Demonstration of cellular aging and senescence in serially passaged long-term cultures of human trabecular osteoblasts

Mesenchymal stem cells: biological characteristics and potential clinical applications

Mesenchymal stem cells (MSC) are clonogenic, non-hematpoietic stem cells present in the bone marrow and are able to differentiate into multiple mesoderm-type cell lineages, for example, osteoblasts, chondrocytes, endothelial-cells and also non-mesoderm-type lineages, for example, neuronal-like cells. Several methods are currently available for isolation of the MSC based on their physical and physico-chemical characteristics, for example, adherence to plastics or other extracellular matrix components. Because of the ease of their isolation and their extensive differentiation potential, MSC are among the first stem cell types to be introduced in the clinic. Several studies have demonstrated the possible use of MSC in systemic transplantation for systemic diseases, local implantation for local tissue defects, as a vehicle for genes in gene therapy protocols or to generate transplantable tissues and organs in tissue engineering protocols. Before their widespread use in therapy, methods allowing the generation of large number of cells without affecting their differentiation potential as well as technologies that overcome immunological rejection (in case allogenic transplantation) must be developed.

Mesenchymal stem cells: cell biology and potential use in therapy

Mesenchymal stem cells are clonogenic, non-haematopoietic stem cells present in the bone marrow and are able to differentiate into multiple mesoderm-type cell lineages e.g. osteoblasts, chondrocytes, endothelial-cells and also non-mesoderm-type lineages e.g. neuronal-like cells. Several methods are currently available for isolation of the mesenchymal stem cells based on their physical and immunological characteristics. Because of the ease of their isolation and their extensive differentiation potential, mesenchymal stem cells are among the first stem cell types to be introduced in the clinic. Recent studies have demonstrated that the life span of mesenchymal stem cells in vitro can be extended by increasing the levels of telomerase expression in the cells and thus allowing culture of large number of cells needed for therapy. In addition, it has been shown that it is possible to culture the cells in xeno-free environment without affecting their growth or differentiation potential. Finally, the mesenchymal stem cells seems to be hypoimmunogenic and thus allogenic mesenchymal stem cells transplantation is possible. It is envisaged that mesenchymal stem cells can be used in systemic transplantation for generalized diseases, local implantation for local tissue defects, as a vehicle for genes in gene therapy protocols or to generate transplantable tissues and organs in tissue engineering protocols. The results of these initial trials are very encouraging and several clinical trials are under way to study the efficacy and long-term safety of therapeutics based on mesenchymal stem cells.

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.

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.

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.

Smooth muscle cells and the pathogenesis of cerebral microvascular disease ("angiomyopathies")

Many forms of human cerebral microvascular disease result from abnormal proliferation and/or degeneration of smooth muscle cells (SMC) in the vessel wall of arteries and arterioles. Human cerebral microvessel-derived smooth muscle cells (MV-SMC) in culture can be used to study the pathogenesis of microvascular disease. Primary cultures were established from nonneoplastic human brain specimens surgically resected and characterized as to their growth properties and phenotype. The cultures have been used to study various factors that may be relevant in the pathogenesis of microangiopathies, in particular cerebral amyloid angiopathy (CAA), to help determine mechanisms of SMC degeneration in these disorders. Factors investigated have included cellular growth rate, response to hypoxia and amyloidogenic peptides, and telomerase activity. MV-SMC appear to behave differently than aortic SMC with regard to proliferation and telomerase activity. These differences may play a role in the responses to MV-SMC in the evolution of CAA and other microangiopathies (cerebral arteriosclerosis/lipohyalinosis) and provide insight into mechanisms of degeneration of these cells within vessel walls.

Cellular senescence and tissue aging in vivo

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

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.

In vitro osteogenic differentiation of rat bone marrow cells subcultured with and without dexamethasone

The aim of our study was to investigate the osteogenic potential of subcultured rat bone marrow cells. Rat bone marrow (RBM) cells were cultured with or without dexamethasone. Subsequently, osteogenic differentiation and expression was studied. When cells were cultured continuously in the presence of dexamethasone, cultures initially showed high alkaline phosphatase expression and abundant mineralization. Expression of differentiation markers decreased with passaging. After cells were passaged three times, no alkaline phosphatase activity and calcification were found. Primary cells cultured without dexamethasone showed low alkaline phosphatase and no calcification, and remained fibroblast-like. When these cells were subcultured in the presence of dexamethasone, the cells did show osteogenic differentiation. Nevertheless, this occurred at a significant lower level than with cells continuously cultured with dexamethasone. In addition, no differentiation was found after second passage. Our results indicate that subcultured undifferentiated RBM cells show osteogenic differentiation after addition of dexamethasone. Expression of alkaline phosphatase and mineralization is higher in cells continuously supplemented with dexamethasone. Still, even when dexamethasone is added continuously, RBM cells loose their osteogenic potential after several passages. Therefore, we conclude that subculture of undifferentiated rat bone marrow cells results in the loss of osteogenic potential of these cells.

