitro model to assess variations and/or induction of aging parameters under basal conditions or following sublethal oxidative stress by H2O2. DNA sensitivities to oxidatively-induced breakage, rather than basal levels of damaged DNA, were significantly different between cultures at low and high population doubling level (PDL): old cells maintained most of their DNA integrity even at high concentrations of H2O2, while young cells showed more extensive DNA damage which developed in a dose-dependent fashion. However, young cells pretreated with low doses of H2O2 exhibited increased resistance against further oxidative damage to DNA thus reproducing a senescent-like profile of sensitivity. In turn, DNA from old cultures incubated in a NAD precursor-free medium was more prone to H2O2-induced strand breaks mimicking DNA sensitivity of young cells. The extent of oxidatively-induced DNA damage in MRC5 populations correlated inversely with the levels of glutathione peroxidase (GPx) activity that almost doubled when cells passed from the young to the senescent stage. In addition, H2O2-pretreatment of young cells induced an increase in GPx expression approaching old cell values and promoted also the premature appearance of neutral beta-galactosidase activity and decreased c-fos expression upon serum stimulation, both of which were assumed to be characteristic traits of the senescent phenotype.

Relationship between donor age and the replicative lifespan of human cells in culture: a reevaluation

Normal human diploid fibroblasts have a finite replicative lifespan in vitro, which has been postulated to be a cellular manifestation of aging in vivo. Several studies have shown an inverse relationship between donor age and fibroblast culture replicative lifespan; however, in all cases, the correlation was weak, and, with few exceptions, the health status of the donors was unknown. We have determined the replicative lifespans of 124 skin fibroblast cell lines established from donors of different ages as part of the Baltimore Longitudinal Study of Aging. All of the donors were medically examined and were declared "healthy," according to Baltimore Longitudinal Study of Aging protocols, at the time the biopsies were taken. Both long- and short-lived cell lines were observed in all age groups, but no significant correlation between the proliferative potential of the cell lines and donor age was found. A comparison of multiple cell lines established from the same donors at different ages also failed to reveal any significant trends between proliferative potential and donor age. The rate of [3H]thymidine incorporation and the initial rates of growth during the first few subcultivations were examined in a subset of cell lines and were found to be significantly greater in fetal lines than in postnatal lines. Cell lines established from adults did not vary significantly either in initial growth rate or in [3H]thymidine incorporation. These results clearly indicate that, if health status and biopsy conditions are controlled, the replicative lifespan of fibroblasts in culture does not correlate with donor age.

Mitogenic factors accelerate later-age diseases: insulin as a paradigm

Some genes are expressed differently in earlier and later generations of most cell lines. Many diseases become clinically expressed only later in life, and show clustering of the age at onset in the affected siblings, which may be related to the changing expression with age of the genes involved. Because insulin and its receptor are extremely ancient and well preserved structures with almost universal mitogenic effects, insulin may serve a paradigm of this process. It is suggested that by stimulating cell proliferation, hyperinsulinemia speeds up the appearance of later generations of cells with different expression of the genes. Insulin resistance, accompanying any hyperinsulinemia and considered to be a pathogenetic factor of some common later-age diseases, involves only some biochemical, but not mitogenic effects of the hormone. In humans, high levels of insulin in blood are encountered both physiologically after meals and in many pathological conditions: insulin therapy inevitably causes peripheral hyperinsulinemia; in type 2 diabetes hyperinsulinemia precedes hyperglycemia by many years; hyperinsulinemia is an independent risk factor of atherosclerosis, of type 2 diabetes itself, of some forms of dementia and other diseases; obesity is an obligatory hyperinsulinemic condition. The opposite of hyperalimentation, i.e. calorie restriction (at least, in rodents) may exert its life-prolonging effects through decreasing insulinemia and therefore the rate of cell proliferation. Insulin is only one example, and different mitogens regulate proliferation of different cells. It is likely that growth factors in general accelerating the replication of cells, play a role in speeding up the appearance of later-age diseases involving these cells.

Selective killing of preneoplastic and neoplastic cells by methotrexate with leucovorin

Three sublines of NIH 3T3 cells had the properties of non-neoplastic, preneoplastic, and neoplastic cells, respectively. The closer the cells were to neoplastic behavior, characterized by continuing growth at high density, the slower they multiplied at lower density. Under the conditions of high population density and low calf serum concentration used in the assay for transformed focus formation, the transformed or neoplastic cells were much more sensitive to killing by methotrexate (MTX) than were non-neoplastic cells in the same culture. This differential sensitivity of neoplastic cells was far more pronounced in molecular, cellular, and developmental biology medium 402 (MCDB 402) than in DMEM. It is associated with the presence in MCDB 402 of folinic acid, known clinically as leucovorin, which is a reduced form of the folic acid present in DMEM. Although leucovorin had been shown to selectively spare normal bone marrow and intestine in animals from the killing effect of MTX on tumor cells, we demonstrate the preferential killing of neoplastic over non-neoplastic cells of the same derivation. Neither neoplastic nor non-neoplastic cells were killed once they had stopped multiplying at their respective saturation densities. The development of the light foci characteristic of the preneoplastic cells was less sensitive to MTX than the formation of the dense foci produced by the fully neoplastic cells. The system should serve as a valuable model to establish basic principles and optimal conditions for selective killing of neoplastic cells by chemotherapeutic drugs.

A novel technique for transforming the theft of mortal human cells into praiseworthy federal policy

A revolution has occurred in the attitude of biologists toward their intellectual property rights. What today is patentable and highly profitable was, 20 years ago, unpatentable and given away for nothing. The history of this revolution began in the early 1960s when we made the first effort to have self-duplicating cell strains patented. The application was denied because patent law at that time did not include living matter. Because of the demand for our normal human diploid cell strain, WI-38, by NIH grantees, NIH support was provided to distribute WI-38 gratis to hundreds of recipients. These included vaccine and cell manufacturers who profited enormously from the direct sale of WI-38 or its use as a substrate for many human virus vaccines. When federal support for the distribution of WI-38 ended, but demand did not, I continued to distribute it for costs similar to those made by the American Type Culture Collection. When I took the first initiative and asked NIH to have the then unique question of title to a self-duplicating system resolved, they sent an accountant who accused me of theft of government property. I replied with a lawsuit that, after six years of litigation, we won with an out-of-court settlement. During these six years the United States Supreme Court ruled that living matter could be patented. Also, the biotechnology industry was launched by biologists who, like me, started companies using cells or microorganisms developed with federal support. This use of intellectual property rights by the nascent biotechnology industry was ultimately embraced by the entire biological community and by a directive from the President of the United States. This revolution has now evolved to the point where government biologists themselves may profit from research in federal laboratories, and the NIH itself aggressively seeks private commercial alliances. Universities have also pursued similar alliances to the extent that today the distinction between a research university and a commercial organization is only in the eyes of the Internal Revenue Service.