Telomere length and replicative aging in human vascular tissues

Angiotensin II Induces Premature Senescence of Vascular Smooth Muscle Cells and Accelerates the Development of Atherosclerosis via a p21-Dependent Pathway

BACKGROUND

Angiotensin II (Ang II) has been reported to contribute to the pathogenesis of various human diseases including atherosclerosis, and inhibition of Ang II activity has been shown to reduce the morbidity and mortality of cardiovascular diseases. We have previously demonstrated that vascular cell senescence contributes to the pathogenesis of atherosclerosis; however, the effects of Ang II on vascular cell senescence have not been examined.

METHODS AND RESULTS

Ang II significantly induced premature senescence of human vascular smooth muscle cells (VSMCs) via the p53/p21-dependent pathway in vitro. Inhibition of this pathway effectively suppressed induction of proinflammatory cytokines and premature senescence of VSMCs by Ang II. Ang II also significantly increased the number of senescent VSMCs and induced the expression of proinflammatory molecules and of p21 in a mouse model of atherosclerosis. Loss of p21 markedly ameliorated the induction of proinflammatory molecules by Ang II, thereby preventing the development of atherosclerosis. Replacement of p21-deficient bone marrow cells with wild-type cells had little influence on the protective effect of p21 deficiency against the progression of atherogenesis induced by Ang II.

CONCLUSIONS

We demonstrated that Ang II promotes vascular inflammation by inducing premature senescence of VSMCs both in vitro and in vivo. Our results suggest a critical role of p21-dependent premature senescence of VSMCs in the pathogenesis of atherosclerosis.

Coronary artery disease and a functional polymorphism of hTERT

Genetic variation, a -1327T/C polymorphism, of human telomerase reverse transcriptase (hTERT) is associated with leukocyte telomere length in healthy subjects, but clinical significances of this functional polymorphism are not clear. Recently, the relationship between the telomere system and coronary artery disease (CAD) was reported. We investigated the association between the -1327T/C polymorphism and (a) susceptibility to CAD and (b) telomere length in CAD patients. In a case-control study, 104 patients confirmed by coronary angiography and 115 age- and sex-matched controls were enrolled. There was a higher frequency of the -1327C/C genotype in CAD patients (51.9%) compared with controls (36.5%, p = 0.0218). Among the 104 CAD patients, leukocyte telomere length in the -1327C/C genotype (7.62+/-2.19 kb, mean+/-SD) was shorter than that in the -1327T/C and -1327T/T genotypes (8.74+/-2.92, p = 0.0287). These findings suggest that the -1327C/C genotype is a genetic risk factor for CAD and relates to shorter telomere length among CAD patients.

Lack of age-associated telomere shortening in long- and short-lived species of sea urchins

The red sea urchin, Strongylocentrotus franciscanus, can live in excess of 100 years while the sea urchin Lytechinus variegatus has an estimated lifespan of only 3-4 years. In an effort to understand the molecular mechanism underlying the difference in their longevity we characterized telomere biology in these species of sea urchins. Telomerase activity was found throughout early stages of development in L. variegatus and is maintained in adult tissues of L. variegatus and S. franciscanus. Terminal restriction fragment analysis indicated a lack of age-associated telomere shortening. These data suggest that long- and short-lived sea urchins do not utilize telomerase repression as a mechanism to suppress neoplastic transformation.

Vascular Smooth Muscle Cells Undergo Telomere-Based Senescence in Human Atherosclerosis

Although human atherosclerosis is associated with aging, direct evidence of cellular senescence and the mechanism of senescence in vascular smooth muscle cells (VSMCs) in atherosclerotic plaques is lacking. We examined normal vessels and plaques by histochemistry, Southern blotting, and fluorescence in situ hybridization for telomere signals. VSMCs in fibrous caps expressed markers of senescence (senescence-associated β-galactosidase [SAβG] and the cyclin-dependent kinase inhibitors [cdkis] p16 and p21) not seen in normal vessels. In matched samples from the same individual, plaques demonstrated markedly shorter telomeres than normal vessels. Fibrous cap VSMCs exhibited markedly shorter telomeres compared with normal medial VSMCs. Telomere shortening was closely associated with increasing severity of atherosclerosis. In vitro, plaque VSMCs demonstrated morphological features of senescence, increased SAβG expression, reduced proliferation, and premature senescence. VSMC senescence was mediated by changes in cyclins D/E, p16, p21, and pRB, and plaque VSMCs could reenter the cell cycle by hyperphosphorylating pRB. Both plaque and normal VSMCs expressed low levels of telomerase. However, telomerase expression alone rescued plaque VSMC senescence despite short telomeres, normalizing the cdki/pRB changes. In vivo, plaque VSMCs exhibited oxidative DNA damage, suggesting that telomere damage may be induced by oxidant stress. Furthermore, oxidants induced premature senescence in vitro, with accelerated telomere shortening and reduced telomerase activity. We conclude that human atherosclerosis is characterized by senescence of VSMCs, accelerated by oxidative stress-induced DNA damage, inhibition of telomerase and marked telomere shortening. Prevention of cellular senescence may be a novel therapeutic target in atherosclerosis.

Matrix metalloproteinase 2 and tissue inhibitors of metalloproteinases regulate human aortic smooth muscle cell migration during in vitro aging

As a direct correlation between aging and the risk of onset of vascular disease has been universally accepted, we prepared an in vitro aging model consisting in sequential passages of human aortic smooth muscle cells (AoSMC) in order to evaluate the cell behavior changes during aging. Because matrix metalloproteinases (MMP) are actively involved in matrix remodeling and disease outcome, in our model we found active MMP-2 only in the conditioned medium of young AoSMCs, whereas aged cells showed only the inactive zymogen form of MMP-2 (pro-MMP-2). We ascribed the pro-MMP-2 activation in young cells to an increase in membrane type 1 matrix metalloproteinase (MT1-MMP) content. Furthermore, we found that transcripts coding for tissue inhibitor of metalloproteinases (TIMPs) were up-regulated in aged cells, and this increase of TIMPs could also prevent pro-MMP-2 activation in aged cells. Moreover, we demonstrated that young AoSMCs possess higher migratory capabilities than aged cells. The young AoSMC migration can be inhibited by adding TIMP-1 and TIMP-2 to the cells reproducing aged AoSMC migratory behavior. Finally, the role of MMP-2 and TIMP-2 in AoSMC migration was confirmed silencing MMP-2 and TIMP-2 in young and aged AoSMCs, respectively; therefore, in this study we showed that these enzymes play a pivotal role in the regulation of the AoSMC migration during in vitro aging.

Telomere length in the colon declines with age: a relation to colorectal cancer?