Nicotinic regulation of c-fos and osteopontin expression in human-derived osteoblast-like cells and human trabecular bone organ culture

Long-term in vivo studies have highlighted smoking as a risk factor in postmenopausal osteoporosis, bone fracture incidence, and increased nonunion rates. In contrast, there are few data postulating the effects of smoking at the cellular level in human skeletal tissue. In this study, we present novel evidence demonstrating that the nicotinic receptor alpha4 subunit is present in human primary bone cells by using reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, we demonstrate direct cellular effects of nicotine on primary human bone cells and blockage of these effects with a nicotinic receptor antagonist, D-tubocurarine. Nicotine effects on cell proliferation were biphasic with toxic, antiproliferative effects at high levels of nicotine (>1 mmol/L) and stimulatory effects at very low levels (0.01-10 micromol/L) after 72 h. This nicotine-induced increase in cell proliferation was inhibited in a dose-dependent manner by the addition of D-tubocurarine. In addition, proliferation effects from low-level treatment correlated with an upregulation of expression of the AP-1 transcription factor, c-fos, within 1 h, which was blocked by incubation with D-tubocurarine. To determine in situ bone cell responses within their trabecular matrix, cores of human bone isolated from biopsies were perfused with 0.1 micromol/L nicotine for 24 h. Western analysis of proteins isolated from the cores highlighted an increase in osteopontin, a bone matrix protein implicated in regulating resorption, which was partially inhibited by the addition of D-tubocurarine. To conclude, our results suggest that nicotine has a direct effect on human bone cells in modulating proliferation, upregulation of the c-fos transcription factor, and the synthesis of the bone matrix protein, osteopontin.

Effect of L-lysine and L-arginine on primary osteoblast cultures from normal and osteopenic rats

A therapeutic role of amino acids L-lysine (Lys) and L-arginine (Arg) in osteoporosis and fracture healing was demonstrated previously by in vivo studies. In the present study, primary cultures of osteoblasts were used to investigate the effect of amino acids on gene expression (alkaline phosphatase activity, ALP; osteocalcin, OC; type I collagen), nitric oxide production (NO) and proliferation (MTT) of cells. Cells were isolated from the distal femurs of normal and osteopenic rats. Normal and osteopenic bone-derived cells were divided into four groups: control, Lys (0.587 mg/mL/d), Arg (0.625 mg/mL/d), and Lys + Arg (0.587 + 0.625 mg/mL/d). No evidence of differences between normal and osteopenic bone-derived cultures in basal conditions was observed. A significant (P = 0.002) increase of 10.4% in NO production was observed in normal bone-derived osteoblasts treated with Lys + Arg when compared to the control group at 7 days. At the same time, normal bone-derived osteoblasts treated with Arg and Lys + Arg showed significant increases in type I collagen synthesis of 25.3% and 28.4%, respectively, when compared to the control group. Osteopenic bone-derived osteoblasts showed significant (P = 0.002) increases of 27.6% in MTT and 28.7% in cell count at 48 hours when treated with Lys + Arg in comparison with the control group. At 7 days, NO production and type I collagen synthesis increased significantly (P< 0.005) both in osteopenic bone-derived osteoblasts treated with Arg (NO: 18.5%; type I collagen: 34.4%) and Lys + Arg (NO: 23.7%; type I collagen: 20.9%) compared to the control group. Finally, a significant (P = 0.025) decrease of 5.8% in OC level was observed in osteopenic bone-derived osteoblasts treated with Arg. Results suggest that the potential therapeutic effect of Lys and Arg on bone could be related, at least in part, to an improvement of NO production and type I collagen synthesis by osteoblasts both in normal and in osteopenic bone. In osteopenic bone-derived osteoblasts this synthetic phase is preceded by an initial increase of cell proliferation.

