Telomere length and replicative aging in human vascular tissues

Short telomeres protect from diet‐induced atherosclerosis in apolipoprotein E‐null mice

By imposing a replicative defect in most somatic cells, gradual telomere attrition during aging is thought to progressively impair cellular function and viability and may contribute to age-related disease. Immune cells play important roles in all phases of atherosclerosis, a multifactorial disease that prevails within the elderly. Because shorter telomeres have been found in circulating blood leukocytes of human patients with advanced coronary atherosclerosis, it has been suggested that telomere shortening may predispose the organism to atheroma development. In this study, we assessed the impact of telomere attrition on atherogenesis induced by dietary cholesterol in apolipoprotein E (apoE)-deficient mice, a well-established model of experimental atherosclerosis that recapitulates important aspects of the human disease. Our study shows that late-generation mice doubly deficient in apoE and telomerase RNA experience telomere attrition and a substantial reduction of atherosclerosis compared with control mice with intact telomerase, in spite of sustained hypercholesterolemia in response to the atherogenic diet. Short telomeres impaired the proliferation of both lymphocytes and macrophages, an important step in atherosclerosis development. Therefore, telomere exhaustion resulting in replicative immunosenescence may serve as a mechanism for restricting atheroma progression.

Lymphocyte count and mortality risk in older persons. The Leiden 85-Plus Study

OBJECTIVES

To investigate whether a low peripheral blood lymphocyte count is associated with increased mortality risk in older persons and to determine whether this association could be ascribed to ill health.

DESIGN

A cohort study with a total follow-up period of 1,602 person years.

SETTING

Leiden, the Netherlands.

PARTICIPANTS

Four hundred thirty-six community-dwelling residents aged 85 and older.

MEASUREMENTS

Health status and leukocyte total and differential counts were assessed at baseline. Lymphocyte subsets were measured with a fluorescence-activated cell sorter. Age- and sex-adjusted mortality risks were estimated using Cox proportional hazard regression analysis.

RESULTS

There was no association between lymphocyte count and mortality in persons with ill health (mortality risk lowest vs highest quartile=1.16; 95% confidence interval (CI)=0.85-1.58, P=.35), but mortality was dependent on lymphocyte count if disease was excluded (mortality risk lowest vs highest quartile=2.14; 95% CI=1.08-4.23, P=.03). A similar increase in mortality risk was found when the cluster designation (CD)4+, CD8+, and CD16+ lymphocyte subsets were analyzed. Within individuals, low values of the lymphocyte subsets were related and there was no compensatory increase in CD16+ lymphocyte counts. A low lymphocyte count was not associated with specific causes of death.

CONCLUSION

A low lymphocyte count was associated with an increased mortality risk in older persons without apparent disease. This association was not only found for the total lymphocyte count but also for the CD4+, CD8+, and CD16+ lymphocyte subset counts.

Telomere length and telomerase activity during expansion and differentiation of human mesenchymal stem cells and chondrocytes

Chondrocyte ex vivo expansion currently performed to replace damaged articular surfaces is associated with a loss of telomeric repeats similar to decades of aging in vivo. This might affect the incidence or time of onset of age-related disorders within transplanted cells or tissues. This study examined whether more immature progenitor cells, such as mesenchymal stem cells (MSC), which can be expanded and subsequently differentiated into chondrocytes is advantageous regarding telomere-length related limitations of expansion protocols. Primary chondrocytes and bone-marrow-derived MSC were isolated from 12 donors, expanded separately to 4 x 10(6) cells, and (re-)differentiated as three-dimensional chondrogenic spheroids. Cells were collected during expansion, after three-dimensional culturing and chondrogenic differentiation, and sequential analyses of telomere length and telomerase activity were performed. Surprisingly, telomeres of expanded MSC were significantly shorter than those from expanded chondrocytes from the same donor (11.4+/-2.5 vs. 13.4+/-2.2 kb) and tended to remain shorter after differentiation in chondrogenic spheroids (11.9+/-1.8 vs. 13.0+/- kb). While telomere lengths in native chondrocytes and MSC were not related to the age of the donor, significant negative correlations with age were observed in expanded (136 bp/year), three-dimensionally reconstituted (188 bp/year), and redifferentiated (229 bp/year) chondrocytes. Low levels of telomerase activity were found in MSC and chondrocytes during expansion and after (re-)differentiation to chondrogenic spheroids. In terms of replicative potential, as determined by telomere length, ex vivo expansion followed by chondrogenic differentiation of MSC did not provide a benefit compared to the expansion of adult chondrocytes. However, accelerated telomere shortening with age during expansion and redifferentiation argues for an "age phenotype" in chondrocytes as opposed to MSC and suggests an advantage for the use of MSC especially in older individuals and protocols requiring extensive expansion

From Weismann's theory to present day gerontology 1889-2003

From Weismann's theory to present day gerontology--Weismann's theory was based on the concept that through natural selection the division potential of somatic cells become finite thus limiting the regeneration of the soma and the life span of the organism. Indeed, the somatic cells of some animals have a finite division potential but what became apparent is that the implications for aging are more complex. Experiments showed that at each cell division the genetic information received by each daughter cell differs; cells are this way progressively modified through division creating a functional drift that is responsible in part for the continuous modifications going on in the organism from its very beginning to its extinction. Comparative biology showed that the finite or the infinite division potential of somatic cells has a complex connotation with developmental characteristics of the respective organism with implications for longevity that are far from being understood.

Aortic stiffness and carotid intima-media thickness: two independent markers of subclinical vascular damage in young adults?

BACKGROUND

Previous reports have shown that carotid intima-media thickness (CIMT) and arterial stiffness are strong predictors of subsequent cardiovascular disease (CVD) morbidity and mortality, and are well related to an unfavourable cardiovascular risk profile in middle-aged and older subjects. These similarities suggest that arterial stiffness may play a role in the development of atherosclerosis or vice versa. However, studies show conflicting results and are limited to elderly subjects. To study this issue further, we evaluated the relation of arterial stiffness to subclinical atherosclerosis in 524 healthy young adults, aged 27-30 years.

METHODS AND RESULTS

Aortic stiffness was assessed using pulse wave velocity (PWV) and CIMT was used as measure of subclinical atherosclerosis. The positive crude correlation between for mean arterial pressure adjusted PWV and CIMT (Pearson's correlation coefficient: 0.11; P=0.016) attenuated after adjustment for common determinants of both measurements like gender and age (partial correlation coefficient: 0.03; P=0.512). Furthermore, multivariate linear regression models showed that male gender, age and blood pressure were independent determinants of both CIMT and PWV while body mass index and LDL-cholesterol were independent determinants of CIMT only.

CONCLUSIONS

These observations suggest that in healthy young adults arterial stiffness and CIMT reflect two separate entities of vascular damage.

Molecular mechanisms of replicative senescence in endothelial cells

As human somatic cells age, they stop replicating and enter an irreversible state of growth arrest known as replicative senescence. Senescent cells are viable, metabolically active, and display altered gene and protein expression compared to proliferating cells. Endothelial cells, both in vitro and in vivo, are known to undergo senescence. As endothelial cells are a critical component of the vasculature, senescence of these cells can have a significant impact of vascular integrity, function, and overall homeostasis. This review will summarize recent work to understand the molecular mechanisms of endothelial cell senescence and the resulting alterations in gene/protein expression in these cells. Endothelial cell senescence will then be discussed in the context of disease development with a focus on atherosclerosis, an important age-associated disease of the vasculature.