Telomeres shorten with age, which may be linked to genomic instability and an increased risk of cancer. To explore this association, we analyzed telomere length in normal colorectal tissue of individuals at different ages using quantitative-fluorescence in situ hybridization (Q-FISH) and quantitative-PCR (Q-PCR). Using Q-FISH, we also examined the histologically normal epithelium adjacent to, or distant from, colon adenomas and cancers, in addition to the neoplasms. Q-FISH and Q-PCR showed that telomere length was inversely associated with age until approximately ages 60 to 70; surprisingly, beyond this age, telomere length was positively associated with age. This association was found exclusively in epithelial, and not in stromal, cells. Peripheral blood lymphocytes showed an inverse association between telomere length and age, but without any apparent increase in telomere length in the oldest individuals. Telomere length in larger adenoma lesions (>2 cm) was significantly shorter than in normal adjacent (P = 0.004) or normal distant (P = 0.05) tissue from the same individuals. However, telomere length in histologically normal epithelium adjacent to cancers or in adenomas <2 cm was not statistically different from that of the normal distant mucosa or from normal controls, evidence that a telomere-shortening field effect was not present. We suggest that the positive association between telomere length and age in the oldest patients is a consequence of selective survival of elderly patients with long colonocyte telomeres.

Telomeres and telomerase biology in vertebrates: progress towards a non-human model for replicative senescence and ageing

Studies on telomere and telomerase biology are fundamental to the understanding of human ageing and age-related diseases such as cancer. However, human studies of whole body ageing are hampered by the lack of suitable fully reflective animal model systems, the wild-type mouse model being unsuitable due to differences in telomere biology. Here we summarise recent data on the biology of telomeres, telomerase, and the tumour suppressor protein p53 in various animals, and examine their possible roles in replicative senescence, ageing, and tumourigenesis. The advantages and disadvantages of various animals as model systems for whole body ageing in humans are discussed.

Degenerative Aortic Valve Stenosis, but not Coronary Disease, Is Associated With Shorter Telomere Length in the Elderly

OBJECTIVE

The mechanisms responsible for the age-related increase in the incidence of calcific aortic valve stenosis (CAS) are unclear but may include telomere-driven cellular senescence. Because telomere length varies widely among individuals of the same age, we hypothesized that patients with shorter telomeres would be prone to develop CAS late in life.

METHODS AND RESULTS

Mean telomere length was measured in leukocytes from a cohort of 193 patients > or =70 years of age with and without CAS. Pilot experiments performed in 30 patients with CAS and controls pair-matched for age, sex, and presence or absence of coronary disease demonstrated significantly shorter telomeres in the CAS group both by Southern blot hybridization (5.75+/-0.55 kbp versus 6.27+/-0.7 kbp, P=0.0023) and by a quantitative polymerase chain reaction-based technique (relative telomere length 0.88+/-0.19 versus 1.0+/-0.19, P=0.01). This finding was then confirmed in the whole cohort (CAS n=64, controls n=129, relative telomere length=0.86+/-0.16 versus 0.94+/-0.12, P=0.0003). Both groups were comparable for potential confounding characteristics. Subgroup analysis according to the presence or absence of coronary disease demonstrated no association of this disorder with telomere length.

CONCLUSIONS

In the elderly, calcific aortic stenosis, but not coronary disease, is associated with shorter leukocyte telomere length.

Apoptosis signal-regulating kinase 1 mediates cellular senescence induced by high glucose in endothelial cells

Vascular ageing is accelerated in patients with diabetes. However, the underlying mechanism remains unclear. Here, we show that high glucose induces activation of apoptosis signal-regulating kinase 1 (ASK1), an apoptosis-inducing signal that mediates endothelial cell senescence induced by hyperglycemia. High glucose induced a time-dependent increase in the levels of ASK1 expression and its activity in human umbilical vein endothelial cells (HUVECs). Incubation of endothelial cells with high glucose increased the proportion of cells expressing senescence-associated beta-galactosidase (SA-beta-gal) activity. However, transfection with an adenoviral construct including a dominant negative form of ASK1 gene significantly inhibited SA-beta-gal activity induced by high glucose. In addition, infection with an adenoviral construct expressing the constitutively active ASK1 gene directly induced an increase in the levels of SA-beta-gal activity. Activation of the ASK1 signal also enhanced plasminogen activator inhibitor-1 (PAI-1) expression in HUVECs. Induction of senescent endothelial cells in aortas and elevation of plasma PAI-1 levels were observed in streptozotocin (STZ) diabetic mice, whereas these changes induced by STZ were attenuated in ASK1-knockout mice. Our results suggest that hyperglycemia accelerates endothelial cell senescence and upregulation of PAI-1 expression through activation of the ASK1 signal. Thus, ASK1 may be a new therapeutic target to prevent vascular ageing and thrombosis in diabetic patients.

Aldosterone and telomere length in white blood cells

BACKGROUND

Aldosterone accelerates cardiovascular aging by mechanisms that generate reactive oxygen species. Telomere length in white blood cells (WBCs) may be a bioindicator that registers the accruing burden of systemic oxidative stress. The aim of the present study was, therefore, to examine the relationship between plasma aldosterone and telomere length in WBCs.

METHODS

We studied 75 normotensive and never-treated mildly hypertensive men whose blood was drawn for the measurements of plasma aldosterone concentration and the terminal restriction fragment (TRF) length in WBCs.

RESULTS

The slope of the TRF-age relationship in the entire cohort showed a decrease in telomere length of 26 +/- 5 base pairs per year (r = -0.46, p <.001). Age-adjusted TRF length was the longest in the lowest aldosterone quartile (6.74 +/- 0.12 kb) and shortest in the highest aldosterone quartile (6.36 +/- 0.11 kb), with intermediate TRF lengths in the second and third aldosterone quartiles (analysis of variance [ANOVA] trend test, p =.025). In telomeric attrition equivalence, participants in the upper aldosterone quartile were 15 years older than their peers in the lowest quartile.

CONCLUSIONS

The inverse relationship between aldosterone and WBC telomere length suggests not only that aldosterone is pro-oxidant but that elevated concentrations of this hormone might be linked to a higher rate of telomere attrition and perhaps increased biological aging in humans.

Cellular senescence in vivo: its relevance in ageing and cardiovascular disease

In most somatic mammalian cell types extensive replication and various types of cellular insults induce a permanent form of growth arrest called senescence. Senescence has been comprehensively characterised in cell culture, but its occurrence in vivo has only recently started to become appreciated. In this mini-review, we examine the evidence for the occurrence of senescence in vivo, with particular emphasis on the cardiovascular system. We also describe the senescent phenotype and discuss its pathophysiological implications. We examine findings in animal models of ageing and human genetic disorders that argue for and against a role of senescence in age-related pathologies in general and vascular disease in particular.

Estradiol enhances recovery after myocardial infarction by augmenting incorporation of bone marrow-derived endothelial progenitor cells into sites of ischemia-induced neovascularization via endothelial nitric oxide synthase-mediated activation of matrix metalloproteinase-9

BACKGROUND

Recent data have indicated that estradiol can modulate the kinetics of endothelial progenitor cells (EPCs) via endothelial nitric oxide synthase (eNOS)-dependent mechanisms. We hypothesized that estradiol could augment the incorporation of bone marrow (BM)-derived EPCs into sites of ischemia-induced neovascularization, resulting in protection from ischemic injury.

