Cellular and molecular mechanisms of stress-induced premature senescence (SIPS) of human diploid fibroblasts and melanocytes

An inhibitor of cyclin-dependent kinase, stress-induced p21Waf-1/Cip-1, mediates hepatocyte mito-inhibition during the evolution of cirrhosis

During the evolution of cirrhosis, there is a relative decrease in volume percentage of hepatocytes and a relative increase in biliary epithelial cells and myofibroblasts. This is recognized histopathologically as a ductular reaction and leads to gradual distortion of the normal hepatic architecture. The final or decompensated stage of cirrhosis is characterized by a further decline in hepatocyte proliferation and loss of functional liver mass that manifests clinically as ascites, encephalopathy, and other signs of liver failure. In this report, we tested the hypothesis that p21-mediated hepatocyte mito-inhibition accelerates the evolution of cirrhosis using an established mouse model of decompensated biliary cirrhosis, p21-deficient mice, and liver tissue from humans awaiting liver replacement. Despite the same insult of long-term (12-week) bile duct ligation, mice prone to decompensation showed significantly more oxidative stress and hepatocyte nuclear p21 expression, which resulted in less hepatocyte proliferation, an exaggerated ductular reaction, and more advanced disease compared with compensation-prone controls. Mice deficient in p21 were better able than wild-type controls to compensate for long-term bile duct ligation because of significantly greater hepatocyte proliferation, which led to a larger liver mass and less architectural distortion. Mito-inhibitory hepatocyte nuclear p21 expression in humans awaiting liver replacement directly correlated with pathological disease stage and model of end-stage liver disease scoring. In conclusion, stress-induced upregulation of hepatocyte p21 inhibits hepatocyte proliferation during the evolution of cirrhosis. These findings have implications for understanding the evolution of cirrhosis and associated carcinogenesis. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

The impact of telomere erosion on memory CD8+ T cells in patients with X-linked lymphoproliferative syndrome

Patients with X-linked lymphoproliferative syndrome (XLP) experience excessive T cell proliferation after primary Epstein-Barr virus (EBV) infection, due to mutations in the signalling lymphocyte activation molecule (SLAM) associated protein (SAP) molecule. We examined the impact of dysfunctional proliferative control on the extent of CD8+ T cell differentiation in XLP patients who recovered from primary EBV infection. Although these young patients have normal numbers of lytic and latent EBV-epitope-specific CD8+ T cells, they were extremely differentiated as defined by loss of CCR7 and CD27, low telomerase activity and very short telomeres. This was not a direct effect arising from the loss of SAP, but was due to excessive T cell stimulation due to this defect. Thus, transduction of XLP CD8+ T cells with the catalytic component of telomerase (hTERT), but not SAP, prevented telomere loss and considerably extended proliferative lifespan in vitro. These results indicate that excessive proliferation in CD8+ T cells in XLP patients may lead to end-stage differentiation and loss of functional EBV-specific CD8+ T cells through replicative senescence. This may contribute to the defective immunity found in XLP patients who survive acute EBV infection who develop EBV-related B cell lymphomas before the fourth decade of life.

Human papillomavirus oncoprotein E7 targets the promyelocytic leukemia protein and circumvents cellular senescence via the Rb and p53 tumor suppressor pathways

Cellular senescence can be triggered by a variety of signals, including loss of telomeric integrity or intense oncogenic signaling, and is considered a potent, natural tumor suppressor mechanism. Previously, it was shown that the promyelocytic leukemia protein (PML) induces cellular senescence when overexpressed in primary human fibroblasts. The mechanism by which the PML IV isoform elicits this irreversible growth arrest is believed to involve activation of the tumor suppressor pathways p21/p53 and p16/Rb; however, a requirement for either pathway has not been demonstrated unequivocally. To investigate the individual contributions of p53 and Rb to PML-induced senescence, we used oncoproteins E6 and E7 from human papillomaviruses (HPVs), which predominantly target p53 and Rb. We show that E7, but not E6, circumvents PML-induced senescence. Using different E7 mutant proteins, dominant negative cyclin-dependent kinase 4, and p16 RNA interference, we demonstrate that Rb-related and Rb-independent mechanisms of E7 are necessary for subversion of PML-induced senescence and we identify PML as a novel target for E7. Interaction between E7 and a functional prosenescence complex composed of PML, p53, and CBP perturbs transcriptional activation of p53, thus highlighting a significant effect also on the p53 tumor suppressor pathway. Given the importance of HPV in the pathogenesis of cervical cancer, our results warrant a more detailed analyses of PML in HPV infections.

Potential involvement of oxidative stress in cartilage senescence and development of osteoarthritis: oxidative stress induces chondrocyte telomere instability and downregulation of chondrocyte function

Oxidative stress leads to increased risk for osteoarthritis (OA) but the precise mechanism remains unclear. We undertook this study to clarify the impact of oxidative stress on the progression of OA from the viewpoint of oxygen free radical induced genomic instability, including telomere instability and resulting replicative senescence and dysfunction in human chondrocytes. Human chondrocytes and articular cartilage explants were isolated from knee joints of patients undergoing arthroplastic knee surgery for OA. Oxidative damage and antioxidative capacity in OA cartilage were investigated in donor-matched pairs of intact and degenerated regions of tissue isolated from the same cartilage explants. The results were histologically confirmed by immunohistochemistry for nitrotyrosine, which is considered to be a maker of oxidative damage. Under treatment with reactive oxygen species (ROS; 0.1 μmol/l H₂O₂) or an antioxidative agent (ascorbic acid: 100.0 μmol/l), cellular replicative potential, telomere instability and production of glycosaminoglycan (GAG) were assessed in cultured chondrocytes. In tissue cultures of articular cartilage explants, the presence of oxidative damage, chondrocyte telomere length and loss of GAG to the medium were analyzed in the presence or absence of ROS or ascorbic acid. Lower antioxidative capacity and stronger staining of nitrotyrosine were observed in the degenerating regions of OA cartilages as compared with the intact regions from same explants. Immunostaining for nitrotyrosine correlated with the severity of histological changes to OA cartilage, suggesting a correlation between oxidative damage and articular cartilage degeneration. During continuous culture of chondrocytes, telomere length, replicative capacity and GAG production were decreased by treatment with ROS. In contrast, treatment with an antioxidative agent resulted in a tendency to elongate telomere length and replicative lifespan in cultured chondrocytes. In tissue cultures of cartilage explants, nitrotyrosine staining, chondrocyte telomere length and GAG remaining in the cartilage tissue were lower in ROS-treated cartilages than in control groups, whereas the antioxidative agent treated group exhibited a tendency to maintain the chondrocyte telomere length and proteoglycan remaining in the cartilage explants, suggesting that oxidative stress induces chondrocyte telomere instability and catabolic changes in cartilage matrix structure and composition. Our findings clearly show that the presence of oxidative stress induces telomere genomic instability, replicative senescence and dysfunction of chondrocytes in OA cartilage, suggesting that oxidative stress, leading to chondrocyte senescence and cartilage ageing, might be responsible for the development of OA. New efforts to prevent the development and progression of OA may include strategies and interventions aimed at reducing oxidative damage in articular cartilage.

