What does cell death have to do with aging?

High glucose-induced replicative senescence: point of no return and effect of telomerase

Primary human cells enter senescence after a characteristic number of population doublings (PDs). In the current study, human skin fibroblasts were propagated in culture under 5.5mM glucose (normoglycemia); addition of 16.5mM D-glucose to a concentration of 22 mM (hyperglycemia); and addition of 16.5mM L-glucose (osmotic control). Hyperglycemia induced premature replicative senescence after 44.42+/-1.5 PDs compared to 57.9+/-3.83 PDs under normoglycemia (p<0.0001). L-Glucose had no effect, suggesting that the effect of hyperglycemia was not attributed to hyperosmolarity. Activated caspase-3 measurement showed a significantly higher percentage of apoptotic cells in high glucose medium. Telomerase overexpression circumvented the effects of hyperglycemia on replicative capacity and apoptosis. The "point of no return," beyond which hyperglycemia resulted in irreversible progression to premature replicative senescence, occurred after exposure to hyperglycemia for as few as 20 PDs. These results may provide a biochemical basis for the relationship between hyperglycemia and those complications of diabetes, which are reminiscent of accelerated senescence.

The genetics of aging

Once thought to be an extremely complex conundrum of weak genetic and environmental effects, exceptional longevity is beginning to yield genetic findings. Numerous lower organism and mammalian models demonstrate genetic mutations that increase life-span markedly. These variations, some of them evolutionarily conserved, inform us about biochemical pathways that significantly impact upon longevity. Centenarian studies have also proven useful as they are a cohort that, relative to younger age groups, lacks genotypes linked to age-related lethal diseases and premature mortality. Pedigree studies have demonstrated a significant familial component to the ability to survive to extreme old age and a recent study demonstrates a locus on chromosome 4 linked to exceptional longevity indicating the likely existence of at least one longevity enabling gene in humans. Thus, a number of laboratories are making substantial and exciting strides in the understanding of the genetics of aging and longevity which should lead to the discovery of genes and ultimately drugs that slow down the aging process and facilitate people's ability to delay and perhaps escape age-associated diseases.

Cell signaling in aging and apoptosis

Alterations in apoptotic potential, due to perturbations in cell signaling cascades, could underlie age-related organ-specific cellular degeneration and death. While increased apoptosis could lead to cell loss, as in neuronal degeneration, loss of apoptosis competence might well result in the loss of phenotypic fidelity of somatic cells, which could explain to some extent, the age-related increase in cancer incidence. Results from our laboratory indicate that after subjecting young and old rats to genotoxic stress in the form of methyl methanesulfonate (MMS), an apoptotic response is quickly mounted in the liver of the young animals but virtually absent in the same organ of old animals (Nature Med. 8 (2002) 3). To address the possible molecular signaling defect(s) responsible for the age-related dysfunction of apoptosis in response to MMS, mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinases (ERKs), c-Jun NH(2)-terminal kinases (JNKs) and p38 MAPKs, were evaluated in the liver of young and old rats after MMS treatment. The results demonstrated distinct age-specific patterns of MMS-induced MAPKs activation, suggesting that the balance between cell survival and apoptosis after genotoxic stress may be impaired during aging. These results are discussed in terms of the relative importance in aging of biological redundancy, a concept put forward by the late Bernard Strehler, and cellular fidelity.

