Telomere shortening during aging of human osteoblasts in vitro and leukocytes in vivo: lack of excessive telomere loss in osteoporotic patients

Telomere length as a predictive biomarker in osteoporosis (Review)

Telomeres are the ends of chromosomes that protect them from DNA damage. There is evidence to suggest that telomere shortening appears with advanced age. Since aging is a significant risk factor for developing age-related complications, it is plausible that telomere shortening may be involved in the development of osteoporosis. The present review summarizes the potential of telomere shortening as a biomarker for detecting the onset of osteoporosis. For the purposes of the present review, the following scientific databases were searched for relevant articles: PubMed/NCBI, Cochrane Library of Systematic Reviews, Scopus, Embase and Google Scholar. The present review includes randomized and non-randomized controlled studies and case series involving humans, irrespective of the time of their publication. In six out of the 11 included studies providing data on humans, there was at least a weak association between telomere length and osteoporosis, with the remaining studies exhibiting no such association. As a result, telomere shortening may be used as a biomarker or as part of a panel of biomarkers for tracking the onset and progression of osteoporosis.

Telomere length as a marker of changes in body composition and fractures-an analysis of data from the NHANES 2001-2002

Purpose

There has been an association between changes in body composition, fracture incidence, and age in previous studies. Telomere length (TL) has been proposed as a biomarker of aging. However, the relationship between body composition, fractures, and TL has rarely been studied. Therefore, this study aimed to investigate the correlation between TL and body composition and fractures.Patients and methods: 20950 participants from the 2001-2002 National Health and Nutrition Examination Survey (NHANES) were included in the final analysis. In NHANES, body compositions were measured with DXA, and TL was determined with quantitative PCR. Correlation analysis of TL and body composition was conducted using multivariate weighted linear regression and logistic regression models.

Results

The results showed that TL positively correlated with bone mineral density (BMD) and bone mineral content (BMC) in most body parts. However, BMD and BMC were negatively connected with TL in the upper limbs and skull. Fat content was negatively associated with TL, while muscle content was positively linked to TL. In addition, TL's trend analysis results were consistent with the regression model when transformed from a continuous to a classified variable. An increase in TL was associated with a higher incidence of wrist fractures, while a decrease in spine fractures. The above correlation also has a certain degree of sex specificity.

Conclusion

Our study indicate that TL is associated with body composition as well as fractures, but further research is needed to confirm these contrasting associations in the skull, upper limbs, and wrists.

Telomeres and Age-Related Diseases

Telomeres are at the non-coding ends of linear chromosomes. Through a complex 3-dimensional structure, they protect the coding DNA and ensure appropriate separation of chromosomes. Aging is characterized by a progressive shortening of telomeres, which compromises their structure and function. Because of their protective function for genomic DNA, telomeres appear to play an important role in the development and progression of many age-related diseases, such as cardiovascular disease (CVD), malignancies, dementia, and osteoporosis. Despite substantial evidence that links telomere length with these conditions, the nature of these observations remains insufficiently understood. Therefore, future studies should address the question of causality. Furthermore, analytical methods should be further improved with the aim to provide informative and comparable results. This review summarize the actual knowledge of telomere biology and the possible implications of telomere dysfunction for the development and progression of age-related diseases. Furthermore, we provide an overview of analytical techniques for the measurement of telomere length and telomerase activity.

Mendelian randomization study of telomere length and bone mineral density

Purpose: Some epidemiological studies and animal studies have reported a relationship between leukocyte telomere length (LTL) and bone mineral density (BMD). However, the causality underlying the purported relationship has not been determined. Here we performed a two-sample MR analysis to test the causal link between telomere length and BMD.

Results: Our research suggested no causal link of LTL and BMD using IVW method. The weighted median, MR-Egger regression and MR.RAPS method yielded a similar pattern of effects. MR-Egger intercept test demonstrated our results were not influenced by pleiotropy. Heterogeneities among the genetic variants on heel estimated BMD and TB-BMD vanished after excluding rs6028466. “Leave-one-out” sensitivity analysis confirmed the stability of our results.

Conclusion: Our MR analysis did not support causal effect of telomere length on BMD.

Methods: We utilized 5 independent SNPs robustly associated with LTL as instrument variables. The outcome results were obtained from GWAS summary data of BMD. The two-sample MR analysis was conducted using IVW, weighted median, MR-Egger regression and MR.RAPS method. MR-Egger intercept test, Cochran’s Q test and I² statistics and “leave-one-out” sensitivity analysis were performed to evaluate the horizontal pleiotropy, heterogeneities and stability of these genetic variants on BMD.

Can telomere length predict bone health? A review of current evidence

Telomeres are repetitive DNA sequences located at the end of chromosomes that serve as a protective barrier against chromosomal deterioration during cell division. Approximately 50–200 base pairs of nucleotides are lost per cell division, and new repetitive nucleotides are added by the enzyme telomerase, allowing telomere maintenance. Telomere shortening has been proposed as an indicator for biological aging, but its relationship with age-related osteoporosis is ambiguous. We summarize the current evidence on the relationship between telomere length and bone health in experimental and epidemiological studies, which serve as a scientific reference for the development of novel diagnostic markers of osteoporosis or novel therapeutics targeting telomere and telomerase of bone cells to treat osteoporosis.

Telomere length and telomerase activity in osteoporosis and osteoarthritis

Osteoarthritis (OA) and osteoporosis (OP) are associated skeletal pathologies and have as a distinct feature the abnormal reconstruction of the subchondral bone. OA and OP have been characterized as age-related diseases and have been associated with telomere shortening and altered telomerase activity (TA). This review discusses the role of telomeres and telomerase in OA and OP pathologies and focuses on the usability of telomere length (TL) and the rate of telomere shortening as potential disease biomarkers. A number of studies have demonstrated that telomere shortening may contribute to OA and OP as an epigenetic factor. Therefore, it has been claimed that the measurement of TL of chondrocytes and/or peripheral blood cells may be an appropriate marker for the evaluation of the progression of these diseases. However, there is a need to be perform further studies with larger cohorts, with the aim of obtaining objective results and a better understanding of the association between TL, inflammation and aging, in order to provide further insight into the pathophysiology of degenerative joint diseases.