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

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

Mild heat stress stimulates 20S proteasome and its 11S activator in human fibroblasts undergoing aging in vitro

Repeated mild heat shock (RMHS) has been shown to have several beneficial hormetic effects on human skin fibroblast undergoing aging in vitro. Because an age-related decline in proteasome activity is 1 of the reasons for the accumulation of abnormal proteins during aging, we have investigated the effects of RMHS on the 20S proteasome, which is the major proteolytic system involved in the removal of abnormal and oxidatively damaged proteins. Serially passaged human skin fibroblasts exposed to RMHS at 41 degrees C for 60 minutes twice a week had increased 3 proteasomal activities by 40% to 95% in early- and midpassage cultures. RMHS-treated cells also contained a 2-fold higher amount of the proteasome activator 11S, and the extent of the bound activator was double in early- and midpassage cells only. Furthermore, there was no difference in the content of the 19S proteasome regulator in the stressed and the unstressed cells. Therefore, RMHS-induced proteasome stimulation in early- and midpassage fibroblasts appears to be due to an induction and enhanced binding of 11S proteasome activators. In contrast to this, the proteasomal system in late-passage senescent cells appears to be less responsive to the stimulatory effects of mild heat shock.

Apoptosis in stress-induced and spontaneously senescent human fibroblasts

Although apoptosis has been shown in vivo to be involved in the aging process, in vitro studies of age-dependent apoptosis are limited. In this study, apoptosis was examined in normal human fibroblasts exposed to H(2)O(2) to induce premature senescence and in spontaneously senescent human fibroblasts. Although apoptosis was not observed for several days after exposure to H(2)O(2), morphological changes indicating apoptosis were evident in about 5% of cells 7 days after exposure to 80microM H(2)O(2), concomitantly with expression of senescent phenotype. The apoptotic changes were preceded by caspase activation in majority of the exposed cells. As well as H(2)O(2)-induced senescent cells, spontaneously senescent human fibroblasts showed apoptotic changes in about 2% of cells and majority of the senescent cells also showed activation of caspases. The results indicate that the apoptosis pathway is activated in H(2)O(2)-induced and spontaneously senescent human fibroblasts in vitro.

Effects of donor age on proteasome activity and senescence in trabecular meshwork cells

The mechanisms involved in the progressive malfunction of the trabecular meshwork (TM) in glaucoma are not yet understood. To study age-related changes in human TM cells, we isolated primary TM cell cultures from young (ages 9, 14, and 25) and old (ages 66, 70, and 73) donors, and compared levels of oxidized proteins, autofluorescence, proteasome function, and markers for cellular senescence. TM cells from old donors showed a 3-fold increase in oxidized proteins and a 7.5-fold decrease of proteasome activity. Loss of proteasome function was not associated with decreased proteasome content but with partial replacement of the proteolytic subunit PSMB5 with the inducible subunit LMP7. Cells from old donors also demonstrated features characteristic of cellular senescence associated with phosphorylation of p38MAPK but only a modest increase in p53. These data suggest that age-related proteasome inhibition and cellular senescence could contribute to the pathophysiological alterations of the TM in glaucoma.

Human polynucleotide phosphorylase (hPNPaseold-35): a potential link between aging and inflammation

Chronic inflammation is a characteristic feature of aging, and the relationship between cellular senescence and inflammation, although extensively studied, is not well understood. An overlapping pathway screen identified human polynucleotide phosphorylase (hPNPase(old-35)), an evolutionary conserved 3',5'-exoribonuclease, as a gene up-regulated during both terminal differentiation and cellular senescence. Enhanced expression of hPNPase(old-35) via a replication-incompetent adenovirus (Ad.hPNPase(old-35)) in human melanoma cells and normal human melanocytes results in a characteristic senescence-like phenotype. Reactive oxygen species (ROS) play a key role in the induction of both in vitro and in vivo senescence. We now document that overexpression of hPNPase(old-35) results in increased production of ROS, leading to activation of the nuclear factor (NF)-kappaB pathway. Ad.hPNPase(old-35) infection promotes degradation of IkappaBalpha and nuclear translocation of NF-kappaB and markedly increases binding of the transcriptional activator p50/p65. The generation of ROS and activation of NF-kappaB by hPNPase(old-35) are prevented by treatment with a cell-permeable antioxidant, N-acetyl-l-cysteine. Infection with Ad.hPNPase(old-35) enhances the production of interleukin (IL)-6 and IL-8, two classical NF-kappaB-responsive cytokines, and this induction is inhibited by N-acetyl-l-cysteine. A cytokine array reveals that Ad.hPNPase(old-35) infection specifically induces the expression of proinflammatory cytokines, such as IL-6, IL-8, RANTES, and matrix metalloproteinase (MMP)-3. We hypothesize that hPNPase(old-35) might play a significant role in producing pathological changes associated with aging by generating proinflammatory cytokines via ROS and NF-kappaB. Understanding the relationship between hPNPase(old-35) and inflammation and aging provides a unique opportunity to mechanistically comprehend and potentially intervene in these physiologically important processes.

Enhancement of Fas-mediated apoptosis in ageing human keratinocytes

Cellular senescence and apoptosis are two metabolically related and seemingly synergistic processes that are involved in tissue maintenance and homeostasis, anti-tumor protection, and age-related diseases. Despite this apparent co-operativity, senescence can inhibit apoptosis in certain conditions. Here, we describe senescence-apoptosis relationships in human epidermal cells by comparing apoptosis-related effector concentrations in keratinocyte cultures and epidermal skin cells at various stages of ageing. Using western blots, flow cytometry, enzyme-linked immuno-sorbent assay (ELISA) and immunofluorescence, we determined the amounts of apoptotic effectors in aged cells compared to young ones, in parallel with beta-galactosidase activity at neutral pH (senescence-associated beta-galactosidase, SA beta-gal), found to be a good indicator of cellular ageing. We observed increased levels of several Fas-mediated apoptosis effectors (Fas, Fas ligand, FADD, FLICE), both in cell cultures at advanced passages and in skin cells of aged donors (above 45 years). Furthermore, we found that while the pro-apoptotic p53 increased, the anti-apoptotic Bcl-2 declined. In spite of this, the extent of spontaneous apoptosis did not change in senescent keratinocyte cultures. The cells, however, became notably more susceptible to apoptosis when kept in exhausted growth medium, or upon Fas receptor activation by anti-Fas antibody binding. Our results are consistent with recent findings in senescent fibroblasts, showing that the death-signaling pathway is enhanced at senescence.

Nuclear accumulation of glycogen synthase kinase-3 during replicative senescence of human fibroblasts

Activation of the tumor suppressor protein p53 contributes to cellular senescence. As glycogen synthase kinase-3 (GSK3) was recently found to interact with p53 and contribute to the actions of p53, this study examined whether GSK3 accumulated in the nucleus and associated with p53 in senescent cells. Compared with young and middle-aged human WI-38 fibroblasts, senescent cells were found to contain increased nuclear levels of GSK3beta, and also tended to accumulate in the nucleus the other isoform of GSK3, GSK3alpha. Co-immunoprecipitation experiments demonstrated that GSK3beta and p53 formed a complex in the nucleus. Further experiments tested whether inhibition of GSK3 altered the development of senescence using long-term treatment with the selective GSK3 inhibitor lithium. Lithium treatment reduced the senescence-associated accumulation of p53 and caused cells to enter a reversible quiescent state. These results indicate that a portion of the p53 that is activated in senescent cells is modulated by its association with GSK3beta in the nucleus, an association that is known to facilitate the actions of p53 and that may contribute to senescence.