Aging and the role of reactive nitrogen species

The role of reactive oxygen species and its effects on aging has received considerable attention in the past 47 years since Dr. Denham Harman first proposed the "free radical theory of aging." Though not completely understood due to the incalculable number of pathways involved, the number of manuscripts that facilitate the understanding of the underlying effects of reactive radical species on the oxidative stress on lipids, proteins, and DNA and its contribution to the aging process increases nearly exponentially each year. More recently, the role of reactive nitrogen species, such as nitric oxide and its by-products--nitrate (NO3-), nitrite (NO2-), peroxynitrite (ONOO-), and 3-nitrotyrosine--have been shown to have a direct role in cellular signaling, vasodilation, and immune response. Nitric oxide is produced within cells by the actions of a group of enzymes called nitric oxide synthases. Presently, there are three distinct isoforms of nitric oxide synthase: neuronal (nNOS or NOS-1), inducible (iNOS or NOS-2), and endothelial (eNOS or NOS-3), and several subtypes. While nitric oxide (NO*) is a relative unreactive radical, it is able to form other reactive intermediates, which could have an effect on protein function and on the function of the entire organism. These reactive intermediates can trigger nitrosative damage on biomolecules, which in turn may lead to age-related diseases due to structural alteration of proteins, inhibition of enzymatic activity, and interferences of the regulatory function. This paper will critically review the evidence of nitration and the important role it plays with aging. Furthermore, it will summarize the physiological role of nitration as well as the mechanisms leading to proteolytic degradation of nitrated proteins within biological tissues.

The role of apoptosis in the normal aging brain, skeletal muscle, and heart

During aging, there is a significant loss of some postmitotic cells, for example, cardiac and skeletal myocytes. Mitochondrial damage and dysfunction with age may trigger increased apoptosis, and this may explain this increase in cell loss. However, it is still unknown if apoptosis plays an important role in normal aging. In vitro it has been shown that several mitochondrial proteins can influence apoptosis, depending on factors such as the mitochondrial membrane potential and cellular redox status. It remains possible that mitochondrial dysfunction due to chronic oxidative stress with age is a cause of apoptosis in vivo. This cell loss may be due to mitochondrial-triggered apoptosis caused by age-associated increases in oxidant production or increased activation of mitochondrial permeability transition pores. Results from our laboratory and others are reviewed that relate to apoptosis in the normal aging of the brain cortex, heart, and skeletal muscle. Particular attention is paid to the role of cytochrome c release from mitochondria and alterations in the pro- and anti-apoptotic proteins, Bax and Bcl-2, respectively. Our results demonstrate that a tissue-specific adaptation of the Bcl-2/Bax ratio occurs with age and may directly influence the release of cytochrome c.

Termination of lifespan of SV40-transformed human fibroblasts in crisis is due to apoptosis

Normal human fibroblasts in culture have a limited lifespan, ending in replicative senescence. Introduction of SV40 sequences encoding large T antigen and small t antigen into pre-senescent cells results in an extension of lifespan for an additional 20-30 population doublings. Rare clones of SV40-transformed cells are capable of indefinite growth and are described as immortal; however, the great majority of SV40-transformed cells terminate this extended lifespan in cell death, termed "crisis." We have examined the properties of cells in crisis to obtain further insights into mechanism of cell death and immortalization. Populations at the terminal cell passage show a balance between cell replication and cell death over a period of several weeks, with a progressive increase in cells undergoing cell death. During this period, there is less than a 3-fold increase in attached cell number, with two stages being identifiable on the basis of the focal pattern of cell survival. We also demonstrate that cells in crisis are undergoing apoptosis based on TUNEL assay, subG1 DNA content, annexin V reactivity, and activation of caspases 3 and 8. We suggest a model whereby SV40-transformed cells acquire increased sensitivity to apoptosis based on changes in properties which activate caspase 8 in addition to changes previously described involving shortening of telomeric sequences. While only telomere stabilization could be clearly shown to be essential for survival of cells through crisis, the extended period of cell replication and altered gene expression observed in SV40-transformed cells during crisis are compatible with other genetic alterations in immortal cells.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) triggers apoptosis in normal prostate epithelial cells

TRAIL is a pro-apoptotic cytokine believed to selectively kill cancer cells without harming normal ones. However, we found that in normal human prostate epithelial cells (PrEC) TRAIL is capable of inducing apoptosis as efficiently as in some tumor cell lines. At the same time, TRAIL did not cause apoptosis in several other human primary cell lines: aorta smooth muscle cells, foreskin fibroblasts, and umbilical vein endothelial cells. Compared to these primary cells, PrEC were found to contain significantly fewer TRAIL receptors DcR1 and DcR2 which are not capable of conducting the apoptotic signal. This result suggests that the unusual sensitivity of PrEC to TRAIL may result from their deficiency in anti-apoptotic decoy receptors. The protein synthesis inhibitor cycloheximide significantly enhanced TRAIL toxicity toward PrEC as measured by tetrazolium conversion but had little or no effect on other TRAIL-induced apoptotic responses. Although cycloheximide did not further accelerate the processing of caspases 3 and 8, it significantly enhanced cleavage of the caspase 3 substrate gelsolin, indicating that in PrEC a protein(s) with a short half-life may inhibit the activity of the executioner caspases toward specific substrates. As the majority of prostate cancers are derived from epithelial cells, our data suggest the possibility that TRAIL could be a useful treatment for the early stages of prostate cancer.