The age modification to leukocyte telomere length effect on bone mineral density and osteoporosis among Chinese elderly women

Critically short telomeres indicate cellular senescence. Leukocyte telomere length (LTL) is regarded as an aging predictor. Osteoporosis is an age-related disease. The purpose of our study is to examine the association between LTL, and BMD and osteoporosis among an elderly Chinese population. A total of 1017 participants (584 postmenopausal women) with a mean age of 66.4 years were recruited from April 2016 to August 2017. Dual-energy X-ray absorptiometry was used for BMD measurement at skeleton sites of lumbar spine (LS), femoral neck (FN), and total hip (TH). LTL was measured using quantitative real-time polymerase chain reaction. Among women, age significantly modified the effect of LTL on BMD at FN. Additionally, significant age modification was observed for the association between LTL and LS BMD category (indicative of control or osteopenia or osteoporosis), and the number of osteoporotic sites at LS or TH. The corresponding estimates (95% CI) for the relative excess risk due to interaction (RERI) were - 0.07 (- 0.11, - 0.01) and - 0.11 (- 0.16, - 0.03) sequentially in ordinal logistic regression models. The estimated RERIs (95% CI) were - 0.11 (- 0.25, - 0.02) and - 0.23 (- 0.39, - 0.10) in multinomial logistic regression models for LS/FN/TH BMD category, and - 0.20 (- 0.31, - 0.09) and - 0.34 (- 0.49, - 0.21) for FN BMD category. However, similar findings did not show in men. The effect of LTL on BMD and osteoporosis risk is modified by age in elderly women but not in men, suggesting that the predictive role of LTL in bone loss differs by sex.

Telomere biology and age-related diseases

Telomeres are the protective end caps of chromosomes and shorten with every cell division. Telomere length has been proposed as a biomarker of biological age and a risk factor for age-related diseases. Epidemiologic studies show an association between leukocyte telomere length (LTL) and mortality. There is solid evidence that links LTL with cardiovascular disease. Short telomeres promote atherosclerosis and impair the repair of vascular lesions. Alzheimer's disease patients have also a reduced LTL. Telomeres measured in tumor tissue from breast, colon and prostate are shorter than in healthy tissue from the same organ and the same patient. In healthy tissue directly adjacent to these tumors, telomeres are also shorter than in cells that are more distant from the cancerous lesion. A reduced telomere length in cancer tissue from breast, colon and prostate is associated with an advanced disease state at diagnosis, faster disease progression and poorer survival. By contrast, results regarding LTL and cancer are inconsistent. Furthermore, the majority of studies did not find significant associations between LTL, bone mineral density (BMD) and osteoporosis. The present manuscript gives an overview about our current understanding of telomere biology and reviews existing knowledge regarding the relationship between telomere length and age-related diseases.

The effect of donor variation and senescence on endothelial differentiation of human mesenchymal stromal cells

Application of autologous cells is considered for a broad range of regenerative therapies because it is not surrounded by the immunological and ethical issues of allo- or xenogenic cells. However, isolation, expansion, and application of autologous cells do suffer from variability in therapeutic efficacy due to donor to donor differences and due to prolonged culture. One important source of autologous cells is mesenchymal stromal cells (MSCs), which can differentiate toward endothelial-like cells, thus making them an ideal candidate as cell source for tissue vascularization. Here we screened MSCs from 20 donors for their endothelial differentiation capacity and correlated it with the gene expression profile of the whole genome in the undifferentiated state. Cells of all donors were able to form tubes on Matrigel and induced the expression of endothelial genes, although with quantitative differences. In addition, we analyzed the effect of prolonged in vitro expansion on the multipotency of human MSCs and found that endothelial differentiation is only mildly sensitive to expansion-induced loss of differentiation as compared to osteogenic and adipogenic differentiation. Our results show the robustness of the endothelial differentiation protocol and the gene expression data give insight in the differences in endothelial differentiation between donors.

The role of telomeres in musculoskeletal diseases

The telomeric region of repetitive DNA sequences at the end of chromosomes prevents end-to-end fusion of chromosome terminals and deterioration of the doublestrand free ends. Because of the 'end-replication problem', telomeres shorten with each round of cell division, resulting in cell senescence. The enzyme telomerase compensates for telomere shortening by elongating telomeric sequences, thereby prolonging the lifespan of the cell. Studies of articular cartilage and bone tissues have indicated that telomere shortening limits normal cell function and proliferation, while the telomere maintenance mechanisms of osteosarcoma cells facilitate escape from cell death and promote immortality. This article reviews the literature on this topic and provides an extensive discussion of the basic molecular biology and roles of telomeres and telomerase in musculoskeletal diseases such as osteoarthritis, osteoporosis and osteosarcoma. Findings to date suggest that telomeres and telomerase may become novel therapeutic targets for the diagnosis, treatment and prevention of musculoskeletal disorders.

Senescence-associated intrinsic mechanisms of osteoblast dysfunctions

Human aging is associated with bone loss leading to bone fragility and increased risk of fractures. The cellular and molecular causes of age-related bone loss are current intensive topic of investigation with the aim of identifying new approaches to abolish its negative effects on the skeleton. Age-related osteoblast dysfunction is the main cause of age-related bone loss in both men and women beyond the fifth decade and results from two groups of pathogenic mechanisms: extrinsic mechanisms that are mediated by age-related changes in bone microenvironment including changes in levels of hormones and growth factors, and intrinsic mechanisms caused by the osteoblast cellular senescence. The aim of this review is to provide a summary of the intrinsic senescence mechanisms affecting osteoblastic functions and how they can be targeted to abolish age-related osteoblastic dysfunction and bone loss associated with aging.

Regulation of senescence in cancer and aging

Senescence is regarded as a physiological response of cells to stress, including telomere dysfunction, aberrant oncogenic activation, DNA damage, and oxidative stress. This stress response has an antagonistically pleiotropic effect to organisms: beneficial as a tumor suppressor, but detrimental by contributing to aging. The emergence of senescence as an effective tumor suppression mechanism is highlighted by recent demonstration that senescence prevents proliferation of cells at risk of neoplastic transformation. Consequently, induction of senescence is recognized as a potential treatment of cancer. Substantial evidence also suggests that senescence plays an important role in aging, particularly in aging of stem cells. In this paper, we will discuss the molecular regulation of senescence its role in cancer and aging. The potential utility of senescence in cancer therapeutics will also be discussed.

Selection of optimal passage of bone marrow-derived mesenchymal stem cells for stem cell therapy in patients with amyotrophic lateral sclerosis

Mesenchymal stem cells (MSCs) obtained from bone marrow (BM) are currently used as an alternative therapy in amyotrophic lateral sclerosis (ALS) patients. Selection of optimal passages of autologous BM-derived MSCs during long-term in vitro expansion is important for clinical trials in patients with ALS. We isolated and expanded MSCs from the BM of eight ALS patients to analyze the growth kinetics, differentiation potential, cellular surface antigen expression, karyotype modifications and secretion of various cytokines during long-term culture. The morphology and size of the cells changed from small and spindle-like cells to large and polygonal types in later passages. The growth rate of the MSCs was highest in the third passage, followed by a gradual decrease. There were no special modifications of cell surface antigens or the karyotype of the MSCs from the first to the tenth passage. MSCs in the fourth passage were differentiated into adipocytes, osteocytes and chondrocytes. When we analyzed the cultured media of MSCs at the third, fifth, seventh and ninth passages, IL-6, VEGF and IL-8 showed high expression, with more than 50pg/10,000 cells at these passages; however, their expression progressively decreased with additional passages. In addition, secretion of IL-15, GM-CSF, IL-10, PDGF-bb, G-CSF, IL-1beta, basic FGF and IFN-gamma gradually decreased over prolonged culture. We suggest that MSCs at earlier passages are more suitable for stem cell therapy in ALS patients because of their stability and more potent anti-inflammatory and neuroprotective properties.