Chondrocyte senescence, joint loading and osteoarthritis

cellular level is not completely understood, but both aging and loading-induced stresses have been shown to undermine cell functions related to the maintenance and restoration of the cartilage matrix. Based on precedents set by studies of other age-related degenerative diseases, we have focused our laboratory work on senescence as the cause of age-dependent decline in chondrocytes and on the impact of excessive mechanical stresses in promoting senescence. We hypothesized that senescent chondrocytes accumulate with age in articular cartilage and we propose that excessive mechanical stress plays a role in this process by promoting oxidative damage in chondrocytes that ultimately causes them to senesce. To test this hypothesis, we measured cell senescence markers (beta-galactosidase expression, mitotic activity, and telomere length) in human articular cartilage chondrocytes, and determined the effects of chronic exposure to oxidative stress on chondrocyte growth and senescence. In addition, we measured the effects of abnormally high levels of mechanical shear stress on the release of oxidants in cartilage explants. We found that senescent chondrocytes accumulated with age in articular cartilage. In vitro studies showed that chronic oxidative stress caused by repeated exposure to peroxide, or by growth under superphysiologic oxygen tension caused chondrocyte populations to senesce prematurely, before extensive telomere erosion occurred. Mechanical shear stress applied to cartilage explants considerably increased the production of oxidants. These observations support the hypothesis that senescence accounts for age-related decline in chondrocyte function and indicate that mechanically induced oxidative damage plays a role in this process. This suggests that new efforts to prevent the development and progression of osteoarthritis should include strategies that slow the progression of chondrocyte senescence or replace senescent cells.

Aging theories of primary osteoarthritis: from epidemiology to molecular biology

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

Inhibition of phosphatidylinostol 3-kinase uncouples H2O2-induced senescent phenotype and cell cycle arrest in normal human diploid fibroblasts

Exposure of WI38 human diploid fibroblasts (HDFs) to hydrogen peroxide (H2O2) induced premature senescence. The senescent HDFs were permanently arrested and exhibited a senescent phenotype including enlarged and flattened cell morphology and increased senescence-associated beta-galactosidase (SA-beta-gal) activity. The induction of HDF senescence was associated with an activation of p53, increased expression of p21Cip1/WAF1, and hypophosphorylation of retinoblastoma protein (Rb), while no changes in the expression of p16Ink4a, p27Kip1, and p14Arf were observed. Exposure of WI38 cells to H2O2 also selectively activated phosphatidylinostol 3-kinase (PI3 kinase) and mitogen-activated protein kinase (MAPK) kinase (MEK), while no changes in p38 MAPK and Jun kinase (JNK) activities were observed. Selective inhibition of PI3 kinase activity with LY294002 abrogated H2O2-induced cell enlargement and flattened morphology and significantly attenuated the increase in SA-beta-gal activity, but did not affect H2O2-induced cell cycle arrest. In contrast, selective inhibition of MEK and p38 MAPK with PD98059 and SB203580, respectively, produced no significant effect on H2O2-induced senescent phenotype and cell cycle arrest. These findings demonstrate that expression of the senescent phenotype can be uncoupled from cell cycle arrest in prematurely senescent cells induced by H2O2 and does not contribute to the maintenance of permanent cell cycle arrest.

Telomere length mediates the effects of telomerase on the cellular response to genotoxic stress

Telomerase inhibition may be a novel anti-cancer strategy that can be used in combination with conventional therapies, such as DNA damaging agents. There are conflicting reports as to whether and to what extent telomerase and telomere length influence the sensitivity of cells to genotoxins. To understand the relationship between telomere length, telomerase expression, and sensitivity to genotoxic stress, we expressed the catalytic subunit of telomerase, hTERT, in human fibroblasts having different telomere lengths. We show that telomerase confers resistance to ionizing radiation, bleomycin, hydrogen peroxide, and etoposide only in cells with short, presumably near-dysfunctional, telomeres. This resistance depended on the ability of telomerase to elongate the short telomeres, and telomerase did not protect cells with long telomeres. Interestingly, although long telomeres had no effect on sensitivity to etoposide and bleomycin, they exacerbated sensitivity to hydrogen peroxide, supporting the idea that, compared to other types of DNA damage, telomeres are particularly vulnerable to oxidative damage. Our findings identify a mechanism and conditions under which telomerase and telomeres affect the response of human cells to genotoxic agents and may have important implications for anti-cancer interventions.

Inactivation of p16INK4a, with retention of pRB and p53/p21cip1 function, in human MRC5 fibroblasts that overcome a telomere-independent crisis during immortalization

Recent investigations, including our own, have shown that specific strains of fibroblasts expressing telomerase reverse transcriptase (hTERT) have an extended lifespan, but are not immortal. We previously demonstrated that hTERT-transduced MRC5 fetal lung fibroblasts (MRC5hTERTs) bypassed senescence but eventually succumbed to a second mortality barrier (crisis). In the present study, 67 MRC5hTERT clones were established by limiting dilution of a mass culture. Whereas 39/67 clones had an extended lifespan, all 39 extended lifespan clones underwent crisis. 11 of 39 clones escaped crisis and were immortalized. There was no apparent relationship between the fate of clones at crisis and the level of telomerase activity. Telomeres were hyperextended in the majority of the clones analyzed. There was no difference in telomere length of pre-crisis compared with post-crisis and immortal clones, indicating that hyperextended telomeres were conducive for immortalization and confirming that crisis was independent of telomere length. Immortalization of MRC5hTERT cells was associated with repression of the cyclin-dependent kinase inhibitor p16INK4a and up-regulation of pRB. However, the regulation of pRB phosphorylation and the response of the p53/p21cip1/waf1 pathway were normal in immortal cells subject to genotoxic stress. Overexpression of oncogenic ras failed to de-repress p16INK4a in immortal cells. Furthermore, expression of ras enforced senescent-like growth arrest in p16INK4a-positive, but not p16INK4a-negative MRC5hTERT cells. Immortal cells expressing ras formed small, infrequent colonies in soft agarose, but were non-tumorigenic. Overall, these results implicate the inactivation of p16INK4a as a critical event for overcoming telomere-independent crisis, immortalizing MRC5 fibroblasts and overcoming ras-induced premature senescence.