METHODS AND RESULTS

Myocardial infarction (MI) was induced by ligation of the left coronary artery in ovariectomized mice receiving either 17beta-estradiol or placebo. Estradiol induced significant increases in circulating EPCs 2 and 3 weeks after MI in estradiol-treated animals, and capillary density was significantly greater in estradiol-treated animals. Greater numbers of BM-derived EPCs were observed at ischemic sites in estradiol-treated animals than in placebo-treated animals 1 and 4 weeks after MI. In eNOS-null mice, the effect of estradiol on mobilization of EPCs was lost, as was the functional improvement in recovery from acute myocardial ischemia. A decrease was found in matrix metalloproteinase-9 (MMP-9) expression in eNOS-null mice under basal and estradiol-stimulated conditions after MI, the mobilization of EPCs by estradiol was lost in MMP-9-null mice, and the functional benefit conferred by estradiol treatment after MI in wild-type mice was significantly attenuated.

CONCLUSIONS

Estradiol preserves the integrity of ischemic tissue by augmenting the mobilization and incorporation of BM-derived EPCs into sites of neovascularization by eNOS-mediated augmentation of MMP-9 expression in the BM. Moreover, these data have broader implications with regard to our understanding of the role of EPCs in post-MI recovery and on the sex discrepancy in cardiac events.

Telomere length of normal leukocytes is affected by a functional polymorphism of hTERT

Transcriptional regulation of human telomerase reverse transcriptase (hTERT), a catalytic subunit of telomerase, is essential for telomerase activity associated with telomere length. In this study, we investigated the effects of a (-1327)T/C polymorphism within the hTERT promoter region on the hTERT promoter activity and leukocyte telomere length in normal individuals. The promoter activity in the (-1327)T-sequence was significantly higher than that in the (-1327)C-sequence (p = 0.0004). For leukocyte telomere length, the (-1327)T-allele carriers had significantly longer than the (-1327)T-allele non-carriers (p = 0.0007). Also, there was no age-related shortening in leukocyte telomere length in the (-1327)T/T (p = 0.6633) and (-1327)T/C subjects (p = 0.1691), whereas there was clear age-related telomere shortening in the (-1327)C/C subjects (p = 0.0117). These findings suggest that the functional (-1327)T/C polymorphism of hTERT is associated with leukocyte telomere length in normal individuals.

Monocyte telomere shortening and oxidative DNA damage in type 2 diabetes

OBJECTIVE

Telomeres are DNA sequences necessary for DNA replication, which shorten at cell division at a rate related to levels of oxidative stress. Once shortened to a critical length, cells are triggered into replicative senescence. Type 2 diabetes is associated with oxidative DNA damage, and we hypothesized that telomere shortening would characterize type 2 diabetes.

RESEARCH DESIGN AND METHODS

We studied 21 male type 2 diabetic subjects (mean age 61.2 years, mean HbA(1c) 7.9%) selected to limit confounding effects on telomere length and 29 matched control subjects. Telomere length was measured in peripheral venous monocyte and T-cells (naïve and memory) by fluorescent in situ hybridization and oxidative DNA damage by flow cytometry of oxidized DNA bases. Peripheral insulin resistance (homeostasis model assessment) and high-sensitivity C-reactive protein (hsCRP) were measured.

RESULTS

Mean monocyte telomere length in the diabetic group was highly significantly lower than in control subjects (4.0 [1.1] vs. 5.5 [1.1]; P < 0.0001), without significant differences in lymphocyte telomere length. There was a trend toward increased oxidative DNA damage in all diabetes cell types examined and a significant inverse relationship between oxidative DNA damage and telomere length (r = -0.55; P = 0.018) in the diabetic group. Telomere length was unrelated to plasma CRP concentration or insulin resistance.

CONCLUSIONS

Monocyte telomere shortening in type 2 diabetes could be due to increased oxidative DNA damage to monocyte precursors during cell division. This data suggests that monocytes adhering to vascular endothelium and entering the vessel wall in type 2 diabetes are from a population with shorter telomeres and at increased risk of replicative senescence within vascular plaque.

Cellular senescence impairs circadian expression of clock genes in vitro and in vivo

Circadian rhythms are regulated by a set of clock genes that form transcriptional feedback loops and generate circadian oscillation with a 24-hour cycle. Aging alters a broad spectrum of physiological, endocrine, and behavioral rhythms. Although recent evidence suggests that cellular aging contributes to various age-associated diseases, its effects on the circadian rhythms have not been examined. We report here that cellular senescence impairs circadian rhythmicity both in vitro and in vivo. Circadian expression of clock genes in serum-stimulated senescent cells was significantly weaker compared with that in young cells. Introduction of telomerase completely prevented this reduction of clock gene expression associated with senescence. Stimulation by serum activated the cAMP response element-binding protein, but the activation of this signaling pathway was significantly weaker in senescent cells. Treatment with activators of this pathway effectively restored the impaired clock gene expression of senescent cells. When young cells were implanted into young mice or old mice, the implanted cells were effectively entrained by the circadian rhythm of the recipients. In contrast, the entrainment of implanted senescent cells was markedly impaired. These results suggest that senescence decreases the ability of cells to transmit circadian signals to their clocks and that regulation of clock gene expression may be a novel strategy for the treatment of age-associated impairment of circadian rhythmicity.

Role of Telomerase in Hematopoietic Stem Cells

Studies in hematopoietic stem cell (HSC) biology are often focused on "self-renewal" and differentiation. Implicit in the word self-renewal is that the two daughter cells generated by a self-renewal division are identical to the parental cell. Strictly speaking, this is not possible because DNA is continuously damaged and repaired by DNA-repair mechanisms that are not 100% efficient. It is important to note that the efficiency of DNA repair varies greatly among different stem cell types. For example, embryonic stem cells are quite resistant to DNA damage and maintain the length of telomere repeats on serial passage, whereas HSCs are quite sensitive to DNA damage and less able to maintain telomere length. Most likely, differences between stem cell types in DNA repair and telomere maintenance pathways coevolved with cell mass, turnover, reproductive strategy, and life span. This idea has given rise to the notion that many aspects of normal aging could primarily reflect limitations in DNA repair and telomere-maintenance pathways in the (stem) cells of the soma. In humans, levels of telomerase in HSCs are under extremely tight control, as is illustrated by the marrow failure in patients with (mild) telomerase deficiencies. Here, the role of telomerase in human HSC biology is reviewed, and it is proposed that telomerase has an important role in the repair of G-rich DNA.

Activation of telomerase and expression of human telomerase reverse transcriptase in coronary atherosclerosis

INTRODUCTION

Considerable research on telomerase on human neoplastic and normal long-lived proliferative tissues has emerged. We explored the expression of telomerase in atherosclerotic human epicardial coronary arteries.