Leukocyte Telomere Length Is Not Associated With BMD, Osteoporosis, or Fracture in Older Adults: Results From the Health, Aging and Body Composition Study

Short leukocyte telomere length (TL), low BMD, and osteoporosis have been associated with increased inflammation. Previous reports suggest an association between TL, BMD, and osteoporosis in women. We sought to verify these associations and to determine whether TL is related to fracture in a cohort of older men and women. Participants included 2750 community-dwelling older persons from the longitudinal Health, Aging, and Body Composition Study (Health ABC) in who average leukocyte TL was measured at baseline using qPCR. We used unconditional logistic regression to determine the association of TL with prevalent fracture, Cox proportional hazards regression for the association with 7-yr incident fracture, and mixed linear models for the association with BMD, change in BMD, and the number of incident fractures. TL was negatively correlated with age, weight, fasting insulin, and fasting glucose in men and women, and additionally, with C-reactive protein and IL-6 in men. TL was not associated with BMD; change in BMD over 1, 3, or 5 yr; osteoporosis; baseline fracture; or 7-yr incident fracture, before or after adjustment for age, race, smoking, and health characteristics. TL is not associated with BMD, osteoporosis, or fracture in older men or women in this sample.

Biological characterization of long-term cultured human mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) have generated a great deal of interest in clinical applications. The reason is that they may have the plasticity needed to differentiate into multiple lineages and the ability to expand ex vivo. For the therapeutic applications of hMSCs to be of practical use, it is crucial to assess the efficacy and safety of hMSCs in long-term ex vivo expansion. In this study, we cultured hMSCs by population doubling (PD) 60, and investigated their growth, osteogenic and adipogenic differential abilities, change of surface markers, telomerase activity, telomere length, and gene expression related to tumorigenesis. An in vivo tumorigenesis assay was also carried out. In long-term expanded hMSCs, the cells became aged above PD 30 and their adipogenic and osteogenic differentiation potential decreased. Telomerase activity unchanged whereas telomere length decreased and karyotypes were not changed. Gene expressions related to tumorigenesis decreased in proportion as the PD of hMSCs increased. In vivo transplantation of long-term cultured hMSCs to nude mice did not result in tumor formation. These findings suggest that diverse tests for cellular therapy should be considered during the ex vivo culture of hMSCs, particularly when a prolonged and extended propagation period is required.

Cellular senescence: molecular mechanisms, in vivo significance, and redox considerations

Cellular senescence is recognized as a critical cellular response to prolonged rounds of replication and environmental stresses. Its defining characteristics are arrested cell-cycle progression and the development of aberrant gene expression with proinflammatory behavior. Whereas the mechanistic events associated with senescence are generally well understood at the molecular level, the impact of senescence in vivo remains to be fully determined. In addition to the role of senescence as an antitumor mechanism, this review examines cellular senescence as a factor in organismal aging and age-related diseases, with particular emphasis on aberrant gene expression and abnormal paracrine signaling. Senescence as an emerging factor in tissue remodeling, wound repair, and infection is considered. In addition, the role of oxidative stress as a major mediator of senescence and the role of NAD(P)H oxidases and changes to intracellular GSH/GSSG status are reviewed. Recent findings indicate that senescence and the behavior of senescent cells are amenable to therapeutic intervention. As the in vivo significance of senescence becomes clearer, the challenge will be to modulate the adverse effects of senescence without increasing the risks of other diseases, such as cancer. The uncoupled relation between cell-cycle arrest and the senescent phenotype suggests that this is an achievable outcome.

Adult mesenchymal stromal stem cells for therapeutic applications

Mesenchymal stromal stem cells (MSC) can be found in almost any adult organ. They can be isolated and expanded within several weeks up to hundreds of millions of cells. The cell isolation based on the surface antigen expression may significantly enrich for the desired cell population and reduce the time required for cell expansion. MSC display a unique molecular signature which clearly discriminates them from other stem cell types. MSC can be differentiated into the cells of several lineages. Additionally, the unique biological properties of MSC are mediated by strong immunomodulatory activity and by paracrine mechanisms. Potential therapeutic applications of the cells require clinically compliant protocols for cell isolation and expansion. The therapeutic utility of MSC has been evaluated and found to be useful in several pre-clinical animal models as well as in clinical trials.

Telomere length in leukocytes correlates with bone mineral density and is shorter in women with osteoporosis

Telomere length decreases with age and is associated with osteoblast senescence. In 2,150 unselected women, leukocyte telomere length was significantly correlated with bone mineral density. Clinical osteoporosis was associated with shorter telomeres, suggesting that telomere length can be used as a marker of bone aging.

INTRODUCTION

The length of telomeres in proliferative cells diminishes with age. Telomere shortening and telomerase activity have been linked to in vitro osteoblast senescence and to increased secretion of pro-inflammatory cytokines. We explored whether bone mineral density correlates with telomere length in leukocytes.

MATERIALS AND METHODS

The relationship between leukocyte telomere length, bone mineral density (BMD) and osteoporosis (as defined by the World Health Organization) was examined in a cohort of 2,150 women from a population-based twin cohort aged 18-79.

RESULTS

After adjusting for age, body mass index, menopausal status, smoking, hormone replacement therapy status, telomere length was positively correlated with BMD of the spine (p < 0.005), forearm (p < 0.013), but not the femoral neck (p < 0.06). Longer telomeres were associated with reduced the risk of clinical OP at two or more sites (odds ratio = 0.594 95% CI 0.42-0.84 p < 0.003) and in women over the age of 50, clinical osteoporosis was associated with 117 bp shorter telomere length (p < 0.02) equivalent to 5.2 years of telomeric aging.

CONCLUSIONS

Shortened leukocyte telomere length is independently associated with a decrease in BMD and the presence of osteoporosis in women. Our data provide evidence that leukocyte telomere length could be a marker of biological aging of bone.