Replicative senescence enhances apoptosis induced by pemphigus autoimmune antibodies in human keratinocytes

We have recently shown that skin lesions of the autoimmune disease pemphigus vulgaris are associated with Fas-mediated apoptosis. Here, we describe the induction of the Fas-dependent apoptosis pathway in cultured keratinocytes by pemphigus vulgaris autoantibodies (PV-IgG), as seen from a variety of cellular, morphological and biochemical parameters. All apoptotic characters appear stronger and faster in aged cultures than in young, showing increased susceptibility of senescent keratinocytes to PV-IgG-mediated apoptotic death and culture lesions. Together with immunosenescence, this phenomenon may explain the late onset of pemphigus disease.

mRNA level of alpha-2-macroglobulin as an aging biomarker of human fibroblasts in culture

Cellular senescence is a well-established model system for studying the molecular basis of aging. To identify a reliable biomarker for cellular age and further study the gene expression of aging, we profiled the gene expression difference between aged and young cultured human embryonic lung fibroblasts by high-density complementary deoxyribonucleic acid (cDNA) arrays. Among the differentially expressed genes, alpha-2-macroglobulin (alpha(2)M) was selected for further study. Its gene expression level as a function of population doubling level (PDL) in cultured fibroblasts was determined by RT-PCR and northern hybridization. mRNA level of alpha(2)M showed a positive linear-correlation with cumulative PDL. Additional assays revealed that the levels of alpha(2)M increased in irreversible growth arrest induced by sublethal H(2)O(2), but not in quiescent state of cultured fibroblasts induced by serum-deprivation, and remained stable in Hela cells. These results suggest that mRNA level of alpha(2)M can be used as a biomarker of aging in cultured fibroblasts. mRNA level of alpha(2)M showed significant difference between newborn and old human leucocytes, which suggest that the mRNA level of alpha(2)M may be used as a biomarker of aging in vivo.

Expression of connective tissue growth factor, a biomarker in senescence of human diploid fibroblasts, is up-regulated by a transforming growth factor-beta-mediated signaling pathway

Molecular changes associated with cellular senescence in human diploid fibroblasts (HDF), IMR-90, were analyzed by two-dimensional differential proteome analysis. A high percentage of replicative senescent cells were positive for senescence-associated beta-galactosidase activity, and displayed elevated levels of p21 and p53 proteins. Comparison of early population doubling level (PDL) versus replicative senescent cells among the 1000 spots resolved on gels revealed that the signal intensities of six spots were increased fivefold, whereas those of four spots were decreased. Proteome analysis data demonstrated that connective tissue growth factor (CTGF) is an age-associated protein. Up-regulation of CTGF expression in senescent cells was further confirmed by Western blotting and RT-PCR. We postulate that CTGF expression is controlled, in part, by transforming growth factor-beta (TGF-beta), in view of the high levels of TGF-beta isoforms as well as type I and II receptors detected only in late PDL of HDF cells. To verify this hypothesis, we stimulated early PDL cells with TGF-beta1 as well as stress inducing agents such as hydrogen peroxide. As expected, CTGF expression and Smad protein phosphorylation were dramatically increased up to observed levels in normal replicative senescent cells. In vivo experiments disclosed that CTGF, pSmad, and p53 were constitutively expressed at basal levels in up to 18-month-old rat liver, and expression was significantly up-regulated in 24-month-old rat tissue. However, expression patterns were not altered at all periods examined in livers of caloric-restricted rats. In view of both in vitro and in vivo data, we propose that the TGF-beta/Smad pathway functions in the induction of CTGF, a novel biomarker protein of cellular senescence in human fibroblasts.

Alterations of senescence biomarkers in human cells by exposure to CrVI in vivo and in vitro

Heavy metals like CrVI, CdII, PbII and SnII have many applications in industry. They also represent a group of labour pollutants, as they are involved in several physiological disorders, such as carcinogenesis and various tissue dysfunctions. However, limited knowledge exists regarding their effects on ageing. In the current work we provide evidence that workers chronically exposed to CrVI have considerably reduced serum levels of the biomarker of senescence and cell survival, Apolipoprotein J/Clusterin (ApoJ/CLU). Moreover, we have found that both the degree and the time of exposure to CrVI associate negatively with ApoJ/CLU serum levels. To further examine whether CrVI directly affects cellular senescence we treated for 10 weeks two adult skin fibroblasts cultures as well as embryonic fibroblasts with a range of CrVI concentrations that approximate the values recorded in the blood circulation of exposed workers. Cellular treatment with a CrVI concentration that approximates the highest concentration in the blood was extremely toxic and nearly all cells died immediately after the first treatment. Interestingly, continuous treatment with a 10-fold lower CrVI concentration resulted in the induction of premature senescence. More specifically, treated cells were growth arrested, acquired an irregular shape, were positive to beta-galactosidase staining, accumulated oxidized proteins and over-expressed the cyclin-dependent kinase inhibitor p21 and ApoJ/CLU. Similar treatments with three additional labour pollutants resulted in the induction of premature senescence by CdII, but not by SnII or PbII. In summary, our results indicate that exposure to CrVI induces alterations of senescence biomarkers both in vivo and in vitro. They also provide new valuable tools for monitoring CrVI cytotoxic effects in vivo as well as for re-evaluating the maximum permissive values of some labour pollutants, like CrVI and CdII.

Involvement of Rel/nuclear factor-kappaB transcription factors in keratinocyte senescence

After a finite doubling number, normal cells become senescent, i.e., nonproliferating and apoptosis resistant. Because Rel/nuclear factor (NF)-kappaB transcription factors regulate both proliferation and apoptosis, we have investigated their involvement in senescence. cRel overexpression in young normal keratinocytes results in premature senescence, as defined by proliferation blockage, apoptosis resistance, enlargement, and appearance of senescence-associated beta-galactosidase (SA-beta-Gal) activity. Normal senescent keratinocytes display a greater endogenous Rel/NF-kappaB DNA binding activity than young cells; inhibiting this activity in presenescent cells decreases the number of cells expressing the SA-beta-Gal marker. Normal senescent keratinocytes and cRel-induced premature senescent keratinocytes overexpressed manganese superoxide dismutase (MnSOD), a redox enzyme encoded by a Rel/NF-kappaB target gene. MnSOD transforms the toxic O()(2) into H(2)O(2), whereas catalase and glutathione peroxidase convert H(2)O(2) into H(2)O. Neither catalase nor glutathione peroxidase is up-regulated during cRel-induced premature senescence or during normal senescence, suggesting that H(2)O(2) accumulates. Quenching H(2)O(2) by catalase delays the occurrence of both normal and premature cRel-induced senescence. Conversely, adding a nontoxic dose of H(2)O(2) to the culture medium of young normal keratinocytes induces a premature senescence-like state. All these results indicate that Rel/NF-kappaB factors could take part in the occurrence of senescence by generating an oxidative stress via the induction of MnSOD.

Effects of Oxidative Damage and Telomerase Activity on Human Articular Cartilage Chondrocyte Senescence

Senescence compromises the ability of chondrocytes to maintain and repair articular cartilage. We hypothesized that oxidative stress and telomere loss contribute to chondrocyte senescence. To test this hypothesis, we compared the growth of human articular cartilage chondrocytes incubated in 5% O2 and 21% O2. Cells grown in 5% O2 reached 60 population doublings (PD) before senescing, but growth in 21% O2 induced DNA damage and premature senescence at less than 40 PD. Human telomerase reverse transcriptase (hTERT)-transduction failed to prevent chondrocyte senescence in 21% O2, but allowed 1 of 3 chondrocyte strains to exceed 90 PD in 5% O2. These results show that oxidative stress causes premature chondrocyte senescence. They may help explain the increased risk of osteoarthritis with age and after joint trauma and inflammation, and suggest that minimizing oxidative damage will help produce optimal results for chondrocyte transplantation.