Cytochrome c release from mitochondria in the aging heart: a possible mechanism for apoptosis with age

There is a loss of myocytes in the aging heart due to necrosis and apoptosis. Oxidative stress, an apoptosis-inducing signal, may also increase in the aging heart. Cytosol and mitochondria isolated from the left and right ventricle of the hearts of 6-, 16-, and 24-mo-old male Fischer 344 rats were used to measure key markers of apoptosis and to assess oxidative stress. Cytosolic cytochrome c content was significantly elevated in the 16- and 24-mo-old animals compared with the 6-mo-old animals. Furthermore, Bcl-2, an antiapoptotic protein, showed a strong tendency to decrease with age, whereas Bax, a proapoptotic protein, remained unchanged. Apoptotic protease-activating factor 1 levels and caspase-3 activities were not different among the three age groups. Indicative of the chronic oxidative stress with age, heart mitochondria from old animals showed increases in manganese superoxide dismutase and glutathione peroxidase activity and increases in lipid peroxidation. This is the first study to report cytochrome c release from the mitochondria and alterations in Bcl-2 with age in vivo, providing a potential mechanism for the increase in apoptosis seen in the aging heart.

Explaining mortality rate plateaus

We propose a stochastic model of aging to explain deviations from exponential growth in mortality rates commonly observed in empirical studies. Mortality rate plateaus are explained as a generic consequence of considering death in terms of first passage times for processes undergoing a random walk with drift. Simulations of populations with age-dependent distributions of viabilities agree with a wide array of experimental results. The influence of cohort size is well accounted for by the stochastic nature of the model.

Brain ageing in the new millennium

OBJECTIVE

This paper examines the current literature pertaining to brain ageing. The objective of this review is to provide an overview of the effects of ageing on brain structure and function and to examine possible mediators of these changes.

METHODS

A MEDLINE search was conducted for each area of interest. A selective review was undertaken of relevant articles.

RESULTS

Although fundamental changes in fluid intellectual abilities occur with age, global cognitive decline is not a hallmark of the ageing process. Decline in fluid intellectual ability is paralleled by regionally specific age related changes apparent from both structural and functional neuroimaging studies. The histopathological mediators of these changes do not appear to be reduction in neuronal number, which, with the exception of selected hippocampal regions, remain relatively stable across age. At the molecular level, several mechanisms of age related change have been postulated. Such theoretical models await refinement and may eventually provide a basis for therapy designed to reduce effects of the ageing process. The role of possible protective factors such as 'brain reserve', neuroprotective agents and hormonal factors in modifying individual vulnerability to the ageing process has been the focus of a limited number of studies.

CONCLUSION

Our understanding of the functional and structural changes associated with both healthy and pathological ageing is rapidly gaining in sophistication and complexity. An awareness of the fundamental biological substrates underpinning the ageing process will allow improved insights into vulnerability to neuropsychiatric disease associated with advancing age.

Apoptosis and aging: role of the mitochondria

Apoptosis research is a rapidly developing area, but the role of apoptosis is still unclear and controversial. For example, several studies document a significant loss of cardiac and skeletal myocytes during normal aging, possibly by apoptotic mechanisms. This loss in cells may be directly mediated by mitochondrial dysfunction caused by chronic exposure to oxidants and increased activation of mitochondrial permeability transition pores. This review will discuss apoptosis in the context of normal aging of T cells, cardiac myocytes, skeletal muscle, and brain cortex. Particular attention is paid to the role of the mitochondria, because they have been implicated as a major control center regulating apoptosis. Mitochondrial oxidative stress and a decline in mitochondrial energy production in vitro often leads to activation of apoptotic pathways, but whether this occurs in vivo is unclear.