METHODS

Forty discrete human coronary arterial segments obtained from 19 heart transplant recipients were classified into nonatherosclerotic and atherosclerotic groups based on coronary angiography and histological examination. PCR-ELISA-based telomeric repeat amplification protocol (TRAP), and immunohistochemical analyses were conducted to determine the functional activity and cell-specific expression of telomerase.

RESULTS

Seventy percent of atherosclerotic coronary arteries exhibited positive telomerase activity, and the reactivation incidence reached fourfold higher than that of controls (P=.007). The telomerase catalytic protein, human telomerase reverse transcriptase (hTERT), was expressed in 88% of atherosclerotic tissues, a fivefold higher frequency compared with that of the controls. There was also a correlation of hTERT expression with the level of telomerase bioactivity (P=.017) and with the severity of atherosclerotic grade (P<.001). In comparison with the immunostaining of mitotic antigen, Ki-67, we found an association of hTERT expression with actively cycling cells in early lesions but with quiescent cells in late advanced atherosclerotic stages.

CONCLUSIONS

The up-regulation of telomerase and its catalytic hTERT protein during stages of atherosclerotic evolution may implicate a role of telomerase in vascular remodeling underlying atherogenesis.

Major cutbacks at chromosome ends

To distinguish a telomere from a double-strand break, a minimum number of telomere repeats must 'cap' each chromosome end. The length of each repeat array will reflect a unique history of addition and losses. Telomere losses are predicted to occur slowly but surely with every replication cycle (referred to as 'typical' telomere loss) in addition to intermittently and, potentially, rapidly ('sporadic'). Recent studies have shown that sporadic telomere losses can result from failure to properly repair (oxidative) damage to telomeric DNA, from failure to properly process higher-order structures of G-rich DNA and from homologous recombination reactions. Differences in telomere-erosion pathways between normal and malignant cells provide novel targets for the prevention and therapy of disease.

Comparison of telomere length of vocal folds with different tissues: a physiological measurement of vocal senescence

The objective of this article is to determine telomere length, a measure of biological age, in true vocal fold (TVF), false vocal fold (FVF), and five other tissue types, to ascertain whether there is tissue-specific telomere shortening. The study design is that of a prospective, basic science study. Tissue samples were obtained from the TVF, FVF, skin from the back of hand, skin from thigh, aorta, blood, and bone marrow from 12 patients ages 54 to 76 years. Genomic DNA was isolated from each sample, and telomere lengths were calculated with real-time polymerase chain reaction. In our small age group, age was not significantly associated with telomere length across tissue types, nor were there any linear correlations within tissue types and age. Controlling for age, significant differences were found between the following tissues: aorta and blood (P < 0.000), aorta and bone marrow (P = 0.033), aorta and FVF (P = 0.015), aorta and hand skin (P = 0.004), blood and thigh skin (P = 0.012), and blood and TVF (P = 0.048). A significant linear correlation between telomere length and tissue type without considering donor age was established between bone marrow and hand skin (P < 0.05, R2 = 0.766), thigh skin and hand skin (P < 0.01, R2 = 0.926), TVF and blood (P < 0.01, R2 = 0.836), and thigh skin and TVF (P < 0.05, R2 = 0.624). Our findings indicate that surrogate tissue for measurement of telomere length of TVF includes FVF, bone marrow, skin, and aorta. These findings have implications for understanding vocal fold aging at the cellular level.

Arterial aging: is it an immutable cardiovascular risk factor?

Age is the dominant risk factor for cardiovascular diseases. However, until recently, convincing mechanistic or molecular explanations for the increased cardiovascular risks conferred by aging have been elusive. Aging is associated with alterations in a number of structural and functional properties of large arteries, including diameter, wall thickness, wall stiffness, and endothelial function. Emerging evidence indicates that these age-associated changes are also accelerated in the presence of cardiovascular diseases, and that these changes are themselves risk factors for the appearance or progression of these diseases. In this review, the evidence demonstrating that arterial aging is accelerated in cardiovascular diseases and that accelerated arterial aging is a risk factor for adverse cardiovascular outcomes is briefly reviewed, and selected advances in vascular biology that provide insights into the mechanisms that may underlie the increased risks conferred by arterial aging are summarized. Remarkably, a host of biochemical, enzymatic, and cellular alterations that are operative in accelerated arterial aging have also been implicated in the pathogenesis and progression of arterial diseases. These vascular alterations are thus putative candidates that could be targeted by interventions aimed at attenuating arterial aging, similar to the lifestyle and pharmacological interventions that have already been proven effective. Therefore, the notion that aging is a chronological process and that its risky components cannot be modulated is no longer tenable. It is our hope that a greater appreciation of the links between arterial aging and cardiovascular diseases will stimulate further investigation into strategies aimed at preventing or retarding arterial aging.

Replicative senescence in sheep fibroblasts is a p53 dependent process

Studies on telomere and telomerase biology are fundamental to the understanding of human ageing, and age-related diseases such as cancer. However, human studies are hampered by the lack of fully reflective animal model systems. Here we describe basic studies of telomere length and telomerase activity in sheep tissues and cells. Terminal restriction fragment lengths from sheep tissues ranged from 9 to 23 kb, with telomerase activity present in testis but suppressed in somatic tissues. Sheep fibroblasts had a finite lifespan in culture, after which the cells entered senescence. During in vitro growth the mean terminal restriction fragment lengths decreased in size at a rate of 210 and 350 bp per population doubling (PD). Senescent skin fibroblasts had increased levels of p53 and p21WAF1 compared to young cells. Incubation of senescent cells with siRNA duplexes specific for p53 suppressed p53 expression and allowed the cells to re-enter the cell cycle. Five PDs beyond senescence the siRNA-treated cells reached a second proliferative barrier. This study shows that telomere biology in sheep is similar to that in humans, with senescence in sheep GM03550 fibroblasts being a telomere-driven, p53-(p21WAF1)-dependent process. Therefore sheep may represent an alternative model system for studying telomere biology, replicative senescence, and by implication human ageing.

Cancer and aging: the importance of telomeres in genome maintenance

Telomeres are the specialized DNA-protein structures that cap the ends of linear chromosomes, thereby protecting them from degradation and fusion by cellular DNA repair processes. In vertebrate cells, telomeres consist of several kilobase pairs of DNA having the sequence TTAGGG, a few hundred base pairs of single-stranded DNA at the 3' end of the telomeric DNA tract, and a host of proteins that organize the telomeric double and single-stranded DNA into a protective structure. Functional telomeres are essential for maintaining the integrity and stability of genomes. When combined with loss of cell cycle checkpoint controls, telomere dysfunction can lead to genomic instability, a common cause and hallmark of cancer. Consequently, normal mammalian cells respond to dysfunctional telomeres by undergoing apoptosis (programmed cell death) or cellular senescence (permanent cell cycle arrest), two cellular tumor suppressor mechanisms. These tumor suppressor mechanisms are potent suppressors of cancer, but recent evidence suggests that they can antagonistically also contribute to aging phenotypes. Here, we review what is known about the structure and function of telomeres in mammalian cells, particularly human cells, and how telomere dysfunction may arise and contribute to cancer and aging phenotypes.