Glucose-6-phosphate dehydrogenase-deficient cells show an increased propensity for oxidant-induced senescence

Glucose-6-phosphate dehydrogenase (G6PD) is involved in the generation of reduced nicotinamide adenine dinucleotide phosphate (NADPH) and the maintenance of cellular redox balance. We previously showed that G6PD-deficient fibroblasts undergo growth retardation and premature cellular senescence. In the present study, we demonstrate abatement of both the intracellular G6PD activity and the ratio NADPH/NADP(+) during the serial passage of G6PD-deficient cells. This was accompanied by a significant increase in the level of 8-hydroxy-2-deoxyguanosine (8-OHdG). This suggests that the lowered resistance to oxidative stress and accumulative oxidative damage may account for the premature senescence of these cells. Consistent with this, the G6PD-deficient cells had an increased propensity for hydrogen peroxide (H(2)O(2))-induced senescence; these cells exhibited such senescent phenotypes as large, flattened morphology and increased senescence-associated beta-galactosidase (SA-beta-Gal) staining. Decreases in both the intracellular G6PD activity and the NADPH/NADP(+) ratio were concomitant with an increase in 8-OHdG level in H(2)O(2)-induced senescent cells. Exogenous expression of G6PD protected the deficient cells from stress-induced senescence. No significant telomere shortening occurred upon repetitive treatment with H(2)O(2). Simultaneous induction of p16(INK4a) and p53 was detected in G6PD-deficient but not in normal fibroblasts during H(2)O(2)-induced senescence. Our findings support the notion that G6PD status, and thus proper redox balance, is a determinant of cellular senescence.

The effect of glutamine on A549 cells exposed to moderate hyperoxia

The use of high oxygen concentrations is frequently necessary in the treatment of acute respiratory distress syndrome (ARDS) and bronchopulmonary dysplasia (BPD). High oxygen concentrations, however, are detrimental to cell growth and cell survival. Glutamine (Gln) may be protective to cells during periods of stress and recently has been shown to increase survival in A549 cells exposed to lethal concentrations of oxygen (95% O2). We found that supplemental Gln enhances cell growth in A549 cells exposed to moderate concentrations of oxygen (60% O2). We therefore evaluated the effect of moderate hyperoxia on the cell cycle distribution of A549 cells. At 48 h there was no significant difference in the cell cycle distribution between 2 mM Gln cells in 60% O2 and 2 mM cells in room air. Furthermore, 2 mM Gln cells in 60% O2 had stable protein levels of cyclin B1 consistent with ongoing cell proliferation. In contrast, at 48 h, cells not supplemented with glutamine (Gln-) in 60% O2 had evidence of growth arrest by both flow cytometry (increased percentage of G1 cells) and by decreased protein levels of cyclin B1. G1 growth arrest in the Gln- cells exposed to 60% O2 was not, however, associated with induction of p21 protein. At 72 and 96 h, Gln- cells in 60% O2, began to demonstrate a partial loss of G1 checkpoint regulation and an increase in apoptosis, indicating an increased sensitivity to oxygen toxicity. Glutathione (GSH) concentrations were then measured. 2 mM Gln cells in 60% O2 were found to have higher concentrations of GSH compared to Gln- cells in 60% O2, suggesting that Gln confers protection to the cell during exposure to hyperoxia through up-regulation of GSH. When cells in 60% O2 were given higher concentrations of Gln (5 and 10 mM), cell growth at 96 h was increased compared to cells grown in 2 mM Gln (P<0.04). Clonal survival was also increased in cells exposed 60% O2 and supplemented with higher concentrations of Gln compared to Gln- cells in 60% O2. These studies suggest that supplemental glutamine may improve cell growth and cell viability and therefore may be beneficial to the lung during exposure to moderate concentrations of supplemental oxygen.

Ionizing radiation and busulfan inhibit murine bone marrow cell hematopoietic function via apoptosis-dependent and -independent mechanisms

OBJECTIVE

Ionizing radiation (IR) and busulfan (BU) are commonly used as preconditioning regimens for bone marrow transplantation (BMT). We examined whether induction of apoptosis in murine bone marrow (BM) hematopoietic cells contributes to IR- and BU-induced suppression of their hematopoietic function.

METHODS

The hematopoietic functions of hematopoietic stem cells (HSCs) and progenitors were analyzed by the cobblestone area-forming cell (CAFC) assay. Apoptosis was determined by measuring 3,3'-dihexyloxacarbocyanine iodide (DiCO6) uptake, annexin V staining, and/or sub-G(0/1) cells. Four cell types were studied: murine BM mononuclear cells (BM-MNCs), linage-negative hematopoietic cells (Lin-) cells), Lin- Scal+ c-kit+ cells, and Lin- Scal- c-kit+ cells by flow cytometry.

RESULTS

Exposure of BM-MNCs to IR (4 Gy) or incubation of the cells with BU (30 microM) resulted in a significant reduction in CAFC frequency (p<0.001). The survival fractions of various day-types of CAFC for the irradiated cells were less than 10%, while that for BU-treated cells was 71.3% on day 7 and progressively declined to 5.3% on day 35. Interestingly, IR significantly induced apoptosis in BM-MNCs, Lin- cells, HSCs, and progenitors, whereas BU failed to increase apoptosis in these cells. In addition, preincubation of BM-MNCs with z-Val-Ala-Asp (OCH3)-fluoromethylketone, methyl ester (z-VAD) attenuated IR-induced reduction in CAFC but not that induced by BU.

CONCLUSION

IR and BU differentially suppress the hematopoietic function of HSCs and progenitors by fundamentally different mechanisms. IR inhibits the function primarily by the induction of HSC and progenitor apoptosis. In contrast, BU suppresses HSC and progenitor function via an apoptosis-independent mechanism.

Ageing across the life span: time to think again

Living organisms are subject to ageing. This natural process has gained greater importance in socially and medically affluent societies. For many, ageing connotes unattractive changes in the appearance of the skin. The gross morphological changes of ageing skin are mirrored by a range of more profound age-associated physiological declines. Thus, skin ageing can be put into other perspectives which lie at the interfaces of molecular biology, cellular biology, oncology and cosmetic dermatology. Genetically programmed replicative senescence and stress-induced premature senescence (SIPS) are two processes that are fundamental to skin ageing. Some iteroparous species can be used as animal models for human ageing. Undoubtedly, scientific understanding of skin ageing is firmly rooted in the distinction between intrinsic and extrinsic types of ageing. However, seven major types of skin ageing can be distinguished: genetic, chronological, solar, behavioural, endocrinological, catabolic and gravitational types. Preventative measures can target each of these.