Mutation frequency and type during ageing in mouse seminiferous tubules

Mutations arise in the germline by errors of replication, recombination and repair, and the movement of transposable elements. Transgenic mice bearing reporter genes such as lacZ have proven useful for measurements of spontaneous and induced mutation frequencies, as well as studies of the effects of ageing. In this study, testicular DNA from lacZ transgenic mice was examined for age-related effects on mutation frequency and type. The recovered transgene was tested for simple substitutions and rearrangements including transposition of endogenous mobile elements. There was no evidence for either an age-related accumulation of mutations, or for the insertion of retrotransposons into the lacZ reporter gene in the testis. We conclude that the frequency of retrotransposition of several mouse mobile elements into the lacZ reporter gene is less than 3.73x10(-8). This is significantly less than the known frequency of approximately 7% of all spontaneous mutations in the mouse being due to retrotransposition of these elements.

Telomeres and telomerase: basic science implications for aging

Life expectancy in the United States and other developed nations has increased remarkably over the past century, and continues to increase. However, lifespan has remained relatively unchanged over this period. As life expectancy approaches maximum human lifespan, further increase in life expectancy would only be possible if lifespan could also be increased. Although little is known about the aging process, increasing lifespan and delaying aging are the research challenges of the new century, and have caused intense debate and research activities among biogerontologists. Many theories have been proposed to explain the aging process. However, damage to deoxyribonucleic acid (DNA) is the centerpiece of most of these. Recently telomere shortening has been described to be associated with DNA damage. Located at the ends of eukaryotic chromosomes and synthesized by telomerase, telomeres maintain the length of chromosomes. The loss of telomeres can lead to DNA damage. The association between cellular senescence and telomere shortening in vitro is well established. In the laboratory, telomerase-negative differentiated somatic cells maintain a youthful state, instead of aging, when transfected with vectors encoding telomerase. Many human cancer cells demonstrate high telomerase activity. Evidence is also accumulating that telomere shortening is associated with cellular senescence in vivo. What causes changes in expression of telomerase in different cell types and premature aging syndromes? Does the key to "youthfulness" lie in our ability to control the expression of telomerase? We have reviewed the contemporary literature to find answers to these questions and explore the association between aging, telomeres, and telomerase.

Replicative senescence of human endothelial cells in vitro involves G1 arrest, polyploidization and senescence-associated apoptosis

Human ageing is characterized by a progressive loss of physiological functions, increased tissue damage and defects in various tissue renewal systems. Age-related decreases of the cellular replicative capacity can be reproduced by in vitro assays of cellular ageing. When diploid human fibroblasts reach their finite lifespan, they enter an irreversible G1 growth arrest status referred to as replicative senescence. While deregulation of programmed cell death (apoptosis) is a key feature of age-related pathology in several tissues, this is not reflected in the standard in vitro senescence model of human fibroblasts, and the role of apoptosis during cellular ageing remains unclear. We have analyzed replicative senescence of human umbilical vein endothelial cells (HUVEC) in vitro and found that senescent HUVEC also arrest in the G1 phase of the cell cycle but, unlike fibroblasts, accumulate with a 4N DNA content, indicative of polyploidization. In contrast to human fibroblasts, senescent endothelial cells display a considerable increase in spontaneous apoptosis. The data imply that age-dependent apoptosis is a regular feature of human endothelial cells and suggest cell type specific differences in human ageing.