T-cell homeostasis in humans with thymic hypoplasia due to chromosome 22q11.2 deletion syndrome

Patients with chromosome 22q11.2 deletion syndrome (DiGeorge syndrome/velocardiofacial syndrome) typically exhibit thymic hypoplasia, conotruncal cardiac defects, and hypoparathyroidism. The immunodeficiency that results from the thymic hypoplasia has been extensively described and consists primarily of T-cell lymphopenia. A curious feature of the T-cell lymphopenia is that the age-related rate of decline of T-cell numbers is slower in patients than controls. This leads to T-cell numbers in adulthood that are minimally decreased compared with controls. This suggests that homeostatic mechanisms might be acting to preserve the peripheral blood T-cell numbers in patients. We characterized changes in CD4/CD45RA and CD4/CD45RO T-cell populations in patients and controls of various ages and determined T-cell recombination excision circles and telomere length within the CD4/CD45RA population. Patients had evidence of accelerated conversion of naive to memory cells and had evidence of more extensive replicative history within the CD4/CD45RA compartment compared with controls. Oligoclonal T-cell receptor (TCR) Vbeta families and missing Vbeta families were seen more often in patients than controls. These data are consistent with homeostatic proliferation of T cells in patients with limited T-cell production due to thymic hypoplasia.

Invited review: Theories of aging

Several factors (the lengthening of the average and, to a lesser extent, of the maximum human life span; the increase in percentage of elderly in the population and in the proportion of the national expenditure utilized by the elderly) have stimulated and continue to expand the study of aging. Recently, the view of aging as an extremely complex multifactorial process has replaced the earlier search for a distinct cause such as a single gene or the decline of a key body system. This minireview keeps in mind the multiplicity of mechanisms regulating aging; examines them at the molecular, cellular, and systemic levels; and explores the possibility of interactions at these three levels. The heterogeneity of the aging phenotype among individuals of the same species and differences in longevity among species underline the contribution of both genetic and environmental factors in shaping the life span. Thus, the presence of several trajectories of the life span, from incidence of disease and disability to absence of pathology and persistence of function, suggest that it is possible to experimentally (e.g., by calorie restriction) prolong functional plasticity and life span. In this minireview, several theories are identified only briefly; a few (evolutionary, gene regulation, cellular senescence, free radical, and neuro-endocrineimmuno theories) are discussed in more detail, at molecular, cellular, and systemic levels.

The role of replicative senescence in chronic allograft nephropathy

Strong evidence suggests that replicative senescence is involved in vivo because senescent cells have been detected in human tissues associated with physiological and pathological aging processes. Chronic allograft nephropathy (CAN) appears to be a major determinant of long-term survival in kidney transplantation. Several mechanisms are potentially involved; the aim of this study was to assess the impact of replicative senescence in CAN. Replicative senescent cells were detected on renal tissue cryosection using expression of a specific marker, senescence-associated beta-galactosidase (SA-beta-Gal) at pH 6. A total of 80 frozen renal samples (67 cases of CAN and 13 controls) were studied. To validate this marker, we measured in situ telomere length in cells expressing or not expressing SA-beta-Gal using a validated quantitative fluorescence in situ hybridization technique. The presence of senescent cells was correlated with clinicopathologic data. Telomere length was significantly lower in cells expressing SA-beta-Gal than in cells that did not. Replicative senescence was present in 45 out of 67 (67%) biopsy specimens and was significantly associated with the severity of CAN. No correlation with the notion of a previous episode of acute tubular necrosis, acute rejection, extrarenal epuration, duration of cold ischemia, and the delay between transplantation and biopsy was observed. However, the age of the donor, but not that of the recipient, was correlated with the occurrence of senescent cells. These results suggest that replicative senescence is a mechanism that might be involved in the development of CAN. The age of the donor appears to be the major determinant factor in replicative senescence.

Telomere shortening impairs organ regeneration by inhibiting cell cycle re-entry of a subpopulation of cells

Telomere shortening limits the regenerative capacity of primary cells in vitro by inducing cellular senescence characterized by a permanent growth arrest of cells with critically short telomeres. To test whether this in vitro model of cellular senescence applies to impaired organ regeneration induced by telomere shortening in vivo, we monitored liver regeneration after partial hepatectomy in telomerase-deficient mice. Our study shows that telomere shortening is heterogeneous at the cellular level and inhibits a subpopulation of cells with critically short telomeres from entering the cell cycle. This subpopulation of cells with impaired proliferative capacity shows senescence-associated beta-galactosidase activity, while organ regeneration is accomplished by cells with sufficient telomere reserves that are capable of additional rounds of cell division. This study provides experimental evidence for the existence of an in vivo process of cellular senescence induced by critical telomere shortening that has functional impact on organ regeneration.

Aging, progenitor cell exhaustion, and atherosclerosis

BACKGROUND

Atherosclerosis is largely attributed to chronic vascular injury, as occurs with excess cholesterol; however, the effect of concomitant vascular aging remains unexplained. We hypothesize that the effect of time in atherosclerosis progression is related to obsolescence of endogenous progenitor cells that normally repair and rejuvenate the arteries.

METHODS AND RESULTS

Here we show that chronic treatment with bone marrow-derived progenitor cells from young nonatherosclerotic ApoE-/- mice prevents atherosclerosis progression in ApoE-/- recipients despite persistent hypercholesterolemia. In contrast, treatment with bone marrow cells from older ApoE-/- mice with atherosclerosis is much less effective. Cells with vascular progenitor potential are decreased in the bone marrow of aging ApoE-/- mice, but cells injected from donor mice engraft on recipient arteries in areas at risk for atherosclerotic injury.

CONCLUSIONS

Our data indicate that progressive progenitor cell deficits may contribute to the development of atherosclerosis.

Interleukin-1 receptor antagonist (IL-1RN) genotype modulates the replicative capacity of human endothelial cells

Endothelial cells (ECs) undergo a finite number of cell divisions before growth arrest or replicative senescence, modulated in part by the proinflammatory cytokine, interleukin-1 (IL-1). IL-1 and its family members are expressed in human atherosclerotic vessels, mainly in the endothelium. EC replicative senescence and IL-1 have been associated with atherosclerosis. Genetic variants at the IL-1 locus have been associated with a variety of coronary phenotypes. In this study, we examined the relationship between the interleukin-1 receptor antagonist variable number tandem repeat allele 2 (IL-1RN*2*2) and EC replicative capacity. A significant decrease in EC cumulative population doublings (CPDs) was associated with the rare allele (IL-1RN*2*2) at IL-1RN, 8.56+/-0.97 (n=7) versus 13.14+/-1.00 (IL-1RN*1*1, n=20), P=0.0118. Proliferation of IL-1RN*2*2 ECs detected by Ki67 expression was also significantly reduced particularly at later passage, passage 6: 21.76+/-0.93% (n=6) versus 48.10+/-8.81% (IL-1RN*1*1, n=7) (P=0.0323) and passage 8: 22.48+/-3.08% (n=6) versus 42.29+/-3.06% (IL-1RN*1*1, n=7) (P=0.0028). IL-1RN*2 carriage was associated with increased numbers of senescent ECs. Basal apoptosis, telomerase activity, and telomere length were not different with respect to IL-1RN genotype. Addition of exogenous IL-1ra (1 ng/mL) increased CPDs in a number of human umbilical vein endothelial cell cultures and increased proliferating cells from 12.11+/-1.21% to 27.82+/-2.82% (P=0.0216, IL-1RN*2*2, passage 8, n=2). These data suggest genetic control of EC proliferation and life span by the IL-1 locus and imply that IL-1ra may have a function connected with EC growth.