The effect of aging on distraction osteogenesis in the rat

The effect of age on bone formation in the limb lengthening model of distraction osteogenesis (DO) was investigated in two studies using Sprague-Dawley (SD) rats from two colonies at various ages (CAMM: 9 vs 24 months, Harlan: 4 vs 24 months). External fixators were placed on the right tibiae of 30 male SD rats (20 CAMM, 10 Harlan) and mid-diaphyseal osteotomies were performed. Distraction was performed at 0.2 mm bid for 20 days (CAMM) or 14 days (Harlan). The experimental (DO) and control (contra-lateral) tibiae were removed for high-resolution radiography and decalcified histology. Videomicroscopy was used to quantitate radiodensity, histology (matrix type) and relative areas of cell proliferation, which was identified by proliferating cell nuclear antigen (PCNA) immunochemistry. Both studies demonstrated an age-related decrease in the percent mineralized bone (radiodensity) in the distraction gap (CAMM 9 vs 24 months: 68% vs 51%, P < 0.003; Harlan 4 vs 24 months: 95% vs 36%, P < 0.001) and no significant colony or distraction time-specific difference was seen between the two colonies of 24-month-old rats. Histology was performed on the Harlan rats. The DO gaps in the 24-month-old rats demonstrated less endosteal new bone compared to the 4-month-old rats (P < 0.01), but equivalent periosteal new bone. In 4-month-old rats, PCNA-immunostained cells were organized along the primary matrix front (where the first deposition of osteoid occurs) extending across both periosteal and endosteal surfaces. In 24-month-old rats, PCNA+ cells were organized in zones along the periosteal new bone fronts only and irregularly scattered throughout the endosteal gap within a fibrovascular non-ossifying matrix. These results indicate that 24-month-old rats have a relative deficit in endosteal bone formation which may not be related to cell proliferation but rather to cell organization. This model reflects the clinical situation where radiographic findings in older patients demonstrate significant delays in mineralization during DO. We believe this model of DO in aged rats presents unique in vivo opportunities to test hypotheses concerning (1) the effects of aging on bone repair, (2) the effects of pharmacological agents on bone repair in a geriatric setting, and (3) to study the mechanisms underlying DO.

Telomere erosion and senescence in human articular cartilage chondrocytes

Aging and the degeneration of articular cartilage in osteoarthritis are distinct processes, but a strong association exists between age and the incidence and prevalence of osteoarthritis. We hypothesized that this association is due to in vivo replicative senescence, which causes age-related declines in the ability of chondrocytes to maintain articular cartilage. For this hypothesis to be tested, senescence-associated markers were measured in human articular chondrocytes from donors ranging in age from 1 to 87 years. These measures included in situ staining for senescence-associated beta-galactosidase activity, (3)H-thymidine incorporation assays for mitotic activity, and Southern blots for telomere length determinations. We found that senescence-associated beta-galactosidase activity increased with age, whereas both mitotic activity and mean telomere length declined. These findings indicate that chondrocyte replicative senescence occurs in vivo and support the hypothesis that the association between osteoarthritis and aging is due in part to replicative senescence. The data also imply that transplantation procedures performed to restore damaged articular surfaces could be limited by the inability of older chondrocytes to form new cartilage after transplantation.

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.

Effect of serial passage on gene expression in MC3T3-E1 preosteoblastic cells: a microarray study

The osteoblastic function of mouse preosteoblastic MC3T3-E1 cells, as measured by alkaline phosphatase activity and osteocalcin secretion, decreases after serial passage. To uncover genes responsible for decreased osteoblastic function in high-passage cells, we have studied passage-dependent change of gene expression in MC3T3-E1 cells. Changes in the expression pattern of 2000 selected genes were examined simultaneously by comparing mRNA levels between MC3T3-E1 cells at passage 20 and passage 60 using the cDNA microarray analysis. Significant changes in the steady-state abundance of 27 mRNAs were observed in response to different passage numbers, including 17 known genes, 4 ESTs with homology to known genes, and 6 genes with no previously described function or homology. Northern blot analysis was used to verify and quantify the expression of selected genes, and revealed a significant higher level of up- and down-regulation compared to microarray data. These results indicate the existence of a significant change in gene expression in osteoblastic cells undergoing serial passages. Such changes might be responsible for a reduction in bone regeneration in older osteoblasts. Potential roles of selected genes in bone aging are discussed.

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.