Delayed wound healing in aged rats is associated with increased collagen gel remodeling and contraction by skin fibroblasts, not with differences in apoptotic or myofibroblast cell populations

Aging has been anecdotally reported to result in prolonged wound healing. Measurement of punch biopsy wound closure in young (4 month old) and old (36 month old) rats indicated there was a significant delay in wound closure by old rats during the early phase of repair, after which closure rates were equivalent. The delay in granulation tissue accumulation in older animals could involve premature programmed cell death (apoptosis); however, apoptotic fibroblasts in sponge granulation tissue and tissue culture were less abundant in samples from old rats relative to young rats. Myofibroblasts express alpha-smooth muscle actin, and they are believed to be important in wound contraction. There were no significant differences in overall abundance or distribution of alpha-smooth muscle actin containing myofibroblasts in granulation tissue and in cultured granulation tissue fibroblasts regardless of the age of the donor rat. The spatial distribution of myofibroblasts and apoptotic cells was distinct. Fibroblasts from granulation tissue and skin explants were placed in a collagen gel contraction assay prior to the 5th passage to determine their in vitro contractility. While granulation tissue fibroblasts from young and old rats showed similar collagen gel contractility, skin fibroblasts from old rats displayed greater collagen gel contractile behavior than young skin fibroblasts. Greater gel contractility of fibroblasts from old rats appeared to result, in large part, from the ability of those cells to cause generalized gel degradation. Gelatin zymography indicated a greater abundance of matrix metalloproteinase-2 in supernatants from gels containing skin fibroblasts from old rats. Taken together, these results suggest that the age-associated healing delay in the rat may not be related to the appearance or abundance of distinct myofibroblast or apoptotic cell populations. Proteolysis may have a significant role in delayed wound healing in aged animals.

Immunosenescence and cancer: a review

The human neoplastic pathologies are age-dependent. The increased occurrence of tumors observed with advancing age may be determined by the accumulation of certain phenomena promoting different phases of neoplastic processes. In these events, important roles can be attributed to mutations of the genome that accumulate during aging and to the immunosenescence. It may be hypothesized that certain tumors controlled by the immune system may become more frequent in the elderly as a consequence of the decreased functionality of this important defense system of the organism. Nevertheless, the problems of the interrelationships between the immunosenescence and tumors are seriously contradictory. Therefore, on the one hand, one has to establish how much the immunodeficit of the elderly patient may be responsible for the neoplastic pathology, while on the other hand, one cannot neglect important environmental and pathophysiological factors in these cases.

Translating basic aging research into geriatric health care

Aging processes are amenable to molecular genetic analyses. Two aspects of such research have been selected for discussion in this paper because of current great interest and their relevance to human aging. Studies on telomeres have revealed new insights on the control of cellular replicative senescence and provided a means to extend the cell's life span during in vitro cultivation. Emerging studies on genetic biomarkers have identified genes that appear to be associated with longevity or with risk factors for aging-related diseases, and raised considerations of ways to reduce disease expression. An interchange between basic scientists and clinicians would encourage new thoughts on the feasibility of translating these fundamental studies into interventions that promote healthier longevity.

Cell darwinism, apoptosis, free radicals and haematological malignancies

Haematopoiesis can be interpreted as an ecosystem composed of billions of cells interacting according to Darwinian rules. Mutation, by promoting cell diversity, ensures versatility in coping with internal and external challenges. Most mutated cells are eliminated through apoptosis. However, if mutation generates relative resistance to apoptosis it may result in growth advantage for the mutated cells. The probability of monoclonality and malignancy is significantly increased if the normal multiclonal environment is damaged by a pathologic proapoptotic process that spares the apoptosis resistant clones. Paroxysmal nocturnal haemoglobinuria, myelodysplastic syndromes, chronic myeloid leukaemia, secondary acute leukaemias and immunosuppression-related non-Hodgkin's lymphomas can be interpreted as 'opportunistic' clonal and malignant diseases. Free radicals (FRs) are closely linked to apoptosis and have been incriminated in oncogenesis. Conditions associated with increased FR formation or impaired FR disposal may provide the enhanced apoptotic background against which an apoptosis-resistant clone may gain growth advantage.

Apoptosis in Cardiovascular Aging Research: Future Directions

Apoptosis is the process by which cells are induced to activate their own suicide. Programmed cell death occurs in a wide variety of cell types, including cardiovascular tissues. There is increasing evidence of a relationship between apoptosis and cardiovascular disease, particularly ischemic heart disease and congestive heart failure, the most frequent heart diseases in the older population. Research directed to the treatment and possible prevention of apoptosis may provide a means of decreasing the incidence of cardiac failure and increasing the survival of endothelial and smooth muscle cells in the elderly. (c) 2000 by CVRR, Inc.