Equine telomeres and telomerase in cellular immortalisation and ageing

To determine the role of telomeres in cellular ageing in equids, we analysed telomere lengths in peripheral blood derived DNA samples from a panel of donkeys (Equus asinus) ranging from 2 to 30 years of age. The average telomere lengths ranged from 7 to 21 kbp and a statistically significant inverse correlation between telomere lengths and donor age was demonstrated. Similarly, telomere lengths in primary fibroblasts isolated from a horse (Equus equus) demonstrated telomeric loss with in vitro ageing when cultured to senescence. We extended this study to evaluate activity of the enzyme telomerase in various equine cell cultures, normal equine tissues and equine benign tumour samples. Initially a panel of equine immortalised and primary cell cultures were evaluated for telomerase activity using a standard telomere repeat amplification protocol (TRAP) assay. High levels of telomerase activity were detected in equine immortalised cells with no activity evident in primary cell cultures. Similarly, no telomerase activity could be detected in normal equine tissues or equine benign tumour samples of the sarcoid or papilloma type. We conclude that telomere attrition may contribute to ageing in equids. However, it would appear that telomerase does not play a major role in the development of the most common benign tumours of the horse.

Telomeres in the chicken: genome stability and chromosome ends

Telomeres are the complex nucleoprotein structures at the termini of linear chromosomes. Telomeric DNA consists of a highly conserved hexanucleotide arranged in tandem repeats. Telomerase, a ribonucleoprotein of the reverse transcriptase family, specifies the sequence of telomeric DNA and maintains telomere array length. Numerous studies in model organisms established the significance of telomere structure and function in regulating genome stability, cellular aging, and oncogenesis. Our overall research objectives are to understand the organization of the telomere arrays in chicken in the context of the unusual organization and specialized features of this higher vertebrate genome (which include a compact genome, numerous microchromosomes, and high recombination rate) and to elucidate the role telomeres play in genome stability impacting cell function and life span. Recent studies found that the chicken genome contains three overlapping size classes of telomere arrays that differ in location and age-related stability: Class I 0.5 to 10 kb, Class II 10 to 40 kb, and Class III 40 kb to 2 Mb. Some notable features of chicken telomere biology are that the chicken genome contains ten times more telomeric DNA than the human genome and the Class III telomere arrays are the largest described for any vertebrate species. In vivo, chicken telomeres (Class II) shorten in an age-related fashion and telomerase activity is high in early stage embryos and developing organs but down-regulates during late embryogenesis or postnatally in most somatic tissues. In vitro, chicken cells down-regulate telomerase activity unless transformed. Knowledge of chicken telomere biology contributes information relevant to present and future biotechnology applications of chickens in vivo and chicken cells in vitro.

Human arteries engineered in vitro

There is a pressing need to develop methods to engineer small-calibre arteries for bypass surgery. We hypothesized that the rate-limiting step that has thwarted previous attempts to engineer such vessels from non-neonatal tissues is the limited proliferative capacity of smooth muscle cells (SMCs), which are the main cellular component of these vessels. Ectopic expression of the human telomerase reverse transcriptase subunit (hTERT) has been shown recently to extend the lifespan of certain human cells. We therefore introduced hTERT into human SMCs and found that the resulting cells proliferated far beyond their normal lifespan but retained characteristics of normal control SMCs. Importantly, using these non-neonatal SMCs, we were able to engineer mechanically robust human vessels, a crucial step towards creating arteries of clinical value for bypass surgery.

White cell telomere length and risk of premature myocardial infarction

OBJECTIVE

Biological age may be distinct from chronological age and contribute to the pathogenesis of age-related diseases. Mean telomeres lengths provide an assessment of biological age with shorter telomeres, indicating increased biological age. We investigated whether subjects with premature myocardial infarction (MI) had shorter leukocyte telomeres.

METHODS AND RESULTS

Mean terminal restriction fragment (TRF) length, a measure of average telomere size, was compared in leukocyte DNA of 203 cases with a premature MI (<50 years) and 180 controls. Age- and sex-adjusted mean TRF length of cases was significantly shorter than that of controls (difference 299.7+/-69.3 base pairs, P<0.0001) and on average equivalent to controls 11.3 years older. The difference in mean TRF length between cases and controls was not accounted for by other coronary risk factors. Compared with subjects in the highest quartile for telomere length, the risk of myocardial infarction was increased between 2.8- and 3.2-fold (P<0.0001) in subjects with shorter than average telomeres.

CONCLUSIONS

The findings support the concept that biological age may play a role in the etiology of coronary heart disease and have potentially important implications for our understanding of its genetic etiology, pathogenesis, and variable age of onset.

Critical periods in human growth and their relationship to diseases of aging

It has long been recognized that there are "critical periods" during mammalian development when exposure to specific environmental stimuli are required in order to elicit the normal development of particular anatomical structures or their normal functioning. The responses of the organism to these stimuli depend on a specific level of anatomical maturation and a state of rapid anatomical and/or functional change. This discussion of critical periods in growth is not confined to the classic definition of a narrow time frame of development during which a particular environmental threshold or limit must exist for normal growth and function to ensue. Using both auxological and epidemiological approaches, we suggest a lifespan perspective which encompasses accumulating and interacting risks that are manifest from prenatal life onward. By understanding the process of growth development, and by scrutinizing the growth process, early variations that lead to later disease can be identified. Here we review a significant amount of the evidence that links exposure during growth to later morbidity and mortality. The fetus appears to respond to insults during the prenatal period through the process of "programming," which has short-term survival advantages but may have a long-term disadvantage in that it is associated with cardiovascular disease, hypertension, type II diabetes, and later obesity. Low birth weight combined with rapid postnatal growth during infancy also appears to be associated, for instance, with later childhood and adult sequelae in terms of glucose tolerance and obesity. Independent of birth weight, the timing of adiposity rebound during mid-childhood also predicts later obesity. The timing, magnitude, and duration of adolescent growth and maturationare associated with critical body composition changes, including the normal acquisition of body fat and bone mineralization. In particular, the acquisition of appropriate peak bone mass is critical in determining the later risk of osteoporosis. A putative causal mechanism linking early growth variation to later chronic disease risk through telomeric attrition is discussed. The obligatory loss of telomeric DNA with each cell division serves as a mitotic clock and marks the rate of growth and repair processes in the cell. Although much more work is required, existing studies support the notion that telomere shortening is not only a clock of cellular division, but also marks relative growth rate, as well as contributing to common degenerative processes of aging through its impact on cellular senescence.