Donor variation and loss of multipotency during in vitro expansion of human mesenchymal stem cells for bone tissue engineering

The use of multipotent human mesenchymal stem cells (hMSCs) for tissue engineering has been a subject of extensive research. The donor variation in growth, differentiation and in vivo bone forming ability of hMSCs is a bottleneck for standardization of therapeutic protocols. In this study, we isolated and characterized hMSCs from 19 independent donors, aged between 27 and 85 years, and investigated the extent of heterogeneity of the cells and the extent to which hMSCs can be expanded without loosing multipotency. Dexamethasone-induced ALP expression varied between 1.2- and 3.7-fold, but no correlation was found with age, gender, or source of isolation. The cells from donors with a higher percentage of ALP-positive cells in control and dexamethasone-induced groups showed more calcium deposition than cells with lower percentage of ALP positive cells. Despite the variability in osteogenic gene expression among the donors tested, ALP, Collagen type 1, osteocalcin, and S100A4 showed similar trends during the course of osteogenic differentiation. In vitro expansion studies showed that hMSCs can be effectively expanded up to four passages (approximately 10-12 population doublings from a P0 culture) while retaining their multipotency. Our in vivo studies suggest a correlation between in vitro ALP expression and in vivo bone formation. In conclusion, irrespective of age, gender, and source of isolation, cells from all donors showed osteogenic potential. The variability in ALP expression appears to be a result of sampling method and cellular heterogeneity among the donor population.

Telomere length is paternally inherited and is associated with parental lifespan

Telomere length (TL) is emerging as a biomarker for aging and survival. To evaluate factors influencing this trait, we measured TL in a large homogeneous population, estimated the heritability (h(2)), and tested for parental effects on TL variation. Our sample included 356 men and 551 women, aged 18-92 years, from large Amish families. Mean TL in leukocytes was measured by quantitative PCR (mean: 6,198 +/- 1,696 bp). The h(2) of TL was 0.44 +/- 0.06 (P < 0.001), after adjusting for age, sex, and TL assay batch. As expected, TL was negatively correlated with age (r = -0.40; P < 0.001). There was no significant difference in TL between men and women, consistent with our previous findings that Amish men lived as long as Amish women. There was a stronger and positive correlation and association between TL in the offspring and paternal TL (r = 0.46, P < 0.001; beta = 0.22, P = 0.006) than offspring and maternal TL (r = 0.18, P = 0.04; beta = -0.02, P = 0.4). Furthermore, we observed a positive correlation and association between daughter's TL and paternal lifespan (r = 0.20, P < 0.001; beta = 0.21, P = 0.04), but not between daughter's TL and maternal lifespan (r = -0.01, beta = 0.04; both P = not significant). Our data, which are based on one of the largest family studies of human TL, support a link between TL and aging and lifespan and suggest a strong genetic influence, possibly via an imprinting mechanism, on TL regulation.

Telomeres

The role that telomere biology may play in the human ageing process is of a significant interest to many laboratories around the world. In this, the first of a series of yearly reviews on telomeres and ageing, I review a small selection of papers published between July 2005 and June 2006 that maybe of direct relevance to the gerontology research community.

Telomere configuration influences the choice of telomere maintenance pathways

Telomere maintenance is required for chromosome stability, and telomeres are typically replicated by the action of telomerase. In yeast cells that lack telomerase, telomeres are maintained by alternative type I and type II recombination mechanisms. Previous studies identified several proteins to control the choice between two types of recombinations. Here, we demonstrate that configuration of telomeres also plays a role to determine the fate of telomere replication in progeny. When diploid yeasts from mating equip with a specific type of telomeric structure in their genomes, they prefer to maintain this type of telomere replication in their descendants. While inherited telomere structure is easier to be utilized in progeny at the beginning stage, the telomeres in type I diploids can gradually switch to the type II cells in liquid culture. Importantly, the TLC1/tlc1 yeast cells develop type II survivors suggesting that haploid insufficiency of telomerase RNA component, which is similar to a type of dyskeratosis congenital in human. Altogether, our results suggest that both protein factors and substrate availability contribute to the choice among telomere replication pathways in yeast.

Long-term culture in dexamethasone unmasks an abnormal phenotype in osteoblasts isolated from osteoporotic subjects

We have shown that osteoblastic cells derived from trabecular bone explants of osteoporotic subjects (OP cells) exhibited an altered alkaline phosphatase (ALP) response to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] compared to control (CON) cells. Our hypothesis that OP cells have other intrinsic abnormalities was investigated using our cell models representing two different stages of differentiation. OP and CON cells were cultured in the absence (-DEX) or presence (+DEX) of 10 nM dexamethasone (DEX) in 10% fetal calf serum (FCS) prior to exposure to serum-free medium containing 1 nM of PTH and/or 17-beta estradiol (E2). Both OP and CON cells responded to DEX with a two-fold increase in basal ALP activity. While E2 or PTH+E2 had no effect on OP cells, both treatments inhibited ALP activity in CON cells (p<0.05). OP and CON cells grown in DEX also expressed PTH-stimulated adenylate cyclase (AC) activities higher than those of (-DEX) cells. OP+DEX cells, however, exhibited activities which were 8-fold higher than those of CON+DEX cells (p<0.001). In OP+DEX cells, E2 stimulated basal AC activity (p<0.05) but did not affect PTH-stimulated activity. In contrast, in CON+DEX cells, E2 had no effect on basal activity but inhibited PTH-stimulated AC activity (p<0.001). Osteocalcin production was 4-fold lower in OP+DEX cells compared to OP-DEX and CON cells (p<0.05) while osteocalcin mRNA levels were significantly lower in OP+DEX and CON+/-DEX cells compared to OP-DEX cells (p<0.05). E2 did not affect osteocalcin protein or mRNA levels in either OP or CON cells. No differences in mRNA levels were found for estrogen receptor-alpha (ER-a) in OP+/-DEX cells whereas these levels were significantly higher in CON+DEX compared to CON-DEX cells (p<0.05). These results indicate that DEX amplified the differences between OP and CON cells and confirm the presence of intrinsic osteoblastic abnormalities in patients with osteoporosis that persist in culture.

Telomere length versus hormonal and bone mineral status in healthy elderly men

Telomeres, the termini of linear chromosomes, exert a key role in the process of cellular ageing. Progressive telomere shortening is implicated in senescence in vitro and ample evidence exists to support the hypothesis that telomere length is correlated with chronological age and ageing phenotypes in vivo. In this study, we assessed whether mean telomere length of peripheral blood leukocytes predicts age-associated bone loss and/or is related to sex steroid status in an elderly healthy male population (71-86 years). Out of this population, we selected 110 samples for telomere restriction fragment (TRF) length analysis. Fasting blood was analysed for testosterone, estradiol, sex hormone binding globulin and biochemical markers of bone turnover. Also, the bioavailable fractions of sex steroids were calculated. Bone mineral density was measured at baseline and longitudinal follow-up was available for 84 men. We found that mean TRF length was inversely correlated with age (r=-0.19; P=0.049). Although no correlations were found with sex steroids or BMD at baseline, age corrected mean TRF length was associated with longitudinal bone loss for different distal forearm sites (P<0.05). Further studies are required to confirm our results, yet in this study, the predictive value of telomere length for bone loss appears to be substantial, hence underscoring the role of telomere length as a biomarker of ageing phenotypes.