CDK2 translational down-regulation during endothelial senescence

Here we report for the first time that loss of CDK2 activity, by translational inhibition and through CDK2 inhibition by p21(Cip1/Waf1), may be responsible for endothelial senescence. We show that expression of dominant-negative p53 extends human umbilical vein endothelial cell (HUVEC) lifespan past senescence. HUVEC expressing telomerase can completely bypass senescence and become immortal (i-HUVEC). Surprisingly, early passage i-HUVEC, like senescent HUVEC, express high levels of the CDK inhibitors p16(INK4a) and p21(Cip1/Waf1). Expression of p16(INK4a) can persist for over 280 population doublings, while p21(Cip1/Waf1) expression was eventually lost in five of six i-HUVEC lines. Senescent HUVEC contain undetectable CDK2 activity, which results from a dramatic reduction of CDK2 protein levels and inhibition of remaining CDK2 by p21(Cip1/Waf1). The decreased CDK2 levels in senescent HUVEC are not due to decreased transcription or protein stability; rather, CDK2 translation declines during senescence. Bypass of endothelial senescence by telomerase entails the restoration of CDK2 translation and activity. These results suggest that p16(INK4a) does not play a role in endothelial senescence. Rather, CDK2 translational down-regulation may be a key regulatory event in replicative senescence of endothelial cells. Understanding the mechanisms regulating endothelial senescence will be critical in determining the role of endothelial senescence in tumor growth.

Paternal age is positively linked to telomere length of children

Telomere length is linked to age-associated diseases, with shorter telomeres in blood associated with an increased probability of mortality from infection or heart disease. Little is known about how human telomere length is regulated despite convincing data from twins that telomere length is largely heritable, uniform in various tissues during development until birth and variable between individuals. As sperm cells show increasing telomere length with age, we investigated whether age of fathers at conception correlated with telomere length of their offspring. Telomere length in blood from 125 random subjects was shown to be positively associated with paternal age (+22 bp yr -1, 95% confidence interval 5.2-38.3, P = 0.010), and paternal age was calculated to affect telomere length by up to 20% of average telomere length per generation. Males lose telomeric sequence faster than females (31 bp yr -1, 17.6-43.8, P < 0.0001 vs. 14 bp yr -1, 3.5-24.8, P < 0.01) and the rate of telomere loss slows throughout the human lifespan. These data indicate that paternal age plays a role in the vertical transmission of telomere length and may contribute significantly to the variability of telomere length seen in the human population, particularly if effects are cumulative through generations.

IL-15 Activates Telomerase and Minimizes Telomere Loss and May Preserve the Replicative Life Span of Memory CD8+T Cells In Vitro

The preservation of the replicative life span of memory CD8(+) T cells is vital for long-term immune protection. Although IL-15 plays a key role in the homeostasis of memory CD8(+) T cells, it is unknown whether IL-15 regulates the replicative life span of memory CD8(+) T cells. In this study, we report an analysis of telomerase expression and telomere length in human memory phenotype CD8(+) T cells maintained by IL-15 in vitro. We demonstrate that IL-15 is capable of activating telomerase in memory CD8(+) T cells via Jak3 and PI3K signaling pathways. Furthermore, IL-15 induces a sustained level of telomerase activity over long periods of time, and in turn minimizes telomere loss in memory CD8(+) T cells after substantial cell divisions. These findings suggest that IL-15 activates stable telomerase expression and compensates telomere loss in memory phenotype CD8(+) T cells, and that telomerase may play an important role in memory CD8(+) T cell homeostasis.

Telomere loss, senescence, and genetic instability in CD4+ T lymphocytes overexpressing hTERT

Little is known about the long-term consequences of overexpression of the human telomerase reverse transcriptase (hTERT) gene in T lymphocytes. To address this issue, we transduced polyclonal as well as clonally derived populations of naive and memory CD44 T cells from 2 healthy donors (aged 24 and 34 years) with retroviral vectors encoding green fluorescence protein (GFP) and hTERT (GFP-hTERT) or GFP alone. After transduction, cells were sorted on the basis of GFP expression and cultured in vitro until senescence. T cells transduced with hTERT exhibited high stable telomerase activity throughout the culture period. Relative to GFP controls, minor changes in overall gene expression were observed yet the proliferative lifespan of the hTERT-transduced populations was significantly increased and the rate of telomere loss was lower. Nevertheless, hTERT-transduced cells showed progressive telomere loss and had shorter telomeres at senescence than controls (2.3 +/- 0.3 kilobase [kb] versus 3.4 +/- 0.1 kb). Furthermore, a population of cells with 4N DNA consisting of binucleated cells with connected nuclei emerged in the hTERT-transduced cells prior to senescence. We conclude that overexpression of hTERT in CD4+ T cells provides a proliferative advantage independent of the average telomere length but does not prevent eventual genetic instability and replicative senescence.

Chronic oxidative stress compromises telomere integrity and accelerates the onset of senescence in human endothelial cells

Replicative senescence and oxidative stress have been implicated in ageing, endothelial dysfunction and atherosclerosis. Replicative senescence is determined primarily by telomere integrity. In endothelial cells the glutathione redox-cycle plays a predominant role in the detoxification of peroxides. The aim of this study was to elucidate the role of the glutathione-dependent antioxidant system on the replicative capacity and telomere dynamics of cultured endothelial cells. Human umbilical vein endothelial cells were serially passaged while exposed to regular treatment with 0.1 microM tert-butyl hydroperoxide, a substrate of glutathione peroxidase, or 10 microM L-buthionine-[S,R]-sulphoximine, an inhibitor of glutathione synthesis. Both treatments induced intracellular oxidative stress but had no cytotoxic or cytostatic effects. Nonetheless, treated cultures entered senescence prematurely (30 versus 46 population doublings), as determined by senescence-associated beta-galactosidase staining and a sharp decrease in cell density at confluence. In cultures subjected to oxidative stress terminal restriction fragment (TRF) analysis demonstrated faster telomere shortening (110 versus 55 bp/population doubling) and the appearance of distinct, long TRFs after more than 15-20 population doublings. Fluorescence in situ hybridisation analysis of metaphase spreads confirmed the presence of increased telomere length heterogeneity, and ruled out telomeric end-to-end fusions as the source of the long TRFs. The latter was also confirmed by Bal31 digestion of genomic DNA. Similarly, upregulation of telomerase could not account for the appearance of long TRFs, as oxidative stress induced a rapid and sustained decrease in this activity. These findings demonstrate a key role for glutathione-dependent redox homeostasis in the preservation of telomere function in endothelial cells and suggest that loss of telomere integrity is a major trigger for the onset of premature senescence under mild chronic oxidative stress.