Slow-down of age-dependent telomere shortening is executed in human skin keratinocytes by hormesis-like-effects of trace hydrogen peroxide or by anti-oxidative effects of pro-vitamin C in common concurrently with reduction of intracellular oxidative stress

The cellular life-span of cultivated human skin epidermis keratinocytes NHEK-F was shown to be extended up to 150% of population doubling levels (PDLs) by repetitive addition with two autooxidation-resistant derivatives of ascorbic acid (Asc), Asc-2-O-phosphate (Asc2P), and Asc-2-O-alpha-glucoside (Asc2G), respectively, but to be not extended with Asc itself. In contrast, hydrogen peroxide (H(2)O(2)) as dilute as 20 microM which was non-cytotoxic to the keratinocytes, or at 60 microM being marginally cytotoxic achieved the cellular longevity, unexpectedly, up to 160 and 120% of PDLs, respectively, being regarded as a hormesis-like stimulatory effect. The lifespan-extended cells that were administered with Asc2P, Asc2G, or 20 microM H(2)O(2) were prevented from senescence-induced symptoms such as PDL-dependent enlargement of a cell size of 14.7 microm finally up to 17.4 microm upon Hayflick's limit-called loss of proliferation ability as estimated with a channelizer, and retained young cell morphological aspects such as thick and compact shape and intense attachment to the culture substratum even upon advanced PDLs, whereas other non-extended cells looked like thin or fibrous shape and large size upon lower PDLs. The PDL-dependent shortening of telomeric DNA of 11.5 kb finally down to 9.12-8.10 kb upon Hayflick's limit was observed in common for each additive-given cells, but was decelerated in the following order: 20 microM H(2)O(2) > Asc2P = Asc2G > 60 microM H(2)O(2) > Asc = no additive, being in accord with the order of cell longevity. Intracellular reactive oxygen species (ROS) was diminished by Asc2P, Asc2G or 20 microM H(2)O(2), but not significantly by Asc or 60 microM H(2)O(2) as estimated by fluorometry using the redox indicator dye CDCFH. There was no appreciable difference among NHEK keratinocytes that were administered with or without diverse additives in terms of telomerase activity per cell, which was 1.40 x 10(4)-4.48 x 10(4) times lower for the keratinocytes than for HeLa cells which were examined as the typical tumor cells. Thus longevity of the keratinocytes was suggested to be achieved by slowdown of age-dependent shortening of telomeric DNA rather than by telomerase; telomeres may suffer from less DNA lesions due to the continuous and thorough repression of intracellular ROS, which was realized either by pro-vitamin C such as Asc2P or Asc2G that exerted an antioxidant ability more persistent than Asc itself or by 20 microM H(2)O(2) which diminished intracellular ROS assumedly through a hormesis-like effect.

Replicative senescence and the art of counting

The idea that aging is largely the result of (endogenous) stress appears to be at odds with the concept of biological 'clocks', which seem to programme and terminate cellular aging processes. Here, data are reviewed that show that telomeres, the major clock identified in human cells so far, do in fact measure stress and damage accumulation much more than simple mitotic time. Telomere shortening is significantly stress-dependent due to a telomere-specific damage repair deficiency. This identifies telomere-driven human cell replicative senescence as a stress response with high potential importance for human aging.

Stress-induced premature senescence in hTERT-expressing ataxia telangiectasia fibroblasts

In addition to replicative senescence, normal diploid fibroblasts undergo stress-induced premature senescence (SIPS) in response to DNA damage caused by oxidative stress or ionizing radiation (IR). SIPS is not prevented by telomere elongation, indicating that, unlike replicative senescence, it is triggered by nonspecific genome-wide DNA damage rather than by telomere shortening. ATM, the product of the gene mutated in individuals with ataxia telangiectasia (AT), plays a central role in cell cycle arrest in response to DNA damage. Whether ATM also mediates signaling that leads to SIPS was investigated with the use of normal and AT fibroblasts stably transfected with an expression vector for the catalytic subunit of human telomerase (hTERT). Expression of hTERT in AT fibroblasts resulted in telomere elongation and prevented premature replicative senescence, but it did not rescue the defect in G(1) checkpoint activation or the hypersensitivity of the cells to IR. Despite these remaining defects in the DNA damage response, hTERT-expressing AT fibroblasts exhibited characteristics of senescence on exposure to IR or H(2)O(2) in such a manner that triggers SIPS in normal fibroblasts. These characteristics included the adoption of an enlarged and flattened morphology, positive staining for senescence-associated beta-galactosidase activity, termination of DNA synthesis, and accumulation of p53, p21(WAF1), and p16(INK4A). The phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK), which mediates signaling that leads to senescence, was also detected in both IR- or H(2)O(2)-treated AT and normal fibroblasts expressing hTERT. These results suggest that the ATM-dependent signaling pathway triggered by DNA damage is dispensable for activation of p38 MAPK and SIPS in response to IR or oxidative stress.

Increased expression of p16(INK4a) and p27(Kip1) cyclin-dependent kinase inhibitor genes in aging human kidney and chronic allograft nephropathy

BACKGROUND

The goal of the current study was to examine the potential value of p16(INK4a) and p27(Kip1) cyclin-dependent kinase inhibitor (CDKI) genes in the process of human kidney aging in vivo, and in the development of chronic allograft nephropathy (CAN).

METHODS

Expression of p16(INK4a) and p27(Kip1) CDKI genes was evaluated and compared in 20 normal human kidney tissues of different ages (range, 21 to 80 years) and in 9 chronically rejected kidney grafts. Age dependency of marker expression was analyzed by the Pearson correlation and linear regression.

RESULTS

Expression of p16 in cortical tubular (CTS) and interstitial (CIS) cells of normal kidney was age dependent (correlation coefficients: 0.608 and 0.726, 95% confidence interval [CI]: 0.227 to 0.828 and 0.417 to 0.884, respectively). Cortical tubular expression of p27 was also correlated with increasing age (0.672, 95% CI: 0.327 to 0.859). Linear regression analyses confirmed the linearity of marker relationship with age (coefficient of determination R(2):0.370, 0.452, and 0.527 for CIS p16, CTS p27, and CTS p16, respectively). The mean chronological and predicted graft ages (53 +/- 21 and 76 +/- 8.9 years, respectively) were significantly different (P = 0.0126). The glomeruli, tubules, and interstitial cells of rejected grafts expressed significantly higher levels of p16 and p27 than normal kidneys. Expression of p16 in glomerular and cortical interstitial cells was higher in grade 3 of CAN than in grade 2 (P = 0.013 and 0.004, respectively).

CONCLUSION

The results of the current study show that expression of p16(INK4a) and p27(Kip1) CDKI genes is increased in cortical cells of the aging human kidney and in chronic allograft rejection, supporting the senescence theory of CAN.

The role of chondrocyte senescence in the pathogenesis of osteoarthritis and in limiting cartilage repair

BACKGROUND

With increasing age, the prevalence of osteoarthritis increases and the efficacy of articular cartilage repair decreases. As chondrocytes age, they synthesize smaller, less uniform aggrecan molecules and less functional link proteins, their mitotic and synthetic activity decline, and their responsiveness to anabolic mechanical stimuli and growth factors decreases. These observations led us to hypothesize that progressive cell senescence decreases the ability of chondrocytes to maintain and to restore articular cartilage.