[Telomerase activity in peripheral blood leukocytes from patients with essential hypertension]

BACKGROUND AND OBJECTIVE

Primary or secondary activation of the immune mechanisms that lead to proliferation and dysfunction of specific cellular groups appears to be involved in the pathogenesis and complications of essential hypertension. In view of the evidence that, on one hand, telomeric length determines the replicative capacity and life span of cells and, on the other hand, idiopathic hypertensive patients have peripheral white cell replicative disorders, we decided to investigate the relationship between the influence of telomerase activity in peripheral leukocytes as an indirect marker of telomeric length and the presence of arterial hypertension.

PATIENTS AND METHOD

Telomerase activity in peripheral white blood cells was measured in healthy individuals, in effectively treated hypertensive patients and in a non-well controlled hypertensive group. White blood cells were separated through a density gradient, then lysed and their DNA amplified by a polimerase chain reaction (PCR). Telomerase activity was determined with an ELISA specific kit.

RESULTS

The white blood cell count was higher in the hypertensive than the control group (p < 0.05). Telomerase activity was positive in all three groups but higher in patients under 45 year-old with bad controlled hypertension as compared with healthy individuals and patients under 45 year-old with well controlled hypertension (p < 0.05); in the latter group, telomerase activity was significantly lower than in the other groups (p < 0.05).

CONCLUSIONS

Our results indicate that there exists a relationship between telomerase activity in peripheral leukocytes, the proliferation of these white blood cells and the presence of essential arterial hypertension.

An analysis of replicative senescence in dermal fibroblasts derived from chronic leg wounds predicts that telomerase therapy would fail to reverse their disease-specific cellular and proteolytic phenotype

The accumulation of senescent fibroblasts within tissues has been suggested to play an important role in mediating impaired dermal wound healing, which is a major clinical problem in the aged population. The concept that replicative senescence in wound fibroblasts results in reduced proliferation and the failure of refractory wounds to respond to treatment has therefore been proposed. However, in the chronic wounds of aged patients the precise relationship between the observed alteration in cellular responses with aging and replicative senescence remains to be determined. Using assays to assess cellular proliferation, senescence-associated staining beta-galactosidase, telomere length, and extracellular matrix reorganizational ability, chronic wound fibroblasts demonstrated no evidence of senescence. Furthermore, analysis of in vitro senesced fibroblasts demonstrated cellular responses that were distinct and, in many cases, diametrically opposed from those exhibited by chronic wound fibroblasts. Forced expression of telomerase within senescent fibroblasts reversed the senescent cellular phenotype, inhibiting extracellular matrix reorganizational ability, attachment, and matrix metalloproteinase production and thus produced cells with impaired key wound healing properties. It would appear therefore that the distinct phenotype of chronic wound fibroblasts is not simply due to the aging process, mediated through replicative senescence, but instead reflects disease-specific cellular alterations of the fibroblasts themselves.

Telomerase: a potential diagnostic and therapeutic tool in canine oncology

In recent years there has been considerable interest in telomerase as a target for therapeutic intervention in oncology. This largely stems from the vast number of studies that have demonstrated expression and activity of the enzyme telomerase in the majority of human cancer tissues with little or no activity detectable in normal somatic tissues. These studies have led to an interest in the role of telomerase in cancers associated with domesticated species, in particular tumors that affect dogs. This article reviews the biology of telomerase and the biological significance of telomerase activity in canine tumors and discusses the clinical implications of telomerase expression in canine cancers with regard to therapeutics and diagnostics.

Induction of telomerase activity and maintenance of telomere length in virus-specific effector and memory CD8+ T cells

Acute viral infections induce extensive proliferation and differentiation of virus-specific CD8+ T cells. One mechanism reported to regulate the proliferative capacity of activated lymphocytes is mediated by the effect of telomerase in maintaining the length of telomeres in proliferating cells. We examined the regulation of telomerase activity and telomere length in naive CD8+ T cells and in virus-specific CD8+ T cells isolated from mice infected with lymphocytic choriomeningitis virus. These studies reveal that, compared with naive CD8+ T cells, which express little or no telomerase activity, Ag-specific effector and long-lived memory CD8+ T cells express high levels of telomerase activity. Despite the extensive clonal expansion that occurs during acute lymphocytic choriomeningitis virus infection, telomere length is maintained in both effector and memory CD8+ T cells. These results suggest that induction of telomerase activity in Ag-specific effector and memory CD8+ T cells is important for the extensive clonal expansion of both primary and secondary effector cells and for the maintenance and longevity of the memory CD8+ T cell population.

Granulocyte colony-stimulating factor administration upregulates telomerase activity in CD34+ haematopoietic cells and may prevent telomere attrition after chemotherapy

Hematopoietic reconstitution could be associated with premature ageing of the transplanted cells and a high frequency of myelodysplastic syndrome and secondary leukaemia. Telomere length decreases with cell divisions and age, and at a crucial length it is associated with chromosomal instability and cell senescence. Telomerase is a reverse transcriptase enzyme that adds nucleotides to chromosomal ends. Most somatic cells lack telomerase activity yet haematopoietic stem cells retain low levels of telomerase. Some studies have found that chemotherapy and stem cell transplantation lead to the accelerated shortening of telomere length. As granulocyte colony-stimulating factor (G-CSF) is routinely used in the mobilization of stem cells for transplantation, we evaluated its effects on telomerase activity and regulation, and on telomere dynamics, in normal donors and selected lymphoma patients. Administration of G-CSF increased telomerase activity in CD34+ haematopoietic cells compared with controls. In marrow-derived CD34+ cells, telomerase activity increased sevenfold, compared with a 14-fold increase in peripheral-blood-mobilized CD34+ cells. A parallel increase in the expression of human telomerase enzyme reverse transcriptase RNA and protein kinase C alpha occurred. In addition, G-CSF administration to five lymphoma patients after consecutive courses of CHOP (cyclophosphamide, doxorubicin, vincristine, prednisone) chemotherapy, resulted in telomere length preservation or elongation, as opposed to marked attrition in patients who did not receive growth factors. We conclude that the in vivo administration of G-CSF prevents or attenuates telomere attrition associated with chemotherapy administration. This attenuation may contribute to the preservation of telomere integrity inG-CSF-primed transplanted stem cells.

Endothelial cells maintain a reduced redox environment even as mitochondrial function declines

Human umbilical vein endothelial cells (HUVECs) are an endothelial model of replicative senescence. Oxidative stress, possibly due to dysfunctional mitochondria, is believed to play a key role in replicative senescence and atherosclerosis, an age-related vascular disease. In this study, we determined the effect of cell division on genomic instability, mitochondrial function, and redox status in HUVECs that were able to replicate for approximately 60 cumulative population doublings (CPD). After 20 CPD, the nuclear genome deteriorated and the protein content of the cell population increased. This indicated an increase in cell size, which was accompanied by an increase in oxygen consumption, ATP production, and mitochondrial genome copy number and approximately 10% increase in mitochondrial mass. The antioxidant capacity increased, as seen by an increase in reduced glutathione, glutathione peroxidase, GSSG reductase, and glucose-6-phosphate dehydrogenase. However, by CPD 52, the latter two enzymes decreased, as well as the ratio of mitochondrial-to-nuclear genome copies, the mitochondrial mass, and the oxygen consumption per milligram of protein. Our results signify that HUVECs maintain a highly reducing (GSH) environment as they replicate despite genomic instability and loss of mitochondrial function.