The effect of ageing on parenchymal cell populations in adult female mouse submandibular gland

The submandibular gland shows an array of responses that accompany ageing, which are usually modest. However, the submandibular acinar-cell mucin shows a substantial decline in total amount per gland. In the submandibular gland, there is also a loss of secretory parenchyma. A number of factors that could influence either parenchymal loss or a change in its cellular composition were examined in three ages of adult female mice. The goal was to see if there are ageing-related cellular changes that might have an effect on mucin production or secretion. The factors examined included DNA, protein, rates of cell division and apoptosis, cell volume and cellular composition of the parenchymal population. The parenchymal cell composition showed significant differences during ageing, with a substantial decrease in the percentage of acinar cells and increases in the percentage of both types of ductal cell components. This decline in the proportion of acinar cells in the parenchyma also reflected an overall reduction in the total number of acinar cells in the gland. Thus, the change in proportions of cells may potentially be a direct cause of the ageing-related decline in the submandibular acinar-cell mucin. The alteration in cellular composition was not attributable to changes in the cell-division indices; however, there was an increased rate of apoptosis for acinar cells that was significantly different between 3 and 28 months. The apoptotic rate doubled for acinar cells but showed no significant change in ductal cells. This selective change in the rate of apoptosis with ageing suggests that it is one of the main reasons for the decline in the proportion of acinar cells in the submandibular gland.

A simple method for accurate estimation of apoptotic cells

A simple, sensitive, and reliable "DNA diffusion" assay for the quantification of apoptosis is described. Human lymphocytes and human lymphoblastoid cells, MOLT-4, were exposed to 0, 12.5, 25, 50, or 100 rad of X-rays. After 24 h of incubation, cells were mixed with agarose, microgels were made, and cells were lysed in high salt and detergents. DNA was precipitated in microgels by ethanol. Staining of DNA was done with an intense fluorescent dye, YOYO-1. Apoptotic cells show a halo of granular DNA with a hazy outer boundary. Necrotic cells, resulting from hyperthermia treatment, on the other hand, show an unusually large homogeneous nucleus with a clearly defined boundary. The number of cells with apoptotic and necrotic appearance can be scored and quantified by using a fluorescent microscope. Results were compared with other methods of apoptosis measurement: morphological estimations of apoptosis and DNA ladder pattern formation in regular agarose gel electrophoresis. Validation of the technique was done using some known inducers of apoptosis and necrosis (hyperthermia, hydrogen peroxide, mitoxantrone, novobiocin, and sodium ascorbate).

Apoptosis: a two-edged sword in aging

Here we summarize briefly what is known about both the positive and negative impacts of apoptosis during aging in mammalian systems and also update an earlier review. It is important to understand both of these impacts to devise useful interventions. Such interventions include both physiological and molecular approaches, including transgenic interventions. The critical roles of the mitochondria in both generating reactive oxygen species, and in initiating apoptosis are recognized, suggesting that maintaining mitochondrial function could be an important therapeutic goal, especially in post-mitotic tissues. In contrast, the ability to eliminate unwanted, damaged and dysfunctional cells through apoptosis has anti-aging implications in mitotic tissues.

The NAD+ precursors, nicotinic acid and nicotinamide protect cells against apoptosis induced by a multiple stress inducer, deoxycholate

The bile salt, sodium deoxycholate (NaDOC), is a natural detergent that promotes digestion of fats. At high physiologic levels, NaDOC activates many stress-response pathways and induces apoptosis in various cell types. NaDOC induces DNA damage and activates poly(ADP-ribose) polymerase (PARP), an enzyme that utilizes NAD+ as a substrate to repair DNA. NaDOC also induces oxidative stress, endoplasmic reticulum (ER) stress and contributes to protein malfolding. The NAD+ precursors, nicotinic acid (NA) and nicotinamide (NAM) were found to protect cells against NaDOC-induced apoptosis. NA and NAM also decreased constitutive levels of both activated NF-kappaB and GRP78, two proteins that respond to oxidative stress. However, the mechanism by which NA and NAM protects cells against apoptosis does not involve a reduction in constitutive levels of oxidative stress. NA or NAM treatment increased the protein levels of glyceraldehyde-3-phosphate dehydrogense (GAPDH), a multi-functional enzyme, in the nucleus and cytoplasm, respectively. NAM did not activate the promoter/response elements of 13 stress response genes nor reduce intracellular non-protein thiols, suggesting that it is non-toxic to cells. NAM thus has promise as a dietary supplement to help prevent disorders involving excessive apoptosis.