METHODS

To test this hypothesis, we measured cell senescence markers (beta-galactosidase expression, mitotic activity, and telomere length) in human articular cartilage chondrocytes from twenty-seven donors ranging in age from one to eighty-seven years. We also assessed mitochondrial DNA, membrane potential, and numerical density. To determine if chondrocyte age changes are reversible, we transfected human articular cartilage chondrocytes with the human telomerase gene (hTERT) and human papilloma virus oncogenes (E6 and E7).

RESULTS

Beta-galactosidase expression increased with age (r = 0.84, p = 0.0001), while mitotic activity and telomere length declined (r = -0.77, p = 0.001 and r = -0.71, p = 0.0004, respectively). Decreasing telomere length was closely correlated with increasing expression of beta-galactosidase and decreasing mitotic activity. As the number of population doublings increased, mitochondrial DNA was degraded, mitochondrial membrane potential was lost, and the number of mitochondria per cell declined. Transfection of human articular cartilage chondrocytes from a forty-seven-year-old donor with hTERT and human papilloma virus proto-oncogenes E6 and E7 created a cell line that has completed more than 300 population doublings as compared with an upper limit of twenty-five population doublings for normal cells. Telomere length increased in cells transduced with hTERT.

CONCLUSIONS

These findings help to explain the previously reported age-related declines in chondrocyte synthetic activity, mitotic activity, and responsiveness to anabolic cytokines and mechanical stimuli. They also suggest that in vivo chondrocyte senescence contributes to the age-related increase in the prevalence of osteoarthritis and decrease in the efficacy of cartilage repair. The creation of immortal cells with increased telomere length suggests that the progression of human chondrocytes toward senescence is not inevitable.

Extracellular superoxide dismutase is a major antioxidant in human fibroblasts and slows telomere shortening

There is good evidence that telomere shortening acts as a biological clock in human fibroblasts, limiting the number of population doublings a culture can achieve. Oxidative stress also limits the growth potential of human cells, and recent data show that the effect of mild oxidative stress is mediated by a stress-related increased rate of telomere shortening. Thus, fibroblast strains have donor-specific antioxidant defense, telomere shortening rate, and growth potential. We used low-density gene expression array analysis of fibroblast strains with different antioxidant potentials and telomere shortening rates to identify gene products responsible for these differences. Extracellular superoxide dismutase was identified as the strongest candidate, a correlation that was confirmed by Northern blotting. Over-expression of this gene in human fibroblasts with low antioxidant capacity increased total cellular superoxide dismutase activity, decreased the intracellular peroxide content, slowed the telomere shortening rate, and elongated the life span of these cells under normoxia and hyperoxia. These results identify extracellular superoxide dismutase as an important antioxidant gene product in human fibroblasts, confirm the causal role of oxidative stress for telomere shortening, and strongly suggest that the senescence-like arrest under mild oxidative stress is telomere-driven.

Down-regulation and decreased activity of cyclin-dependent kinase 2 in H2O2-induced premature senescence

Premature senescence of human diploid fibroblasts (HDFs) induced by exposure to H2O2 at subcytotoxic concentration is characterized by many biomarkers of normal senescence such as irreversible growth arrest. Cyclin-dependent kinase inhibitor (CdKI) p21(Waf-1) is overexpressed in H2O2- and tert-butylhydroperoxide-induced premature senescence, likely explaining in part the hypophosphorylation of the retinoblastoma protein. p21(Waf-1) is known to inhibit the kinase activity of the cyclin-dependent kinase (CdK) 4 and 6 cyclin complexes. In this work, we investigated whether the kinase activity of the CdK4 and 6 cyclin complexes can be modulated by CdKI p16(Ink-4a), by changes in the protein level of CdKs and cyclins, or by changes in kinase activity of these CdKs not directly involving CdKIs. RNase protection assay, semi-quantitative RT-PCR, Western blot and kinase assay showed that the mRNA level, protein and kinase activity of CdK2 are decreased at 72h after H2O2 stress. These results suggest that the hypophosphorylation of the retinoblastoma protein is mediated in part by a decrease of the kinase activity of CdK2 not directly involving CdKIs. This CdK2-mediated effect should be considered in addition to the inhibition of cyclin D-CdK4 and 6 complexes by CdKI p21(Waf-1).

Identification of 30 protein species involved in replicative senescence and stress-induced premature senescence

Exposure of human proliferative cells to subcytotoxic stress triggers stress-induced premature senescence (SIPS) which is characterized by many biomarkers of replicative senescence. Proteomic comparison of replicative senescence and stress-induced premature senescence indicates that, at the level of protein expression, stress-induced premature senescence and replicative senescence are different phenotypes sharing however similarities. In this study, we identified 30 proteins showing changes of expression level specific or common to replicative senescence and/or stress-induced premature senescence. These changes affect different cell functions, including energy metabolism, defense systems, maintenance of the redox potential, cell morphology and transduction pathways.

Signal transduction in H2O2-induced senescence-like phenotype in human diploid fibroblasts

A stress-induced senescence-like phenotype is induced by exposure of human diploid fibroblasts to subcytotoxic H2O2 stress. Previous studies showed that TGF-beta1 is responsible for the induction of several biomarkers of replicative senescence within 72 h after stress: senescence-like morphology, senescence-associated beta-galactosidase activity, and an increase in the mRNA steady state level of four senescence-associated genes. Other studies showed that the retinoblastoma protein is responsible for the appearance of these biomarkers in the same conditions. Here we show that sustained p38(MAPK) phosphorylation is responsible for both H2O2-induced overexpression of TGF-beta 1 and subsequent TGF-beta 1-induced appearance of these biomarkers. p38(MAPK) phosphorylation is shown to be necessary for a self-sustained TGF-beta 1 overexpression after H2O2 stress through the activation of ATF-2 transcription factor, thereby creating a regulatory loop between sustained p38(MAPK) activation and sustained TGF-beta 1 overexpression after stress. p38(MAPK) activation is also shown to be responsible in part for the growth arrest observed in stress-induced senescence-like phenotype. At 48 h after stress, ATF-2 starts to interact with hypophosphorylated Rb, which allows the biomarkers of stress-induced senescence-like phenotype to appear. This report gives an overall explanation of how a senescence-like phenotype is established after subcytotoxic H2O2 stress.

Molecular changes accompanying senescence and immortalization of cultured human mammary epithelial cells

Limits on the proliferative potential of cultured normal human cells may be consequences of pathways that exist to suppress tumorigenicity. Human mammary epithelial cells (HMEC) employ several mechanisms to prevent unlimited growth. One mechanism may be activated by stress, and is associated with upregulated expression of p16(INK4a). In serum-free medium, some HMEC arise spontaneously which do not express p16. These "post-selection" HMEC are capable of long-term proliferation, but ultimately cease growth when their telomeres become very short. As they approach a growth plateau, termed agonescence, post-selection HMEC populations accumulate chromosome abnormalities. In contrast to the crisis exhibited by cells lacking functional p53, agonescent cells can be maintained as viable cultures. Although transduction of hTERT, the catalytic subunit of telomerase, into post-selection cells can, by itself, efficiently produce immortality and avoid agonescence, the errors that produce telomerase reactivation during carcinogenesis are not known. The block to endogenous telomerase reactivation in HMEC is extremely stringent. However, if one predisposing error is present, the probability greatly increases that additional error(s) required for immortalization may be generated by genomic instability encountered during agonescence. In p53(+) HMEC immortalized after chemical carcinogen exposure, the events involved in overcoming agonescence can be temporally separated from activation of telomerase. We have used the term "conversion" to describe the gradual process that leads to telomerase activation, telomere length stabilization, decreased p57 (KIP2) expression, and increased ability to grow uniformly well in the presence or absence of TGF beta. In the presence of active p53, conversion may represent a rate-limiting step in immortal transformation.