Cancer and aging: a model for the cancer promoting effects of the aging stroma

The incidence of cancer rises exponentially with age in humans and many other mammalian species. Malignant tumors are caused by an accumulation of oncogenic mutations. In addition, malignant tumorigenesis requires a permissive tissue environment in which mutant cells can survive, proliferate, and express their neoplastic phenotype. We propose that the age-related increase in cancer results from a synergy between the accumulation of mutations and age-related, pro-oncogenic changes in the tissue milieu. Most age-related cancers derive from the epithelial cells of renewable tissues. An important element of epithelial tissues is the stroma, the sub-epithelial layer composed of extracellular matrix and several cell types. The stroma is maintained, remodeled and repaired by resident fibroblasts, supports and instructs the epithelium, and is essential for epithelial function. One change that occurs in tissues during aging is the accumulation of epithelial cells and fibroblasts that have undergone cellular senescence. Cellular senescence irreversibly arrests proliferation in response to damage or stimuli that put cells at risk for neoplastic transformation. Senescent cells secrete factors that can disrupt tissue architecture and/or stimulate nearby cells to proliferate. We therefore speculate that their presence may create a pro-oncogenic tissue environment that synergizes with oncogenic mutations to drive the rise in cancer incidence with age. Recent evidence lends support to this idea, and suggests that senescent stromal fibroblasts may be particularly adept at creating a tissue environment that can promote the development of age-related epithelial cancers.

Comparative analysis of telomere lengths and erosion with age in human epidermis and lingual epithelium

We investigated progressive telomere shortening in normal human epidermis and lingual epithelium during aging, and attempted, in particular, to ascertain whether the telomere shortening that accompanies aging occurs at the same rate in different tissues. We studied telomeric DNA integrity, and estimated annual telomere loss, in 52 specimens of epidermis and 48 specimens of lingual epithelium collected at autopsy from subjects who had died at ages between 0 and 101 y. Most of the DNA samples were measured twice by southern blot hybridization. In addition, the correlation between telomere lengths in the two types of tissues was examined. The telomere reduction rates in epidermis and lingual epithelium were 36 bp and 30 bp per y, respectively, and these were significantly different. The rates obtained by the second measurements in epidermis and lingual epithelium were 39 and 32 bp per y, respectively, and these were also significantly different. The mean telomere lengths in the epidermis of eight neonates and the lingual epithelium of seven neonates were 13.2+/-1.0 and 13.8+/-1.0 kb, respectively. Comparison of telomere lengths in the two tissues for 41 paired samples showed that the mean telomere length in the epidermis (10.7+/-2.3 kb) was less than that in the lingual epithelium (12.4+/-2.5 kb); however, statistical analysis revealed a very significant relationship between epidermal and lingual epithelial telomere length (r=0.842, p<0.0001). These results indicate that the telomeres in epidermis and lingual epithelium are characterized by tissue-specific loss rates.

Replicative aging of human articular chondrocytes during ex vivo expansion

OBJECTIVE

To investigate the contribution of clinical ex vivo expansion protocols to replicative aging of human chondrocytes.

METHODS

Primary human chondrocytes were cultured as monolayers after isolation from 7 articular cartilage specimens. Cells were passaged corresponding to 12-19 cell population doublings (cpd). Aliquots of the cells were collected from each passage and analyzed for telomere length and telomerase activity.

RESULTS

The rate of telomere shortening was heterogeneous, ranging from 147 to 431 bp/cpd (mean +/- SD 305 +/- 122). Telomerase activity was detected at various time points during passaging in 5 of 7 primary chondrocytes analyzed, but not in native human articular cartilage specimens. According to our data, an 8-10-fold ( approximately 3 cpd) ex vivo expansion of articular chondrocytes, as typically performed for transplantation procedures, leads to telomere erosion in the range of 900 bp. This is comparable with 30 years of aging based on the in vivo rate of telomere shortening of 30 bp/year recently found in chondrocytes.

CONCLUSION

If telomere shortening is an important determinant of aging in human articular cartilage, an additional telomere loss due to ex vivo expansion might affect the incidence or time of onset of age-related cartilage disorders. However, given the limited extent of expansion performed in the clinical setting to date, a significant telomere-mediated increase in the risk of malignant transformation or replicative exhaustion of the transplanted cells seems unlikely.

Replicative senescence of human fibroblasts: the role of Ras-dependent signaling and oxidative stress

Replicative senescence of human fibroblasts is a widely used cellular model for human aging. While it is clear that telomere erosion contributes to the development of replicative senescence, it is assumed that additional factors contribute to the senescent phenotype. The free radical theory of aging suggests that oxidative damage is a major cause of aging; furthermore, the expression of activated oncogenes, such as oncogenic Ras, can induce premature senescence in primary cells. The functional relation between the various inducers of senescence is not known. The present study was guided by the hypothesis that constitutive activation of normal, unmutated Ras may contribute to senescence-induced growth arrest in senescent human fibroblasts. When various branches of Ras-dependent signaling were investigated, constitutive activation of the Ras/Raf/MEK/ERK pathway was not observed. To evaluate the role of oxidative stress for the senescent phenotype, we also investigated stress-related protein kinases. While we found no evidence for alterations in the activity of p38, we could detect an increased activity of Jun kinase in senescent fibroblasts. We also found higher levels of reactive oxygen species (ROS) in senescent fibroblasts compared to their younger counterparts. The accumulation of ROS in senescent cells may be related to the constitutive activation of Jun kinase.

Longitudinal studies of telomere length in feline blood cells: implications for hematopoietic stem cell turnover in vivo

OBJECTIVE

To address questions about stem cell turnover in relation to telomere length dynamics, we analyzed telomere length in serial blood samples from cats.

MATERIALS AND METHODS

Lymphocytes and granulocytes from two newborn kittens, a 2-year-old cat, a 10-year-old recipient of a double autologous stem cell transplant, and a 10-year-old control animal were analyzed by fluorescence in situ hybridization and flow cytometry at 2-week intervals over a 1-year period.

RESULTS

At study onset, long telomeres were found in granulocytes and lymphocytes from the two kittens (mean +/- SD: 70.2 +/- 3.1 and 72.5 +/- 3.1 telomere fluorescence units [TFU], respectively) compared with the 2-year-old cat (55.6 +/- 2.5 and 64.1 +/- 4.3 TFU, respectively) and the two adult animals (49.6 +/- 1.5 and 45.4 +/- 0.8 TFU, respectively). The rate of telomere shortening in both granulocytes and lymphocytes was most rapid in the kittens (slope: -16.7 +/- 1.4 and -15.6 +/- 0.2 TFU/year, respectively). As in humans, telomere shortening with age was more rapid in lymphocytes than in granulocytes. An average rate of telomere attrition of -0.52 +/- 0.03 TFU per cell division was calculated for cultured lymphocytes from the two kittens, approximately 5-fold higher than the rate observed in human cells.

CONCLUSIONS

The average telomere length in cats is 5- to 10-fold longer than in humans, but the rate of telomere shortening is much higher both in vivo and in vitro. These observations are compatible with similar stem cell kinetics in both species.