Telomere shortening is an in vivo marker of myocyte replication and aging

To determine whether adult cardiac myocytes are capable of multiple divisions and whether this form of growth is restricted to a subpopulation of cells that retain this capacity with age, telomere lengths were measured in myocyte nuclei isolated from the left ventricle of fetal and neonatal Fischer 344 rats and rats at 4, 12, and 27 months after birth. Two independent methodologies were used for this analysis: laser scanning cytometer and confocal microscopy. In each case, fluorescence intensity of a peptide nucleic acid probe specific for telomeric sequence was evaluated. The two techniques yielded comparable results. Telomeric shortening increased with age in a subgroup of myocytes that constituted 16% of the entire cell population. In the remaining nondividing cells, progressive accumulation of a senescent associated nuclear protein, p16(INK4), was evidenced. In conclusion, a significant fraction of myocytes divides repeatedly from birth to senescence, counteracting the continuous death of cells in the aging mammalian rat heart.

Resistance to apoptosis in human CD8+ T cells that reach replicative senescence after multiple rounds of antigen-specific proliferation

We have established an in vitro culture model of cellular aging in which antigen-specific T cells are stimulated repeatedly to divide until they reach the irreversible state of growth arrest known as "replicative senescence." T lymphocytes that reach replicative senescence in culture show complete loss of CD28 expression, shortened telomeres, undetectable telomerase, and reduced ability to produce heat shock proteins. We now document that in response to treatment with apoptotic stimuli, senescent CD8+ T-cell cultures show reduced apoptosis and diminished caspase 3 activity compared with quiescent early passage cultures from the same donor. Our results suggest that the progressive accumulation of T cells showing many of the hallmarks of replicative senescence during aging, chronic infection, and autoimmune disease may, in part, reflect the diminished capacity of such cells to undergo normal programmed cell death.

Establishment of persistent infection with HIV-1 abrogates the caspase-3-dependent apoptotic signaling pathway in U937 cells

Treatment of 26L cells, a subclone obtained from U937 cells, with TNF-alpha or DNA-damaging agents such as teniposide (VM26) and camptothecin (CPT) induced morphologically and biochemically typical apoptotic changes, including the activation of procaspase-3. The cells persistently infected with HIV-1 (26L/HIV), however, showed a marked resistance to VM26 and CPT, whereas they hardly lost the sensitivity to TNF-alpha. TNF-alpha-induced apoptosis of 26L/HIV cells proceeded without the increase in caspase-3 activity, indicating that signaling for apoptosis in the infected cells proceeded through an alternative caspase-3-independent pathway which could respond to TNF-alpha but not to VM26 and CPT. The evidence that p-toluenesulfonyl-l-lysine chloromethyl ketone (a trypsin-like serine protease inhibitor) blocked VM26- and CPT-induced apoptotic changes but not TNF-alpha-induced apoptosis also supported the existence of the alternative TNF-alpha-inducible pathway. The results also suggest that a TLCK-sensitive protease is involved upstream of the procaspase-3 activation process and that the protease is essential for the progress of VM26- and CPT-induced apoptosis. The similar effect of HIV-1-productive infection on the apoptosis induced by the DNA-damaging agents was also confirmed by utilizing U1 cells, which are latently HIV-1-infected U937 cells. The cells became resistant to these agents after induction of the viral production by pretreatment with PMA. These results suggest that persistent HIV-1 infection blocks an apoptotic pathway triggered by DNA damaging agents through the inhibition of the procaspase-3 activation process.