An Improved Method for Determining Telomere Length and Its Use in Assessing Age in Blood and Saliva

BACKGROUND

It is disputed whether telomeres shorten with age.

OBJECTIVE

The proposition of this study was to answer this question.

METHODS

The question was answered by a simple method consisting of a color reaction combined with long-term electrophoresis and weak voltage.

RESULTS

The telomeres shorten and the shortening is much higher than previously assumed.

CONCLUSION

The telomere theory of ageing and cancer is corroborated and for the first time it is possible to estimate age in blood or saliva.

Effects of Wnt signaling on proliferation and differentiation of human mesenchymal stem cells

Mesenchymal stem cells are pluripotent cells from bone marrow, which can be differentiated into the osteogenic, chondrogenic, and adipogenic lineages in vitro and are a source of cells in bone and cartilage tissue engineering. An improvement in current tissue-engineering protocols requires more detailed insight into the molecular cues that regulate the distinct steps of osteochondral differentiation. Because Wnt signaling has been widely implicated in mesenchymal differentiation, we analyzed the role of Wnt signaling in human mesenchymal stem cell (hMSC) biology by stimulation of the pathway with lithium chloride and Wnt3A-conditioned medium. We demonstrate a role for low levels of Wnt signaling in proliferation of uncommitted hMSCs and confirm that Wnt signaling controls osteoprogenitor proliferation. On the other hand, at high Wnt levels we observed a block in adipogenic differentiation and an increase in the expression of alkaline phosphatase, suggesting a role in the initiation of osteogenesis. The results of this study suggest that bone tissue engineering could benefit from the activation of critical levels of Wnt signaling at defined stages of differentiation. Moreover, our data suggest that hMSCs provide a valid in vitro model to study the role of Wnt signaling in mesenchymal biology.

Maintenance of differentiation potential of human bone marrow mesenchymal stem cells immortalized by human telomerase reverse transcriptase gene despite of extensive proliferation

Human bone marrow mesenchymal stem cells (hMSC) represent a population of stem cells that are capable of differentiation into multiple lineages. However, these cells exhibit senescence-associated growth arrest and phenotypic changes during long-term in vitro culture. We have recently demonstrated that overexpression of human telomerase reverse transcriptase (hTERT) in hMSC reconstitutes telomerase activity and extends life span of the cells [Nat. Biotechnol. 20 (2002) 592]. In the present study, we have performed extensive characterization of three independent cell lines derived from the parental hMSC-TERT cell line based on different plating densities during expansion in culture: 1:2 (hMSC-TERT2), 1:4 (hMSC-TERT4), and 1:20 (hMSC-TERT20). The 3 cell lines exhibited differences in morphology and growth rates but they all maintained the characteristics of self-renewing stem cells and the ability to differentiate into multiple mesoderm-type cell lineages: osteoblasts, adipocytes, chondrocytes, and endothelial-like cells over a 3-year period in culture. Also, surface marker studies using flow cytometry showed a pattern similar to that known from normal hMSC. Thus, telomerization of hMSC by hTERT overexpression maintains the stem cell phenotype of hMSC and it may be a useful tool for obtaining enough number of cells with a stable phenotype for mechanistic studies of cell differentiation and for tissue engineering protocols.

Lifetime high calcium intake increases osteoporotic fracture risk in old age

Caloric restriction prolongs life span. Calcium restriction may preserve bone health. In osteoporosis, bone mineral density (BMD) has significantly decreased, due to a lack of osteoblast bone formation. Traditional osteoporosis prevention is aimed at maximizing BMD, but the lifetime effects of continuously maintaining a high BMD on eventual bone health in old age, have not been studied. Strikingly, in countries with a high mean BMD, fracture rates in the elderly are significantly higher than in countries with a low mean BMD. Studies show that this is not based on genetic differences. Also, in primary hyperparathyroidism, on the brink of osteoporosis, BMD levels may be significantly higher than normal. Maybe, BMD does not represent long term bone health, but merely momentary bone strength. And maybe, maintaining a high BMD might actually wear out bone health. Since osteoporosis particularly occurs in the elderly, and because in osteoporotic bone less osteoblasts are available, the underlying process may have to do with ageing of osteoblastic cells. In healthy subjects, osteoblastic bone cells respond to the influx of calcium by composing a matrix upon which calcium precipitates. In the process of creating this matrix, 50-70% of the involved osteoblasts die. The greater the influx of calcium, the greater osteoblast activity, and the greater osteoblast apoptosis rate. An increased osteoblast apoptosis rate leads to a decrease in the age-related osteoblast replicative capacity (ARORC). In comparison to healthy bone, in osteoporotic bone the decrease in the replicative capacity of osteoblastic cells is greater. Due to the eventual resulting lack of osteoblast activity, micro-fractures cannot be repaired. Continuously maintaining a high BMD comes with continuously high bone remodeling rates, which regionally exhaust the ARORC, eventually leading to irreparable microfractures. Regarding long time influences on bone health, adequate estrogen levels are known to be protective against osteoporosis. This is generally attributed to its inhibiting influence on osteoclast activity. Instead, its net effects on osteoblast metabolism may be the key to osteoporosis prevention. Adequate estrogen levels inhibit osteoblast activity, calcium apposition and osteoblast apoptosis rate, preserving the ARORC.

CONCLUSION

Regarding osteoporosis prevention, ARORC better than BMD represents bone health. Regarding ARORC, adequate estrogen levels are protective, opposing the similar effects of hyperparathyroidism and a high calcium diet. Tests need to be performed in mice to assess the lifetime effects of a high versus a low calcium diet, on eventual bone fracture toughness.

Replicative senescence: a critical review

Human cells in culture have a limited proliferative capacity. After a period of vigorous proliferation, the rate of cell division declines and a number of changes occur in the cells including increases in size, in secondary lysosomes and residual bodies, nuclear changes and a number of changes in gene expression which provide biomarkers for senescence. Although human cells in culture have been used for over 40 years as models for understanding the cellular basis of aging, the relationship of replicative senescence to aging of the organism is still not clear. In this review, we discuss replicative senescence in the light of current information on signal transduction and mitogenesis, cell stress, apoptosis, telomere changes and finally we discuss replicative senescence as a model of aging in vivo.

Immortalization of human osteoblasts by transferring human telomerase reverse transcriptase gene

In the current study, in order to establish an immortalized osteoblast cell line, human mesenchymal stem cells (hMSCs) had been inducted into osteoblasts directionally by an osteo-inductive conditioned medium, then the osteoblasts were steadily transduced by a retroviral vector containing human telomerase reverse transcriptase (hTERT) gene. The expression of hTERT, the telomerase activity, the telomere lengths, the tumorigenesis and the osteogenesis characteristics of transduced cells at different population doublings (PDs) and the primary normal human osteoblast (hOB) were identified. The results demonstrated that hTERT gene had been transferred into human osteoblasts successfully; the transduced cell line-clone5 expressed telomerase activity and divided vigorously and now have undergone more than 120 PDs; The telomere length of clone5 elongated and was stable; Different eras of clone5 (PD 40 and PD 88) both expressed bone-specific markers, such as alkaline phosphatase (ALP), collagen type I, and osteopontin. And the quantitative assay of ALP activity showed that there were no significant differences among untransduced cells, PD 40 and PD 88 clone5 cells. Furthermore, the immortalized cell line was benign in nude mice tumor formation and soft agar colony formation assay.