Role of telomere in endothelial dysfunction in atherosclerosis

PURPOSE OF REVIEW

Telomeres consist of repeats of G-rich sequence at the end of chromosomes. These DNA repeats are synthesized by enzymatic activity associated with an RNA protein complex called telomerase. In most somatic cells, telomerase activity is insufficient, and telomere length decreases with increasing cell division, resulting in an irreversible cell growth arrest, termed cellular senescence. Cellular senescence is associated with an array of phenotypic changes suggestive of aging. Until recently, cellular senescence has largely been studied as an in-vitro phenomenon; however, there is accumulating evidence that indicates a critical role of telomere function in the pathogenesis of human atherosclerosis. This review attempts to summarize recent work in vascular biology that supports the "telomere hypothesis". We discuss the possible relevance of telomere function to vascular aging and the therapeutic potential of telomere manipulation.

RECENT FINDINGS

It has been reported that many of the changes in senescent vascular cell behavior are consistent with known changes seen in age-related vascular diseases. Introduction of telomere malfunction has been shown to lead to endothelial dysfunction that promotes atherogenesis, whereas telomere lengthening extends cell lifespan and protects against endothelial dysfunction associated with senescence. Indeed, recent studies have demonstrated that telomere attrition and cellular senescence occur in the blood vessels and are associated with human atherosclerosis.

SUMMARY

Recent findings suggest that vascular cell senescence induced by telomere shortening may contribute to atherogenesis and may provide insights into a novel treatment of antisenescence to prevent atherosclerosis.

Cellular senescence and tissue aging in vivo

A long-standing controversy concerns the relevance of cellular senescence, defined and observed as a cell culture phenomenon, to tissue aging in vivo. Here the evidence on this topic is reviewed. The main conclusions are as follows. First, telomere shortening, the principal known mediator of cellular senescence, occurs in many human tissues in aging. Second, it is not clear whether this results in cellular senescence or in some other cell fate (e.g., crisis). Third, rodents probably are not appropriate experimental models for these questions, because of important differences in telomere biology between rodent cells and cells from long-lived mammals (e.g., human or bovine cells). Fourth, better and more comprehensive observations on aging human tissues are needed to answer the question of the occurrence of senescent cells in tissues, and new experimental approaches are needed to elucidate the consequences of telomere shortening in tissues in aging.

Replicative senescence of hematopoietic stem cells during serial transplantation: does telomere shortening play a role?

Hematopoietic stem cells (HSC) have a finite proliferative lifespan, based upon the limited number of times they can be serially transplanted in mice. Telomeres have been shown to shorten during the division of many normal somatic cells in humans, and the attrition of telomeres has been shown to ultimately cause replicative senescence in vitro for a number of different human cell strains. Whereas most human cell types have little to no detectable levels of telomerase activity, hematopoietic cells, including HSC, express low to moderate levels of telomerase, and yet telomeres shorten considerably during replicative aging of these cells. Here we consider the role telomerase may play in the hematopoietic system as well as the effect that over-expression of telomerase reverse transcriptase may have on the replicative capacity of hematopoietic stem cells during transplantation.

The reserve-capacity hypothesis: evolutionary origins and modern implications of the trade-off between tumor-suppression and tissue-repair

Antagonistic pleiotropy, the evolutionary theory of senescence, posits that age related somatic decline is the inevitable late-life by-product of adaptations that increase fitness in early life. That concept, coupled with recent findings in oncology and gerontology, provides the foundation for an integrative theory of vertebrate senescence that reconciles aspects of the 'accumulated damage' 'metabolic rate', and 'oxidative stress' models. We hypothesize that (1) in vertebrates, a telomeric fail-safe inhibits tumor formation by limiting cellular proliferation. (2) The same system results in the progressive degradation of tissue function with age. (3) These patterns are manifestations of an evolved antagonistic pleiotropy in which extrinsic causes of mortality favor a species-optimal balance between tumor suppression and tissue repair. (4) With that trade-off as a fundamental constraint, selection adjusts telomere lengths--longer telomeres increasing the capacity for repair, shorter telomeres increasing tumor resistance. (5) In environments where extrinsically induced mortality is frequent, selection against senescence is comparatively weak as few individuals live long enough to suffer a substantial phenotypic decline. The weaker the selection against senescence, the further the optimal balance point moves toward shorter telomeres and increased tumor suppression. The stronger the selection against senescence, the farther the optimal balance point moves toward longer telomeres, increasing the capacity for tissue repair, slowing senescence and elevating tumor risks. (6) In iteroparous organisms selection tends to co-ordinate rates of senescence between tissues, such that no one organ generally limits life-span. A subsidiary hypothesis argues that senescent decline is the combined effect of (1) uncompensated cellular attrition and (2) increasing histological entropy. Entropy increases due to a loss of the intra-tissue positional information that normally regulates cell fate and function. Informational loss is subject to positive feedback, producing the ever-accelerating pattern of senescence characteristic of iteroparous vertebrates. Though telomere erosion begins early in development, the onset of senescence should, on average, be deferred to the species-typical age of first reproduction, the balance point at which selection on this trade-off should allow exhaustion of replicative capacity to overtake some cell lines. We observe that captive-rodent breeding protocols, designed to increase reproductive output, simultaneously exert strong selection against reproductive senescence and virtually eliminate selection that would otherwise favor tumor suppression. This appears to have greatly elongated the telomeres of laboratory mice. With their telomeric failsafe effectively disabled, these animals are unreliable models of normal senescence and tumor formation. Safety tests employing these animals likely overestimate cancer risks and underestimate tissue damage and consequent accelerated senescence.

Aging and survival of cutaneous microvasculature

The growth and turnover of blood vessels in the skin is fundamental in normal development, wound repair, hair follicle cycling, tumor cell metastasis, and in many different states of cutaneous pathology. Whereas many investigations are focused on mechanisms of angiogenesis in the skin, the influence of cellular aging and replicative senescence (i.e., the inability, after a critical number of population doublings, to replicate) on microvascular remodeling events has received relatively less attention. In this article, we review the clinical and pathologic relationships associated with cutaneous vascular aging and update current knowledge of endothelial cell survival characteristics. A hypothesis is presented in which endothelial cell aging and survival are linked to molecular mechanisms controlling cell proliferation, quiescence, apoptosis, and cellular senescence. We review recent results demonstrating how activation of telomerase in human dermal microvascular endothelial cells affects their durability both in vitro and in vivo and conclude by linking these studies with current concepts involving endothelial cell precursors, control of postnatal somatic cell telomerase activity, and murine model systems.

Endothelial cell senescence in human atherosclerosis: role of telomere in endothelial dysfunction

BACKGROUND

The functional changes associated with cellular senescence may be involved in human aging and age-related vascular disorders. We have shown the important role of telomere and telomerase in vascular cell senescence in vitro. Progressive telomere shortening in vivo has been observed in the regions susceptible to atherosclerosis, implying contributions to atherogenesis. However, whether senescent vascular cells are present in the vasculature and contribute to the pathogenesis of atherosclerosis remains unclear.