What can progeroid syndromes tell us about human aging?

Human genetic diseases that resemble accelerated aging provide useful models for gerontologists. They combine known single-gene mutations with deficits in selected tissues that are reminiscent of changes seen during normal aging. Here, we describe recent progress toward linking molecular and cellular changes with the phenotype seen in two of these disorders. One in particular, Werner syndrome, provides evidence to support the hypothesis that the senescence of somatic cells may be a causal agent of normal aging.

Aging theories of primary osteoarthritis: from epidemiology to molecular biology

Osteoarthritis is the most common disabling condition of humans in the western world. It has been known for a very long time that aging is the most prominent risk factor for the initiation and progression of the disease, but the explanations for this phenomenon have changed over time. The most longstanding theory is that osteoarthritis develops because of continuous mechanical wear and tear. However, osteoarthritis can also be the result of time/age-related modifications to cartilage matrix components. One of the simplest biological explanations for the initiation and progression of osteoarthritic cartilage degeneration is a mere loss of viable cells, due to apoptosis or other mechanisms. Overall, the most likely scenario is that the cells and the matrix of articular cartilage get older over time, and eventually the tissue enters a senescence-like state that makes it more prone to enter the osteoarthritic degeneration pathway. Thus, patients with osteoarthritis might progress more quickly to the senescence phenotype compared to others. Moreover, stressful conditions associated with the osteoarthritic disease process might further promote chondrocyte senescence. Primary osteoarthritis in this model would be a "premature" degeneration of the joint due to a premature chondrocyte senescence. By analogy to neurodegenerative disorders, one could refer to osteoarthritis as the "M. Alzheimer" of articular cartilage. One of the most important implications of this hypothesis is that it points to issues of cellular degeneration as the basis for understanding the initiation and progression of osteoarthritis. Equally important, it emphasizes that whatever treatment we envisage for osteoarthritis, we must take into account that we are dealing with aged/(pre)senescent cells that no longer have the ability of their juvenile counterparts to counteract the many mechanical, inflammatory, and/or other assaults to the tissue.

Measuring vertebrate telomeres: applications and limitations

Telomeres are short tandem repeated sequences of DNA found at the ends of eukaryotic chromosomes that function in stabilizing chromosomal end integrity. In vivo studies of somatic tissue of mammals and birds have shown a correlation between telomere length and organismal age within species, and correlations between telomere shortening rate and lifespan among species. This result presents the tantalizing possibility that telomere length could be used to provide much needed information on age, ageing and survival in natural populations where longitudinal studies are lacking. Here we review methods available for measuring telomere length and discuss the potential uses and limitations of telomeres as age and ageing estimators in the fields of vertebrate ecology, evolution and conservation.

Telomere attrition and accumulation of senescent cells in cultured human endothelial cells

The human umbilical vein endothelial cell (HUVEC) is an important model of the human endothelium that is widely used in vascular research. HUVECs and the adult endothelium share many characteristics including progression into senescence as the cells age. Despite this, the shortening of telomeres and its relationship to the progression into senescence are poorly defined in HUVECs. In this study of several HUVEC lines we show notable consistency in their growth curves. There is a steady decline in the growth rate of HUVECs grown continually in culture and we estimate complete cessation of growth after approximately 70 population doublings. The HUVECs lose telomeric DNA at a consistent rate of 90 base pairs/population doubling and show a progressive accumulation of shortened telomeres (below 5 kilobases). This telomeric loss correlates with the accumulation of senescent HUVECs in culture as assessed by staining for beta-galactosidase activity at pH 6. Although the telomere length of a large population of cells is a relatively crude measure, we suggest that in HUVECs a mean telomere length (as measured by terminal restriction fragment length) of 5 kilobases is associated with entry into senescence. These data demonstrate the strong relationship between telomere attrition and cell senescence in HUVECs. They suggest that DNA damage and subsequent telomere attrition are likely to be key mechanisms driving the development of endothelial senescence in the pathogenesis of vascular disease.

Mathematical modeling of vascular endothelial layer maintenance: the role of endothelial cell division, progenitor cell homing, and telomere shortening

Maintenance of the endothelial cell (EC) layer of the vessel wall is essential for proper functioning of the vessel and prevention of vascular disorders. Replacement of damaged ECs could occur through division of surrounding ECs. Furthermore, EC progenitor cells (EPCs), derived from the bone marrow and circulating in the bloodstream, can differentiate into ECs. Therefore, these cells might also play a role in maintenance of the endothelial layer in the vascular system. The proliferative potential of both cell types is limited by shortening of telomeric DNA. Accelerated telomere shortening might lead to senescent vascular wall cells and eventually to the inability of the endothelium to maintain a continuous monolayer. The aim of this study was to describe the dynamics of EC damage and repair and telomere shortening by a mathematical model. In the model, ECs were integrated in a two-dimensional structure resembling the endothelium in a large artery. Telomere shortening was described as a stochastic process with oxidative damage as the main cause of attrition. Simulating the model illustrated that increased cellular turnover or elevated levels of oxidative stress could lead to critical telomere shortening and senescence at an age of 65 yr. The model predicted that under those conditions the EC layer could display defects, which could initiate severe vascular wall damage in reality. Furthermore, simulations showed that 5% progenitor cell homing/yr can significantly delay the EC layer defects. This stresses the potential importance of EPC number and function to the maintenance of vascular wall integrity during the human life span.

Role of replicative senescence in the progression of fibrosis in hepatitis C virus (HCV) recurrence after liver transplantation

BACKGROUND

Although hepatitis C virus (HCV) recurrence is almost universal after orthotopic liver transplantation (OLT), the impact of viral infection on liver graft is highly variable and difficult to predict. Because of the possible relationship between replicative senescence (RS) and the accelerated development of liver fibrosis, we aimed to assess the potential role of RS in the severity of HCV-related chronic hepatitis recurrence after OLT.

METHODS

One hundred three liver biopsies from 56 patients receiving transplants for HCV-related cirrhosis were studied, including 30 revascularization biopsies and 52 and 21 biopsies performed during and beyond the first year of OLT, respectively. The presence of senescent cells in liver grafts was assessed by the senescence-associated beta-galactosidase (SA-beta-Gal) staining method. Chronic hepatitis was defined by fibrosis stage and necrotico-inflammatory activity grade using the METAVIR score.

RESULTS

A total of 34 of the 103 (33%) frozen liver biopsies displayed SA-beta-Gal-positive cells, including 6 (20%) of the revascularization biopsies, 14 (34%) of the biopsies performed within the first year, and 10 (46%) of the biopsies performed beyond 1 year of follow-up. The presence of senescent cells in revascularization biopsies was significantly associated with the degree of ischemic necrosis at time of OLT (P = 0.01) and hepatitis C recurrence in the first year after OLT (P = 0.05). Furthermore, the presence of RS in the biopsy performed within the first year was associated with further development of fibrosis (P = 0.05).