Telomere Length in Leukocytes and Cultured Gingival Fibroblasts from Patients with Aggressive Periodontitis

BACKGROUND

The association of genetic risk factors with the pathogenesis of aggressive periodontitis (AgP) has been a focus of attention. Telomeres, which are nucleoprotein complexes at the ends of chromosomes, could be a genetic marker for Down's syndrome and Hutchinson-Gilford progeria, in which patients' premature aging is involved in the pathogenesis. It has been reported that these patients tend to experience severe periodontitis. Therefore, we investigated the telomere length of peripheral blood leukocytes (PBL) obtained from patients with AgP and that in the patients' gingival fibroblasts undergoing cellular aging in vitro.

METHODS

Twenty-one patients with AgP and 50 age-matched, periodontally healthy subjects (HS) participated in this study. Genomic DNA from PBL and from human gingival fibroblasts (HGF) was analyzed by Southern blotting for telomere length. The percentage of HGF positive for beta-galactosidase (beta-gal), a marker for cellular senescence, was also investigated.

RESULTS

There was no significant difference in the telomere length (P = 0.20, Student's t test) between the two groups, and wide interindividual variation was found (5.93 to 11.4 kbp, average 8.35 +/- 1.19 kbp). The telomere length from PBL negatively correlated with donor age, but no significant difference in telomere loss between the two groups was observed. With HGF undergoing aging in culture, the mean telomere length of these cells from six patients with AgP and seven HS decreased an average of -67.5 bp and -81.0 bp, respectively. No association was found in the telomere length between PBL and HGF from the same donors (r = 0.56, P = 0.20). A significant association was found between the telomere length and the percentages of beta-gal-positive HGF during cell passages (r = 0.70, P < 0.001).

CONCLUSIONS

These results indicate that patients with AgP do not have excessive telomere loss and thus do not support the notion of the occurrence of a generalized premature cellular aging in patients with AgP. Further studies are required to investigate the association between telomere length and beta-gal in HGF.

Pathogenesis of osteoporosis

There are many pathways that might lead to decreased bone mass, skeletal fragility, and increased fracture risk in osteoporosis. Some of these have been clearly identified, such as estrogen deficiency. Others that were conceived on the basis of experimental findings and recent scientific discoveries such as abnormalities of cytokines, bone growth factors, and osteoblast transcription factors remain interesting but speculative. The recent revolution in genomics and proteomics opens new avenues for pursuing in great depth the pathways leading to osteoporosis. Animal models developed largely in rodents can suggest specific factors that can be further studied in primate models and in osteoporotic patients. Identification of specific pathogenetic mechanisms should lead to new approaches to the diagnosis and management of this disorder.

From the Hayflick mosaic to the mosaics of ageing. Role of stress-induced premature senescence in human ageing

The Hayflick limit-senescence of proliferative cell types-is a fundamental feature of proliferative cells in vitro. Various human proliferative cell types exposed in vitro to many types of subcytotoxic stresses undergo stress-induced premature senescence (SIPS) (also called stress-induced premature senescence-like phenotype, according to the definition of senescence). The known mechanisms of appearance the main features of SIPS are reviewed: senescent-like morphology, growth arrest, senescence-related changes in gene expression, telomere shortening. Long before telomere-shortening induces senescence, other factors such as culture conditions or lack of 'feeder cells' can trigger either SIPS or prolonged reversible G(0) phase of the cell cycle. In vivo, 'proliferative' cell types of aged individuals are likely to compose a mosaic made of cells irreversibly growth arrested or not. The higher level of stress to which these cells have been exposed throughout their life span, the higher proportion of the cells of this mosaic will be in SIPS rather than in telomere-shortening dependent senescence. All cell types undergoing SIPS in vivo, most notably the ones in stressful conditions, are likely to participate in the tissular changes observed along ageing. For instance, human diploid fibroblasts (HDFs) exposed in vivo and in vitro to pro-inflammatory cytokines display biomarkers of senescence and might participate in the degradation of the extracellular matrix observed in ageing.

The disparity between human cell senescence in vitro and lifelong replication in vivo

Cultured human fibroblasts undergo senescence (a loss of replicative capacity) after a uniform, fixed number of approximately 50 population doublings, commonly termed the Hayflick limit. It has been long known from clonal and other quantitative studies, however, that cells decline in replicative capacity from the time of explantation and do so in a stochastic manner, with a half-life of only approximately 8 doublings. The apparent 50-cell doubling limit reflects the expansive propagation of the last surviving clone. The relevance of either figure to survival of cells in the body is questionable, given that stem cells in some renewing tissues undergo >1,000 divisions in a lifetime with no morphological sign of senescence. Oddly enough, these observations have had little if any effect on general acceptance of the Hayflick limit in its original form. The absence of telomerase in cultured human cells and the shortening of telomeres at each population doubling have suggested that telomere length acts as a mitotic clock that accounts for their limited lifespan. This concept assumed an iconic character with the report that ectopic expression of telomerase by a vector greatly extended the lifespan of human cells. That something similar might occur in vivo seemed consistent with initial reports that most human somatic tissues lack telomerase activity. More careful study, however, has revealed telomerase activity in stem cells and some dividing transit cells of many renewing tissues and even in dividing myocytes of repairing cardiac muscle. It now seems likely that telomerase is active in vivo where and when it is needed to maintain tissue integrity. Caution is recommended in applying telomerase inhibition to kill telomerase-expressing cancer cells, because it would probably damage stem cells in essential organs and even increase the likelihood of secondary cancers. The risk may be especially high in sun-exposed skin, where there are usually thousands of p53-mutant clones of keratinocytes predisposed to cancer.

Telomerase expression extends the proliferative life-span and maintains the osteogenic potential of human bone marrow stromal cells

Human bone marrow stromal cells (hMSCs) were stably transduced by a retroviral vector containing the gene for the catalytic subunit of human telomerase (hTERT). Transduced cells (hMSC-TERTs) had telomerase activity, and the mean telomere length was increased as compared with that of control cells. The transduced cells have now undergone more than 260 population doublings (PD) and continue to proliferate, whereas control cells underwent senescence-associated proliferation arrest after 26 PD. The cells maintained production of osteoblastic markers and differentiation potential during continuous subculturing, did not form tumors, and had a normal karyotype. When implanted subcutaneously in immunodeficient mice, the transduced cells formed more bone than did normal cells. These results suggest that ectopic expression of telomerase in hMSCs prevents senescence-associated impairment of osteoblast functions.