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.

UVB-induced premature senescence of human diploid skin fibroblasts

In this work, we show that repeated stresses with UVB (290-320 nm) induce stress-induced premature senescence (SIPS) of skin human diploid fibroblasts (HDFs). HDFs at early cumulative population doublings were exposed three or five times to increasing subcytotoxic doses of UVB with one stress per day. After 2 days of recovery, several biomarkers of replicative senescence were established. First, there was an increase in the proportion of cells positive for senescence-associated beta-galactosidase activity. Second, there was a loss of replicative potential as assessed by a very low level of [3H]-thymidine incorporation. Third, the steady-state level of the mRNA of three senescence-associated genes, i.e. fibronectin, osteonectin and SM22, was increased in HDFs at 72 h after three and five exposures to UVB. In conclusion, these results suggest that it is possible to induce SIPS in HDFs after repeated exposures to subcytotoxic doses of UVB. This model could be used to test whether HDFs in UVB-induced premature senescence are able to promote epithelial cell growth and tumorigenesis in skin, as shown recently with HDFs in H(2)O(2)-induced premature senescence.

Replicative senescence and senescence-like state induced in cancer-derived cells

Studies on the replicative senescence and premature senescence induced by various stresses in normal somatic cells have provided important clues on the role of telomere shortening and mechanisms involved in aging processes and carcinogenesis. Recent work revealed that cancer cells also are induced to undergo replicative senescence state via telomere shortening as well as to enter a senescence-like state by the activation of cell cycle inhibitory pathways. Although less relevant in terms of aging physiology, studies on these phenomena in cancer cells have yielded important information on telomerase regulation and the roles of tumor suppressors in senescence and immortalization, and are expected to generate valuable anti-cancer strategies. Several features of the phenotypes specific for the senescent and senescence-like states induced in cancer cells are discussed.

Expression of human telomerase (hTERT) does not prevent stress-induced senescence in normal human fibroblasts but protects the cells from stress-induced apoptosis and necrosis

Cells subjected to sub-lethal doses of stress such as irradiation or oxidative damage enter a state that closely resembles replicative senescence. What triggers stress-induced premature senescence (SIPS) and how similar this mechanism is to replicative senescence are not well understood. It has been suggested that stress-induced senescence is caused by rapid telomere shortening resulting from DNA damage. In order to test this hypothesis directly, we examined whether overexpression of the catalytic subunit of human telomerase (hTERT) can protect cells from SIPS. We therefore analyzed the response of four different lines of normal human fibroblasts with and without hTERT to stress induced by UV, gamma-irradiation, and H(2)O(2). SIPS was induced with the same efficiency in normal and hTERT-immortalized cells. This suggests that SIPS is not triggered by telomere shortening and that nonspecific DNA damage serves as a signal for induction of SIPS. Although telomerase did not protect cells from SIPS, fibroblasts expressing hTERT were more resistant to stress-induced apoptosis and necrosis. We hypothesize that healing of DNA breaks by telomerase inhibits the induction of cell death, but because healing does not provide legitimate DNA repair, it does not protect cells from SIPS.

Chronic in vivo exposure to glucocorticoids prolongs cellular lifespan: the case of Cushing's syndrome-patients' fibroblasts

Glucocorticoid (GC) hypersecretion constitutes the major hormonal response to stress. In an effort to investigate the impact of a long-lasting exposure to high GC levels in vivo on cellular longevity, we have studied the lifespan of skin fibroblasts from patients suffering from Cushing's syndrome, who are characterised by chronic endogenous GC excess. Interestingly, we have observed that these cells exhibit a significant increase in their proliferative lifespan when cultured in vitro, under standard conditions, compared to fibroblasts from normal donors. In parallel, these cells secrete lower levels of transforming growth factor-beta, known to be implicated in stress-induced premature senescence. Furthermore, they also exhibit an intense stress reaction (near 2-fold, compared to normal cells) in terms of heat-shock protein-70 induction. These results support the hypothesis that stress response may have beneficial consequences in cellular longevity, as well as in tissue homeostasis.

PML a target of translocations in APL is a regulator of cellular senescence

PML is the most frequent fusion partner of the RARalpha in the specific translocations associated with acute promyelocytic leukemia (APL). Models to explain the origin of this leukemia propose a block in cell differentiation due to aberrant repression of retinoic acid responsive genes and/or disruption of the function of the PML-containing nuclear bodies. Recently, PML has been identified as a regulator of replicative senescence and the premature senescence that occurs in response to oncogenic ras. This review discusses the idea that senescence is a general tumor suppressor mechanism related to terminal differentiation and disrupted during the establishment of APL and other cancers. According to this idea the PML-RARalpha fusion protein promotes leukemogenesis not only through repression of retinoic acid responsive genes, but also by way of interfering with several tumor suppressor proteins that cooperate to establish senescence. Retinoids and other drugs effective against APL do so by re-establishment of the senescence program, which also includes features of cell differentiation.

Stress-induced premature senescence and replicative senescence are different phenotypes, proteomic evidence

In this paper, we illustrate how a proteomic analysis can be useful to approach complex biological problems, in this case the concept of stress-induced premature senescence (SIPS). According to the stochastic theories of ageing, damage that accumulate with time in the cellular components are responsible for cellular ageing. As a corollary, some sort of premature senescence should appear if the damage level is artificially increased due to the presence of stressing agents at subcytotoxic level. It has been shown, in several different models, that at a long-term after subcytotoxic stresses of many different natures, human diploid fibroblasts (HDFs) display biomarkers of replicative senescence (RS), which led to the concept of SIPS as compared to telomere-dependent RS. We compared RS and SIPS of HDFs by proteome analysis. SIPS was induced by two very different stressors: tert-butyhydroperoxide or ethanol. First, only a part of the protein expression changes observed in RS were also observed in SIPS. Second, HDFs in SIPS show changes specific either to the long-term effects of t-BHP or ethanol or independent of the nature of the stress. These changes have been termed "molecular scars" of subcytotoxic stresses. This work is also an excellent opportunity to discuss on important methodological issue in proteomics: the absolute requirement to start from reliable and reproducible models, which was the case in this study. We also focus on the data handling and statistical analysis allowing to use two-dimensional gel electrophoresis patterns in a semi-quantitative analysis.

Stress-induced premature senescence and tissue ageing

Various human proliferative cell types exposed in vitro to many types of subcytotoxic stresses undergo stress-induced premature senescence (SIPS). The known mechanisms of appearance the main features of SIPS are reviewed: senescent-like morphology, growth arrest, senescence-related changes in gene expression. All cell types undergoing SIPS in vivo, are likely to participate in the tissular changes observed along ageing. For instance, human diploid fibroblasts 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.