CONCLUSIONS

These data show that RS has a significant impact upon the course of liver transplantation, especially in the long-term progression of fibrosis observed in HCV-infected patients.

The Extent and Significance of Telomere Loss with Age

By imposing a limit on the proliferative life span of some human cell types, telomere loss and the subsequent onset of replicative senescence have been proposed to contribute to age-related disease. Although there is a large body of in vitro data to reveal the mechanisms by which telomere erosion triggers senescence, technical limitations have hampered our ability to understand the full extent of telomere erosion in vivo. Thus far, we have evidence of age-related telomere loss; however, the lack of resolution of existing technologies does not allow us to determine if telomere erosion is extensive enough to trigger replicative senescence in vivo. This coupled with the considerable interindividual heterogeneity, and the overlap in telomere lengths between young and elder individuals, render any correlation weak and the significance unclear. However, recent technical developments, including adaptations of quantitative telomere fluorescence, in situ hybridization (Q-FISH), and the PCR-based single telomere length analysis (STELA), have increased the resolution of telomere length analysis. These technologies promise to provide the evidence required to address the full extent and significance of telomere loss in the human aging process. Here, we review published data on the dynamics of telomere erosion with age in the human body.

Mathematical modeling confirms the length-dependency of telomere shortening

Telomeres, the ends of chromosomes, shorten with each cell division in human somatic cells, because of the end-replication problem, C-strand processing and oxidative damage. On the other hand, the reverse transcriptase telomerase can add back telomeric repeats at the telomere ends. It has been suggested that once telomeres have reached a critical length, cells cease proliferation, also known as senescence. Evidence is accumulating that telomere shortening and subsequent senescence might play a crucial role in life-threatening diseases. So far, mathematical models described telomere shortening as an autonomous process, where the loss per cell division does not depend on the telomere length itself. In this study, published measurements of telomere distributions in human fibroblasts and human endothelial cells were used to show that telomeres shorten in a length-dependent fashion. Thereafter, a mathematical model of telomere attrition was composed, in which a shortening factor and an autonomous loss were incorporated. It was assumed that the percentage of senescence was related to the percentage of telomeres below a critical length. The model was compared with published data of telomere length and senescence of human endothelial cells using the maximum likelihood method. This enabled the estimation of physiologically important parameters and confirmed the length-dependency of telomere shortening.

DNA end joining becomes less efficient and more error-prone during cellular senescence

Accumulation of somatic mutations is thought to contribute to the aging process. Genomic instability has been shown to increase during aging, suggesting an aberrant function of DNA double-strand break (DSB) repair. Surprisingly, DSB repair has not been examined with respect to cellular senescence. Therefore, we have studied the ability of young, presenescent, and senescent normal human fibroblasts to repair DSBs in transfected DNA by using a fluorescent reporter substrate. We have found that the efficiency of end joining is reduced up to 4.5 fold in presenescent and senescent cells, relative to young cells. Sequence analysis of end junctions showed that the frequency of precise ligation was higher in young cells, whereas end joining in old cells was associated with extended deletions. These results indicate that end joining becomes inefficient and more error-prone during cellular senescence. Furthermore, the ability to use microhomologies for end joining was compromised in senescent cells, suggesting that young and senescent cells may use different end joining pathways. We hypothesize that inefficient and aberrant end joining is a likely mechanism underlying the age-related genomic instability and higher incidence of cancer in the elderly.

Telomeres and Cardiovascular Disease

Telomeres—the specialized DNA-protein structures at the ends of eukaryotic chromosomes—are essential for maintaining genome stability and integrity and for extended proliferative life span in both cultured cells and in the whole organism. Telomerase and additional telomere-associated proteins are necessary for preserving telomeric DNA length. Age-dependent telomere shortening in most somatic cells, including vascular endothelial cells, smooth muscle cells, and cardiomyocytes, is thought to impair cellular function and viability of the aged organism. Telomere dysfunction is emerging as an important factor in the pathogenesis of hypertension, atherosclerosis, and heart failure. In this Review, we discuss present studies on telomeres and telomere-associated proteins in cardiovascular pathobiology and their implications for therapeutics.

Are major risk factors for myocardial infarction the major predictors of degree of coronary artery disease in men?

Although numerous cross-sectional studies have reported associations of hypertension, hypercholesterolemia, diabetes, smoking, and/or obesity with the presence of coronary artery disease (CAD), correlations of these risk factors for myocardial infarction (MI) with the degree or progression of CAD have been less consistent. Nevertheless, these risk factors are generally assumed to be major determinants not only of MI, but of the degree of CAD as well. The present study is an attempt to evaluate the relationship of major risk factors for MI to degree of CAD. From 182 men who underwent diagnostic coronary arteriography, the 154 with CAD were selected for study. These 154 patients were divided into 2 groups, those with hypertension, hypercholesterolemia, diabetes, smoking, and/or obesity (n = 121) and those with none of these risk factors (n = 33). The mean degree of CAD in the group with risk factors for MI (44.4%) and in the group without (50.6%) was not significantly different (P =.15); nor was the increase in CAD with age augmented by the presence of these risk factors. On multiple regression analysis, none of these risk factors was associated with degree of CAD. Three other variables that were considered in this study, age, high-density lipoprotein-cholesterol (HDL-C), and free testosterone (FT), did show an independent association with degree of CAD. These findings, together with the findings of previous studies from other laboratories, raise the possibility that in men selected for coronary arteriography, age, HDL-C, and FT may be stronger predictors of degree of CAD than are blood pressure, cholesterol, diabetes, smoking, and body mass index (BMI).

Telomere Shortening in Human Coronary Artery Diseases

BACKGROUND

Increased cell turnover in response to injury is considered to be important in the development of atherosclerotic plaques. Telomere shortening has been shown to be associated with cell turnover. We assessed the telomere length of human coronary endothelial cells to clarify whether there is a relationship between telomere shortening and coronary artery disease (CAD).

METHODS AND RESULTS

Coronary endothelial cells were obtained from 11 patients with CAD who underwent autopsy and 22 patients without CAD who underwent autopsy by scraping off the luminal surface of coronary arteries. DNA extracted from the endothelial cells were blotted and hybridized with telomere-specific oligonucleotide ([TTAGGG]4). The hybridization signal intensity, which represented telomeric DNA content, was standardized with centromeric DNA content (T/C ratio) to estimate telomere length. The T/C ratios were significantly smaller (P<0.0001) in CAD patients than in age-matched non-CAD patients (CAD patients, 0.462+/-0.135; non-CAD patients, 1.002+/-0.212). In 6 individual CAD patients, the T/C ratio at the atherosclerotic lesion was significantly smaller (P<0.05) than that at the non-atherosclerotic portion.

CONCLUSIONS

These findings suggest that focal replicative senescence and telomere shortening of endothelial cells may play a critical role in coronary atherogenesis and CAD.