Evidence of a normal mean telomere fragment length in patients with Ullrich-Turner syndrome

Clinical and epidemiological studies suggest that premature ageing and increased morbidity and mortality is present in Ullrich-Turner syndrome. We studied telomere restriction fragment length (TRFL) in 30 women with Ullrich-Turner syndrome and 30 age-matched control women. All Turner women had the 45,X karyotype verified by karyotyping. We found no difference in the mean TRFL in the young age group (TS: 7011+/-521 vs C: 7285+/-917 bp, P = 0.3), or in the older age group (TS: 7357+/-573 vs C: 7221+/-621 bp, P = 0.6). In conclusion, our data suggest that Ullrich-Turner syndrome is not associated with excessive telomere loss, at least when studied in peripheral blood leucocytes, and thus quite different from other premature ageing syndromes.

Experimental gerontology in the Nordic countries

Research in geriatric medicine developed in the Nordic countries in the 1950s, following the tradition from the United Kingdom. Quite early, longitudinal epidemiological studies of 'normal' ageing emerged. Now there are chairs in geriatric medicine at many of the medical schools. Experimental gerontology came much later, typically scattered in a variety of medical school departments. There is only one chair in gerontology (in Tampere). Two major research undertakings have emerged in recent years, the Danish Centre for Molecular Gerontology, and a cluster of research groups at the Division of Geriatrics at the Karolinska Institutet. Other research groups are found in Denmark at the universities in Aarhus, Copenhagen and Odense; in Finland at the universities in Jyväskylä, Kuopio, Tampere and Turku; and in Norway at the university in Trondheim. These activities are reviewed country-wise.

Replicative senescence in normal liver, chronic hepatitis C, and hepatocellular carcinomas

There is growing evidence that senescent cells accumulate in vivo and are associated with the aging process in parallel with the progressive erosion of telomeres. Because recent data show that telomere shortening is involved in the pathogenesis of liver cirrhosis, we looked for replicative senescence cells in normal livers, chronic hepatitis C, and hepatocellular carcinoma (HCC). Replicative senescent cells were detected on liver tissue cryosections using expression of a specific marker, senescence-associated beta-galactosidase, a cytoplasmic enzyme detected at pH 6. A total of 57 frozen liver samples (15 normal liver, 32 chronic hepatitis C, and 10 HCCs) were studied. Replicative senescence was graded as absent in 56% of cases (32 of 57) and present in 44% (25 of 57). Replicative senescence was considered present in 3 of 15 normal livers (20%), 16 of 32 chronic hepatitis cases (50%), and 6 of 10 HCCs (60%). In the group of nontumoral livers, the presence of senescent cells in liver was associated with older age (P =.03). In the group with chronic hepatitis C, fibrosis stage, but not activity grade, was significantly correlated with the accumulation of replicative senescent cells (P <.001). Finally, beta-Gal staining in nontumoral tissue was strongly correlated with the presence of HCC in the surrounding liver (P <.001). These results suggest that chronic hepatitis C represents a relevant model of accelerated replicative senescence and that accumulation of replicative senescent cells predispose to HCC development. Detection of replicative senescent cells may then serve as a predictive marker of a hepatocellular carcinoma in the surrounding tissue. HUM PATHOL 32:327-332.

Translating basic aging research into geriatric health care

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

Treatment with 1,25-dihydroxyvitamin D3 reduces impairment of human osteoblast functions during cellular aging in culture

Adequate responses to various hormones, such as 1,25-dihydroxyvitamin D(3) (calcitriol) are a prerequisite for optimal osteoblast functions. We have previously characterized several human diploid osteoblastic cell lines that exhibit typical in vitro aging characteristics during long-term subculturing. In order to study in vitro age-related changes in osteoblast functions, we compared constitutive mRNA levels of osteoblast-specific genes in early-passage (< 50% lifespan completed) with those of late-passage cells (> 90% lifespan completed). We found a significant reduction in mRNA levels of alkaline phosphatase (AP: 68%), osteocalcin (OC: 67%), and collagen type I (ColI: 76%) in in vitro senescent late-passage cells compared to early-passage cells, suggesting an in vitro age-related impairment of osteoblast functions. We hypothesized that decreased osteoblast functions with in vitro aging is due to impaired responsiveness to calcitriol known to be important for the regulation of biological activities of the osteoblasts. Thus, we examined changes in vitamin D receptor (VDR) system and the osteoblastic responses to calcitriol treatment during in vitro osteoblast aging. We found no change in the amount of VDR at either steady state mRNA level or protein level with increasing in vitro osteoblast age and examination of VDR localization, nuclear translocation and DNA binding activity revealed no in vitro age-related changes. Furthermore, calcitriol (10(-8)M) treatment of early-passage osteoblastic cells inhibited their proliferation by 57 +/- 1% and stimulated steady state mRNA levels of AP (1.7 +/- 0.1-fold) and OC (1.8 +/- 0.2-fold). Similarly, calcitriol treatment increased mRNA levels of AP (1.7 +/- 0.2-fold) and OC (3.0 +/- 0.3-fold) in late-passage osteoblastic cells. Thus, in vitro senescent osteoblastic cells maintain their responsiveness to calcitriol and some of the observed in vitro age-related decreases in biological markers of osteoblast functions can be reverted by calcitriol treatment.

Cloning Adult Animals - What is the Genetic Age of the Clones?

The rapid progress in cloning research along with its many ramifications will soon have a significant beneficial impact on basic research, agriculture, and biomedicine. However, for the nuclear transfer technology to reach its fullest potential, it is important to understand whether the cloning procedure can reverse cellular aging and generate clones with normal genetic and physiological age, similar to those produced from natural reproduction. Telomere shortening is believed to correlate with cellular aging both in vitro and in vivo. Telomere lengths in cells of cloned individuals thus may reflect their genetic age. However, controversies have developed over whether the eroded telomere in somatic cells used for nuclear transfer can be restored during the cloning process.

Decreased cellular activity and replicative capacity of osteoblastic cells isolated from the periarticular bone of rheumatoid arthritis patients compared with osteoarthritis patients

OBJECTIVE

Periarticular osteopenia is frequently observed in rheumatoid arthritis (RA). Bone loss has been considered to be at least partly due to inadequate bone formation, which in turn, is largely dependent on the number of osteoblasts and the osteoblastic activity. Normal human somatic cells undergo a finite number of cell divisions and ultimately enter a nondividing state called replicative senescence. It has been proposed that the telomere, the terminal sequence of chromosomes, is the mitotic clock that triggers senescence. In the present study, we sought to clarify the relationship between periarticular osteopenia and osteoblast replicative senescence in RA.