METHODS AND RESULTS

Senescence-associated beta-galactosidase (beta-gal) activity was examined in the coronary arteries and the internal mammary arteries retrieved from autopsied individuals who had had ischemic heart diseases. Strong beta-gal stainings were observed in atherosclerotic lesions of the coronary arteries but not in the internal mammary arteries. An immunohistochemical analysis using anti-factor VIII antibody demonstrated that beta-gal stained cells are vascular endothelial cells. To determine whether endothelial cell senescence causes endothelial dysfunction, we induced senescence in human aortic endothelial cells (HAECs) by inhibiting telomere function and examined the expression of intercellular adhesion molecule (ICAM)-1 and endothelial nitric oxide synthase (eNOS) activity. Senescent HAECs exhibited increased ICAM-1 expression and decreased eNOS activity, both of which are alterations implicated in atherogenesis. In contrast, introduction of telomerase catalytic component significantly extended the life span and inhibited the functional alterations associated with senescence in HAECs.

CONCLUSIONS

Vascular endothelial cells with senescence-associated phenotypes are present in human atherosclerotic lesions, and endothelial cell senescence induced by telomere shortening may contribute to atherogenesis.

Cell senescence in human aging and disease

The most common causes of death and suffering, even in most underdeveloped nations, are age-related diseases. These diseases share fundamental and often unappreciated pathology at the cellular and genetic levels, through cell senescence. In cancer, enforcing cell senescence permits us to kill cancer cells without significantly harming normal cells. In other age-related diseases, cell senescence plays a direct role, and we may be able to prevent and reverse much of the pathology. While aging is attributed to "wear and tear," genetic studies show that these effects are avoidable (as is the case in germ cell lines) and occur only when cells down-regulate active (and sufficient) repair mechanisms, permitting degradation to occur. Aging occurs when cells permit accumulative damage by wear and tear, by altering their gene expression rather than vice versa. Using telomerase in laboratory settings, we can currently reset this pattern and its consequences both within cells and between cells. Doing so resets not only cell behavior but the pathological consequences within tissues comprising such cells. We can currently grow histologically young, reconstituted human skin using old human skin cells (keratinocytes and fibroblasts). Technically we could now test this approach in joints, vessels, the immune system, and other tissues. This model is consistent with all available laboratory data and known aging pathology. Within the next decade, we will be able to treat age-related diseases more effectively than ever before.

Telomere shortening with aging in human thyroid and parathyroid tissue

Progressive telomere shortening with aging was studied using normal thyroid tissue specimens from 46 human subjects aged between 0 and 98 yr and normal parathyroid tissue specimens from 21 human subjects aged between 0 and 83 yrs. There has hitherto been no information documented about telomere length in such thyroid and parathyroid tissues. Age-related shortening at rates of 91 and 92 base pairs (bp) per year, respectively, were observed. Telomere lengths of normal thyroid tissues were 16.53 +/- 1.10 (mean +/- SE), 14.31 +/- 0.80, 11.27 +/- 0.68 and 8.73 +/- 1.08 kbp for age groups less than 2, 20-50, 51-80 and more than 80 yr. Telomere lengths of normal parathyroid tissues were 15.80 +/- 1.46 (mean +/- SE), 15.36 +/- 0.86 and 10.93 +/- 0.78 kbp for age groups less than 4, 20-50 and 51-80 yr. Telomere shortening occurred after 50 yr of age in thyroid and parathyroid tissues. Human thyroid and parathyroid tissues do not seem to show the rapid reduction in telomere length early in life that was reported for some human cell types, suggesting that the rate of telomere shortening has tissue-specific characteristics.

Telomere lengths are characteristic in each human individual

BACKGROUND

A great deal of attention has been focused on telomeres in relation to cellular aging, immortality, and cancer. However, there is no simple link between telomeres and tissue turnover. We recently proposed a hypothesis that telomere shortening with aging and telomere lengths in different organs are characteristic for human individuals.

METHODS

To test this, telomere lengths were measured using DNA from cerebral cortex, myocardium, liver, renal cortex and spleen tissues obtained from human subjects ranging in age from neonates to centenarians.

RESULTS

Regression analyses demonstrated telomere reduction rates of 29-60 base pair (bp) per year in the liver, renal cortex and spleen, but no such decrease in the cerebral cortex and myocardium. Significant correlation was found between tissues within individuals, such as cerebral cortex versus (vs) myocardium, cerebral cortex vs liver, cerebral cortex vs renal cortex, myocardium vs liver, myocardium vs renal cortex, and liver vs renal cortex. In most cases, the longest telomeres were observed in the myocardium and the shortest in the liver or renal cortex.

CONCLUSIONS

Telomere lengths did not show clear correlation with tissue renewal times in vivo, but rather were characteristic for individuals.

Replicative aging, telomeres, and oxidative stress

Aging is a very complex phenomenon, both in vivo and in vitro. Free radicals and oxidative stress have been suggested for a long time to be involved in or even to be causal for the aging process. Telomeres are special structures at the end of chromosomes. They shorten during each round of replication and this has been characterized as a mitotic counting mechanism. Our experiments show that the rate of telomere shortening in vitro is modulated by oxidative stress as well as by differences in antioxidative defence capacity between cell strains. In vivo we found a strong correlation between short telomeres in blood lymphocytes and the incidence of vascular dementia. These data suggest that parameters that characterise replicative senescence in vitro offer potential for understanding of, and intervention into, the aging process in vivo.

Enhanced apoptosis in prolonged cultures of senescent porcine pulmonary artery endothelial cells

Senescent or aged endothelial cells in culture remain metabolically active after cessation of division, and are generally believed to eventually die. However, mechanisms underlying the terminal aging of cultured cells, i.e. from senescence to death, are poorly understood. Here, we report that culturing of replicative senescent endothelial cells for a prolonged period of time without passaging leads to enhanced programmed cell death or apoptosis. Senescent (passage 45) and young (passage 3) porcine pulmonary artery endothelial cells (PAEC) were cultured for 0-42 days post confluence. The cells attached to culture dishes and floating in medium were collected at 0, 7, 14, 21, 28, 35 and 42 days post confluence and were assessed for markers of apoptosis. Morphology studies showed that ratios between senescent and young cells attached to dishes declined to 45% after 42 days postconfluence. Apoptotic cells in prolonged cultures of senescent PAEC increased from 5 to 35% as determined by protein mass, DNA breakage, and caspase-3 activation. Steady state levels of Bcl-2, an anti-apoptotic protein, in senescent prolonged cultures decreased to less than 20% for all time points compared with young cells. Relative levels of Bad, a pro-apoptotic protein, in senescent cells were elevated from 60 to 130% during prolonged culturing. These results indicate that terminal cellular aging enhances apoptosis and the levels of Bcl-2/Bad may be associated with the apoptotic process in porcine lung endothelial cells.

The telomeric length and heterogeneity decrease with age in normal human oral keratinocytes

We have examined the telomere length in NHOK explanted from 28 donors between the ages of 21 and 84 years. Genomic DNA was isolated from exponentially replicating NHOK and digested with HinFI to yield terminal restriction fragments (TRF). The TRF length ranged from 4.1 to 7.0 kbp with a mean of 5.3 +/- 0.8 kbp, which was significantly shorter than that (8.9 +/- 1.0 kbp) of normal human oral fibroblasts (NHOF). The TRF length was inversely correlated to the increase of donor age in NHOK (m=-23 bp per year; r=-0.60; P<0.001). Also, the heterogeneity of TRF length in cultured NHOK decreased with increased donor age (r=-0.38, P<0.05). These data indicated that clonogenic NHOK cells had replicated in situ and showed a progressive shortening of TRF length. The short telomere length and decreased telomeric length heterogeneity in immortalized cells suggested that there is a critical minimum for cell survival.