Aging: an overview

The elderly population has rapidly increasing needs for skin care. The demographics of aging allow a profound insight into the future of geriatric dermatology.

Vascular cell senescence and vascular aging

Vascular cells have a finite lifespan when cultured in vitro and eventually enter an irreversible growth arrest called "cellular senescence". A number of genetic animal models carrying targeted disruption of the genes that confer the protection against senescence in vitro have been reported to exhibit the phenotypes of premature aging. Similar mutations have been found in the patients with premature aging syndromes. Many of the changes in senescent vascular cell behavior are consistent with the changes seen in age-related vascular diseases. We have demonstrated the presence of senescent vascular cells in human atherosclerotic lesions but not in non-atherosclerotic lesions. Moreover, these cells express increased levels of pro-inflammatory molecules and decreased levels of endothelial nitric oxide synthase, suggesting that cellular senescence in vivo contributes to the pathogenesis of human atherosclerosis. One widely discussed hypothesis of senescence is the telomere hypothesis. An increasing body of evidence has established the critical role of the telomere in vascular cell senescence. Another line of evidence suggests that telomere-independent mechanisms are also involved in vascular cell senescence. Activation of Ras, an important signaling molecule involved in atherogenic stimuli, induces vascular cell senescence and thereby promotes vascular inflammation in vitro and in vivo. It is possible that mitogenic-signaling pathways induce telomere-dependent and telomere-independent senescence, which results in vascular dysfunction. Further understanding of the mechanism underlying cellular senescence will provide insights into the potential of antisenescence therapy for vascular aging.

Short Telomeres Are Associated With Increased Carotid Atherosclerosis in Hypertensive Subjects

Recent studies have shown that individuals with shorter telomeres present a higher prevalence of arterial lesions and higher risk of cardiovascular disease mortality. As a group, patients with high blood pressure are at an increased risk for cardiovascular diseases. However, some hypertensive patients are more prone than others to atherosclerotic lesions. The main objective of this study was to examine the relationship between telomere length, as expressed in white blood cells, and carotid artery atherosclerotic plaques in hypertensive males. Data from 163 treated hypertensive men who were volunteers for a free medical examination were analyzed. Extracranial carotid plaques were assessed with B-mode ultrasound. Telomere length was measured from DNA samples extracted from white blood cells. The results of this study show that telomere length was shorter in hypertensive men with carotid artery plaques versus hypertensive men without plaques (8.17±0.07 kb versus 8.46±0.07 kb; P <0.01). Multivariate analysis showed that in addition to age, telomere length was a significant predictor of the presence of carotid artery plaques. The findings from this study suggest that in the presence of chronic hypertension, which is a major risk factor for atherosclerotic lesions, shorter telomere length in white blood cells is associated with an increased predilection to carotid artery atherosclerosis.

How the endothelium and its bone marrow-derived progenitors influence development of disease

The association between diseases accompanied by abnormal endothelial/vascular function (atherosclerosis, hypertension, diabetes mellitus, preeclampsia), and conditions characterized by increased tissue growth and normal endothelial/vascular function (cancer, placental size, birth length, adult height) could be caused by inherited characteristics of endothelial cells and their bone marrow-derived precursors. The genotype responsible for normal endothelial/precursor function could be modified by intrauterine and postnatal endothelial injury; telomere shortening caused by increased endothelial precursor proliferation in response to injury can result in premature endothelial senescence and a decreased precursor proliferative potential, thereby leading to an abnormal endothelial/precursor phenotype and the associated diseases. The individual endothelial/precursor phenotype could be established early in life and its changes in response to risk factors for diseases followed over time, thus providing a unique opportunity for identification and early institution of prophylactic and therapeutic interventions in diseases that cause most of the morbidity and mortality in advanced industrialized societies.

Telomere Attrition in White Blood Cell Correlating with Cardiovascular Damage

Aging is a major risk factor for cardiovascular disease. Chronological aging does not always parallel biological aging, but there is no reliable biomarker for the latter. In the present study, we tested the hypothesis that telomere attrition in white blood cells is related to endothelial dysfunction and the extent of atherosclerosis, and thus may serve as a useful marker for biological aging. We evaluated telomere lengths in white blood cells by measuring the mean telomere restriction fragment length (mTRFL), as well as endothelial function by flow mediated dilatation (FMD) in the brachial artery, in patients with various degrees of cardiovascular damage and in normal subjects. Cardiovascular damage was assessed by a cardiovascular damage (CVD) score, with 1 point being given for the presence of each cardiovascular risk factor (hypertension, hyperlipidemia and diabetes) and for each event (angina, myocardial infarction, cerebrovascular event and peripheral vascular disease). Subset analysis of CVD score groups revealed that mTRFL and FMD decreased in the rank order of CVD score. Although mTRFL was inversely correlated with age, telomere index, defined as the ratio of TRFL to TRFL predicted by age, also decreased with increase in CVD score. These results indicate that telomere attrition in white blood cells is more closely associated with endothelial damage and atherosclerosis than is chronological aging, supporting the hypothesis that mTRFL in white blood cells is a useful marker for biological aging of the cardiovascular system.

Optimizing endothelial nitric oxide activity may slow endothelial aging

The capacity of vascular endothelium to generate bioactive nitric oxide (NO) decreases with advancing age, even in healthy subjects with a relatively benign risk factor profile; this phenomenon may reflect decreased expression of NO synthase, as well as increased production of superoxide, and evidently contributes importantly to the increased vascular risk associated with aging. Studies with cultured endothelial cells suggest that the rate of endothelial aging is determined primarily by the rate of cell turnover and the associated progressive shortening of telomeres; endothelial cells transfected with the catalytic subunit of telomerase--which preserves a youthful telomere length--do not show a reduction in NO synthase expression after numerous doublings, in contrast to the marked reduction observed in control cells. Also consistent with this view is the fact that, following balloon denudation of arteries, the regenerated endothelium makes less NO. In the vasculature of adults, the rate of endothelial cell mitosis is evidently a reflection of the rate of endothelial cell apoptosis. Numerous cell culture studies demonstrate that physiological levels of NO protect endothelial cells from apoptosis induced by a wide range of noxious stimuli--including vascular risk factors such as oxidized LDL, angiotensin II, and hyperglycemia. In the human vasculature, endothelial cells with disproportionately short telomeres are found capping atheromatous lesions and in atheroma-prone areas where blood flow is turbulent; these findings evidently reflect increased endothelial cell turnover in regions where NO bioactivity is relatively weak. It can be deduced that lifelong adherence to an "endotheliophilic lifestyle" that optimizes vascular NO production, while minimizing that of superoxide, will literally slow the rate of aging of vascular endothelium, such that, at any given advanced age, the optimal functional capacity of the vascular endothelium will be superior to that of age-matched controls. These considerations underline the desirability of actively promoting vascular health in younger and middle-aged individuals in whom risk for vascular events may still be quite low. The impact of lifelong caloric restriction on endothelial aging requires further study, preferably in primates.