Involvement of Rb family proteins, focal adhesion proteins and protein synthesis in senescent morphogenesis induced by hydrogen peroxide

Early passage human diploid fibroblasts develop senescent morphology prematurely within a week after a 2-hour pulse treatment with low or mild dose H(2)O(2). We test here the role of cell cycle checkpoints, cytoskeletal proteins and de novo protein synthesis in senescent morphogenesis following H(2)O(2) treatment. H(2)O(2) treatment causes transient elevation of p53 protein and prolonged inhibition of Rb hyperphosphorylation. Expression of human papillomaviral E6 gene prevented elevation of p53 but did not affect senescent morphogenesis. Expression of human papillomaviral E7 gene reduced the level of Rb protein and prevented induction of senescent morphology by H(2)O(2). The mutants of the E7 gene, in which the Rb family protein binding site was destroyed, could not reduce Rb protein or prevent H(2)O(2) from inducing senescent morphology. Senescent-like cells showed enhanced actin stress fibers. In untreated cells, vinculin and paxillin preferentially distributed along the edge of the cells. In contrast, vinculin and paxillin distributed randomly and sporadically throughout senescent-like cells. E7 expression prevented enhancement of actin filament formation and redistribution of vinculin or paxillin. Neither wild-type nor E7 cells showed changes in the protein level of actin, vinculin or paxillin measured by western blot after H(2)O(2) treatment. Finally, depletion of methionine in the culture medium after H(2)O(2) treatment prevented senescent morphogenesis without affecting dephosphorylation of Rb protein. Our results suggest that senescent morphology likely develops by a program involving activated Rb family proteins, enhancement of actin stress fibers, redistribution of focal adhesion proteins and de novo protein synthesis.

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.

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.

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

We have compared the telomere length, as assessed by Southern analysis, of telomere restriction fragments (TRFs) generated by RsaI/HinfI digestion of genomic DNA in: (i) in vitro cultured human trabecular osteoblasts undergoing cellular aging; and (ii) peripheral blood leukocytes (PBL) obtained from three groups of women: young (aged 20-26 years, n = 15), elderly (aged 48-85 years, n = 15) and osteoporotic (aged 52-81 years, n = 14). The mean TRF length in human osteoblasts undergoing aging in vitro decreased from an average of 9.32 kilobasepairs (kb) in middle-aged cells to an average of 7.80 kb in old cells. The rate of TRF shortening was about 100 bp per population doubling, which is similar to what has been reported for other cell types, such as human fibroblasts. Furthermore, there was a 30% decline in the total amount of telomeric DNA in senescent osteoblasts as compared with young cells. In the case of PBL, TRF length in the DNA extracted from young women was slightly longer (6.76 +/- 0.64 kb) than that from a group of elderly women (6.42 +/- 0.71 kb). A comparison of TRFs in the DNA extracted from the PBL from osteoporotic patients and from age-matched controls did not show any significant differences (6.47 +/- 0.94 versus 6.42 +/- 0.71 kb, respectively). Therefore, using TRF length as a marker for cellular aging in vitro and in vivo, our data comparing TRFs from osteoporotic patients and age-matched controls do not support the notion of the occurrence of a generalized premature cellular aging in osteoporotic patients.

Cloning and identification of genes that associate with mammalian replicative senescence

Cellular senescence and limited proliferative capacity of normal diploid cells has a dominant phenotype over immortality of cancerous cells, suggesting its regulation by the expression of a set of genes. In order to isolate the genes that associate with senescence, we have employed a clonal system of conditional SV40 T antigen rat embryo fibroblast cell lines which undergo senescence upon T antigen inactivation. Construction of cDNA libraries from two conditional cell lines and application of differential screening and subtractive hybridization techniques have resulted in the cloning of eight senescence-induced genes (SGP-2/Apo J, alpha 1-procollagen, osteonectin, fibronectin, SM22, cytochrome C oxidase, GTP-alpha, and a novel gene) and a senescence-repressed gene (FRS-2). Three of these genes encode for extracellular matrix proteins, others are involved in the calcium-dependent signal transduction pathways, while the SGP-2/Apo J gene may have a cellular protective function. RNA analysis has shown that the senescence-associated genes are overexpressed in both normal rat embryonic fibroblasts and human osteoblasts cell cultures undergoing aging in vitro. In comparison, the expression of these genes in a rat fibroblast immortalized cell line (208F cells) was down-regulated after both its partial and its full transformation by ras oncogenes. Thus, cloning of senescence-associated genes opens up new ways to elucidate and/or to modulate aging and cancer.