Cell senescence and telomere shortening induced by a new series of specific G-quadruplex DNA ligands

Telomeres of human chromosomes contain a G-rich 3'-overhang that adopts an intramolecular G-quadruplex structure in vitro which blocks the catalytic reaction of telomerase. Agents that stabilize G-quadruplexes have the potential to interfere with telomere replication by blocking the elongation step catalyzed by telomerase and can therefore act as antitumor agents. We have identified by Fluorescence Resonance Energy Transfer a new series of quinoline-based G-quadruplex ligands that also exhibit potent and specific anti-telomerase activity with IC50 in the nanomolar concentration range. Long term treatment of tumor cells at subapoptotic dosage induces a delayed growth arrest that depends on the initial telomere length. This growth arrest is associated with telomere erosion and the appearance of the senescent cell phenotype (large size and expression of beta-galactosidase activity). Our data show that a G-quadruplex interacting agent is able to impair telomerase function in a tumor cell thus providing a basis for the development of new anticancer agents.

Efficacy and age-related effects of nitric oxide-releasing aspirin on experimental restenosis

Restenosis after percutaneous transluminal coronary angioplasty is caused by neointimal hyperplasia, which involves impairment of nitric oxide (NO)-dependent pathways, and may be further exacerbated by a concomitant aging process. We compared the effects of NO-releasing-aspirin (NCX-4016) and aspirin (ASA) on experimental restenosis in both adult and elderly rats. Moreover, to ascertain the efficacy of NCX-4016 during vascular aging, we fully characterized the release of bioactive NO by the drug. Sprague-Dawley rats aged 6 and 24 months were treated with NO releasing-aspirin (55 mg/kg) or ASA (30 mg/kg) for 7 days before and 21 days after standard carotid balloon injury. Histological examination and immunohistochemical double-staining were used to evaluate restenosis. Plasma nitrite and nitrate and S-nitrosothiols were determined by a chemiluminescence-based assay. Electron spin resonance was used for determining nitrosylhemoglobin. Treatment of aged rats with NCX-4016 was associated with increased bioactive NO, compared with ASA. NO aspirin, but not ASA, reduced experimental restenosis in old rats, an effect associated with reduced vascular smooth muscle cell proliferation. NCX-4016, but not ASA, was well tolerated and virtually devoid of gastric damage in either adult or old rats. Thus, impairment of NO-dependent mechanisms may be involved in the development of restenosis in old rats. We suggest that an NCX-4016 derivative could be an effective drug in reducing restenosis, especially in the presence of aging and/or gastrointestinal damage.

Effects of telomerase and viral oncogene expression on the in vitro growth of human chondrocytes

Senescent chondrocytes accumulate with aging in articular cartilage, a process that interferes with cartilage homeostasis and increases the risk of cartilage degeneration. We showed previously that chondrocyte telomere length declines with donor age, which suggests that the aging process is telomere dependent. From these results we hypothesized that telomerase should delay the onset of senescence in cultured chondrocytes. Population doubling limits (PDL) were determined for chondrocytes expressing telomerase. We found that telomerase alone did not extend PDL beyond controls that senesced after 25 population doublings. The human papillomavirus 16 oncogenes E6 and E7 were transduced into the same cell population to investigate this telomere-independent form of senescence further. Chondrocytes expressing E6 and E7 grew longer than the telomerase cDNA (hTERT) cells but still senesced at 55 population doublings. In contrast, chondrocytes expressing telomerase with E6 and E7 grew vigorously past 100 population doublings. We conclude that although telomerase is necessary for the indefinite extension of chondrocyte life span, telomere-independent senescence limits PDL in vitro and may play a role in the age-related accumulation of senescent chondrocytes in vivo.

Telomeres, aging and cancer: in search of a happy ending

Telomeres are distinctive structures, composed of a repetitive DNA sequence and associated proteins, that cap the ends of linear chromosomes. Telomeres are essential for maintaining the integrity and stability of eukaryotic genomes. In addition, under some circumstances, telomeres can influence cellular gene expression. In mammals, the length, structure, and function of telomeres have been proposed to contribute to cellular and organismal phenotypes associated with cancer and aging. Here, we discuss what is known about the basis for the links between telomeres, aging and cancer, and some of the known and proposed consequences of telomere dysfunction and maintenance for mammalian cells and organisms.

Immunosenescence of macrophages: reduced MHC class II gene expression

In order to determine the effect of aging on macrophages, we produced bone marrow-derived macrophages in vitro from young and aged mice. We analyzed the effect of aging on the genomic expression of macrophages in these conditions, without the influence of other cell types that may be affected by aging. Macrophages from young and aged mice were present in similar numbers and showed an identical degree of differentiation, cell size, DNA content and cell surface markers. After incubation with interferon-gamma (IFN-gamma), the expression at the cell surface of the MHC class II gene IA complex product and the levels of intracellular IAbeta protein and mRNA were lower in aged macrophages. Moreover, the transcription of IAbeta gene was impaired in aged macrophages. The amount of transcription factors that bound to the W and X boxes, but not to the Y box of the IAbeta promoter gene were lower in aged macrophages. Similar levels of CIITA mRNA were found after IFN-gamma treatment of both young and aged macrophages. This shows that neither the initial cascade that starts after the interaction of IFN-gamma with the receptor, nor the second signals involved in the expression of CIITA, are impaired in aged macrophages. These data could explain, at least in part, the impaired immune response associated to senescence.

Telomerase: diagnostics, cancer therapeutics and tissue engineering

The enzyme telomerase has a key role in controlling the lifespan of human cells. It is absent from most somatic tissues but is reactivated in more than 85% of cancers, making the enzyme ideal as a marker of cancer cells and as a therapeutic target. In the context of normal human cells, the enzyme can extend cellular lifespan without causing cancer-associated changes or altering phenotypic properties. This capability could solve a major obstacle in the use of normal human cells for tissue engineering, that is, the induction of cellular senescence.

Age dependent aneuploidy and telomere length of the human vascular endothelium

RATIONALE

Aneuploidy and telomere length are two major parameters that have been associated with cellular senescence in vitro. In order to explore the role of aneuploidy and telomere length in aging of the human vasculature, we studied these two parameters in direct preparations of endothelial cells of the human abdominal aorta.

METHODS

Using fluorescent in situ hybridization on 'touch prep' slides, we evaluated aneuploidy of two autosomes (chromosomes 6 and 16) and sex chromosomes in non cultured endothelial cells of the abdominal aorta as a function of the donor's age.

RESULTS

We found that the frequency of aneuploidy of vascular endothelial cells significantly increased with age. This was expressed by age-dependent tetrasomy (r(s)=0.56, P=0.006 for chromosome 6; and r(s)=0.54, P=0.008 for chromosome 16), and age dependent loss of the Y chromosome (r(s)=0.85, P=0.0003). In addition, we found that telomere length was inversely correlated with age (r=-0.38, P=0.008). DATA INTERPRETATION: These findings suggest that indicators of cellular senescence, earlier observed in vitro, are also expressed in the human vascular endothelium in vivo. Aneuploidy and telomere attrition might thus play a role in the aging of the human vasculature.