METHODS

We examined age-related changes in cellular activity (alkaline phosphatase activity, osteocalcin and C-terminal type I procollagen secretion, and cAMP response to parathyroid hormone), replicative capacity, and senescent cell expression in osteoblasts from periarticular bone samples obtained from 15 patients with RA and 15 age-matched patients with osteoarthritis (OA). Cellular replicative capacity was analyzed by the mean telomere length and in vitro remaining replicative lifespan of the cells.

RESULTS

In both OA and RA groups, the cell proliferation rate, the levels of osteoblastic markers, mean telomere length, and replicative lifespan in osteoblastic cells gradually decreased with the increasing age of the donor. The percentage of senescent osteoblastic cells in the periarticular bone increased with age in both groups, and the rate of expression of senescent cells was higher in RA patients than in age-matched OA patients. The osteoblastic activities and replicative capacity of osteoblastic cells from RA patients were lower than those from OA patients at any donor age. The age-related decreases in the osteoblastic activity and replicative capacity of osteoblastic cells from periarticular bone were greater in RA patients than in OA patients.

CONCLUSION

Our results suggest that osteoblast replicative senescence in periarticular bones occurs more rapidly with aging in RA than in OA patients and contributes to periarticular osteopenia in RA.

Changes in the insulin-like growth factor-system may contribute to in vitro age-related impaired osteoblast functions

Age-related bone loss is thought to be due to impaired osteoblast functions. Insulin-like growth factors (IGFs) have been shown to be important stimulators of bone formation and osteoblast activities in vitro and in vivo. We tested the hypothesis that in vitro osteoblast senescence is associated with changes in components of the IGF-system including IGF-I, IGF-II, IGF-binding proteins (IGFBPs) and IGFBP-specific proteases. We employed a human diploid osteoblast cell line obtained from trabecular bone explants and that exhibit typical characteristics of in vitro senescence during serial subculturing. Using a non-competitive reverse-transcriptase polymerase-chain reaction (RT-PCR) assay, we found that the constitutive level of IGF-I mRNA decreased progressively to 49.9 +/- 4.9% in old osteoblasts as compared to the levels found in the young cells. No age-related change was found in IGF-II steady-state mRNA levels. Changes in IGFBPs gene expression and protein production were assessed using Northern blot analysis and Western ligand blotting (WLB), respectively. IGFBP-3 mRNA levels decreased to 30% and protein production to 16% in aged osteoblasts as compared to levels found in young cells. We also found age-related decreases in mRNA levels of both IGFBP-4 and IGFBP-5 to 70% and 60% in aged osteoblasts, respectively, compared to young cells. While IGFBP-5 protein was not detected by WLB, IGFBP-4 protein production showed a biphasic change with 50% decrease in middle-aged cells and a subsequent increase in aged osteoblasts to levels similar to those in young osteoblasts. We found an age-related increase in the immunoreactive levels of IGFBP-4 protease, however, no detectable IGFBP-4 or IGFBP-3 protease activities in conditioned media from osteoblast cultures were observed. Our findings demonstrate that osteoblast aging is associated with impaired production of the stimulatory components of the IGF-system, that may be a mechanism contributing to age-related decline in osteoblast functions.

Gene-specific DNA repair of pyrimidine dimers does not decline during cellular aging in vitro

A large number of studies have demonstrated that various kinds of DNA damage accumulate during aging and one of the causes for this could be a decrease in DNA repair capacity. However, the level of total genomic repair has not been strongly correlated with aging. DNA repair of certain kinds of damage is known to be closely connected to the transcription process; thus, we chose to investigate the level of gene-specific repair of UV-induced damage using in vitro aging of human diploid skin fibroblasts and trabecular osteoblasts as model systems for aging. We find that the total genomic repair is not significantly affected during cellular aging of cultures of both human skin fibroblasts and trabecular osteoblasts. Gene-specific repair was analyzed during cellular aging in the dihydrofolate reductase housekeeping gene, the p53 tumor suppressor gene, and the inactive region X(754). There was no clear difference in the capacity of young and old cells to repair UV-induced pyrimidine dimers in any of the analyzed genes. Thus, in vitro senescent cells can sustain the ability to repair externally induced damage.

CBFA1 and topoisomerase I mRNA levels decline during cellular aging of human trabecular osteoblasts

In order to understand the reasons for age-related impairment of the function of bone forming osteoblasts, we have examined the steady-state mRNA levels of the transcription factor CBFA1 and topoisomerase I during cellular aging of normal human trabecular osteoblasts, by the use of semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR). There is a progressive and significant reduction of the CBFA1 steady-state mRNA level down to 50% during cellular aging of human osteoblasts. In comparison to the normal cells, human osteosarcoma cell lines SaOS-2 and KHOS/NP, and the SV40-transformed human lung fibroblast cell line MRC5V2 have 20 to 40% higher levels of CBFA1 mRNA. Similar levels of CBFA1 mRNA are detectable in normal human skin fibroblasts, and these cells also exhibit an age-related decline to the same extent. In addition, the expression of topoisomerase I is reduced by 40% in senescent osteoblasts, and the mRNA levels are significantly higher (40-70%) in transformed osteoblasts and fibroblasts. These changes in gene expression may be among the causes of impaired osteoblast functions, resulting in reduced bone formation during aging.

Studies of the molecular mechanisms in the regulation of telomerase activity

Telomerase, a specialized RNA-directed DNA polymerase that extends telomeres of eukaryotic chromosomes, is repressed in normal human somatic cells but is activated during development and upon neoplasia. Whereas activation is involved in immortalization of neoplastic cells, repression of telomerase permits consecutive shortening of telomeres in a chromosome replication-dependent fashion. This cell cycle-dependent, unidirectional catabolism of telomeres constitutes a mechanism for cells to record the extent of DNA loss and cell division number; when telomeres become critically short, the cells terminate chromosome replication and enter cellular senescence. Although neither the telomere signaling mechanisms nor the mechanisms whereby telomerase is repressed in normal cells and activated in neoplastic cells have been established, inhibition of telomerase has been shown to compromise the growth of cancer cells in culture; conversely, forced expression of the enzyme in senescent human cells extends their life span to one typical of young cells. Thus, to switch telomerase on and off has potentially important implications in anti-aging and anti-cancer therapy. There is abundant evidence that the regulation of telomerase is multifactorial in mammalian cells, involving telomerase gene expression, post-translational protein-protein interactions, and protein phosphorylation. Several proto-oncogenes and tumor suppressor genes have been implicated in the regulation of telomerase activity, both directly and indirectly; these include c-Myc, Bcl-2, p21(WAF1), Rb, p53, PKC, Akt/PKB, and protein phosphatase 2A. These findings are evidence for the complexity of telomerase control mechanisms and constitute a point of departure for piecing together an integrated picture of telomerase structure, function, and regulation in aging and tumor development-Liu, J.-P. Studies of the molecular mechanisms in the regulation of telomerase activity.