Anti-apoptotic role of telomerase in pheochromocytoma cells

Telomerase protects developing neurons against DNA damage-induced cell death

In mitotic cells, telomerase adds repeats of a DNA sequence (TTAGGG) to the ends of chromosomes (telomeres) thereby maintaining their length and preventing cellular senescence. We recently reported that the catalytic subunit of telomerase (TERT) is expressed in neuronal progenitor cells and in early postmitotic neurons in the developing rodent brain. We now report that TERT can protect cultured PC12 cells and embryonic hippocampal neurons against death induced by DNA damage. Overexpression of TERT in PC12 cells increases their resistance to the topoisomerase inhibitors camptothecin and etoposide. Hippocampal neurons in which TERT levels are decreased using antisense technology exhibit increased vulnerability to the DNA-damaging agents. Emerging findings suggest that DNA damage may trigger the death of neurons during brain development and in neurodegenerative disorders. Our data therefore suggest roles for TERT in modulating such cell deaths.

Telomerase and oligodendrocyte differentiation

Myelin in the mammalian central nervous system (CNS) is produced by oligodendrocytes, most of which arise from oligodendrocyte precursor cells (OPCs) during late embryonic and early postnatal development. Both external and internal cues have been implicated in regulating OPC exit from the cell cycle and differentiation into oligodendrocytes. In this study, we demonstrate that differentiation of cultured OPCs into mature oligodendrocytes is associated with lower levels of activity of telomerase, the ribonucleoprotein that synthesizes telomeric DNA at the ends of chromosomes. Differentiation is also associated with lower levels of mRNA encoding the catalytic subunit of telomerase (TERT), whereas no difference is seen in the expression of its telomeric template RNA component (TR). These data suggest a possible role for telomerase during normal growth and differentiation of oligodendrocytes that may be relevant to the mechanism of myelination in the CNS.

c-Myc-mediated regulation of telomerase activity is disabled in immortalized cells

Myc overexpression is a hallmark of human cancer and promotes transformation by facilitating immortalization. This function has been linked to the ability of c-Myc to induce the expression of the catalytic subunit of telomerase, telomerase reverse transcriptase (TERT), as ectopic expression of TERT immortalizes some primary human cell types. c-Myc up-regulates telomerase activity in primary mouse embryonic fibroblasts (MEFs) and myeloid cells. Paradoxically, Myc overexpression also triggers the ARF-p53 apoptotic program, which is activated when MEFs undergo replicative crises following culture ex vivo. The rare immortal variants that arise from these cultures generally suffer mutations in p53 or delete Ink4a/ARF, and Myc greatly increases the frequency of these events. Alternative reading frame (ARF)- and p53-null MEFs have increased telomerase activity, as do variant immortal clones that bypass replicative crisis. Similarly, immortal murine NIH-3T3 fibroblasts and myeloid 32D.3 and FDC-P1.2 cells do not express ARF and have robust telomerase activity. However, Myc overexpression in these immortal cells results in remarkably discordant regulation of TERT and telomerase activity. Furthermore, in MEFs and 32D.3 cells TERT expression and telomerase activity are regulated independently of endogenous c-Myc. Thus, the regulation of TERT and telomerase activity is complex and is also regulated by factors other than Myc, ARF, or p53.

Methionine aminopeptidase-2 regulates human mesothelioma cell survival: role of Bcl-2 expression and telomerase activity

Methionine aminopeptidase-2 (MetAP2) is the molecular target of the angiogenesis inhibitors, fumagillin and ovalacin. Fumagillin can also inhibit cancer cell proliferation, implying that MetAP2 may play a quite complex role in tumor progression. Here, we examined the expression and function of MetAP2 in an in vitro model of human mesothelioma. We found that mesothelioma cells expressed higher MetAP2 mRNA levels than primary normal mesothelial cells. Consistently, fumagillin induced apoptosis, owing to early mitochondrial damage, in malignant, but not in normal mesothelial cells. Transfection of mesothelioma cells with a MetAP2 anti-sense oligonucleotide determined a time-dependent inhibition of cell survival and induced nucleosome formation. Interestingly, mRNA and protein levels of the anti-apoptotic gene bcl-2 as well as telomerase activity were selectively reduced after MetAP2 inhibition in mesothelioma cells, whereas bcl-2 overexpression counteracted the effect of MetAP2 inhibition on telomerase activity and apoptosis. MetAP2 inhibition also increased caspase activity and the caspase inhibitor, zVAD-fmk, prevented fumagillin-induced apoptosis, but it did not alter telomerase activity. These results indicate that MetAP2 is a main regulator of proliferative and apoptotic pathways in mesothelioma cells and suggest that MetAP2 inhibition may represent a potential target for therapeutic intervention in human mesothelioma.

Role of ceramide in mediating the inhibition of telomerase activity in A549 human lung adenocarcinoma cells

This study was designed to analyze whether ceramide, a bioeffector of growth suppression, plays a role in the regulation of telomerase activity in A549 cells. Telomerase activity was inhibited significantly by exogenous C(6)-ceramide, but not by the biologically inactive analog dihydro-C(6)-ceramide, in a time- and dose-dependent manner, with 85% inhibition produced by 20 microm C(6)-ceramide at 24 h. Moreover, analysis of phosphatidylserine translocation from the inner to the outer plasma membrane by flow cytometry and of poly(ADP-ribose) polymerase degradation by Western blotting showed that ceramide treatment (20 microm for 24 h) had no apoptotic effects. Trypan blue exclusion, [(3)H]thymidine incorporation, and cell cycle analyses, coupled with clonogenic cell survival assay on soft agar, showed that ceramide treatment with a 20 microm concentration at 24 h resulted in the cell cycle arrest of the majority of the cell population at G(0)/G(1) with no detectable cell death. These results suggest that the inhibition of telomerase by ceramide is not a consequence of cell death but is correlated with growth arrest. Next, to determine the role of endogenous ceramide in telomerase modulation, A549 cells were transiently transfected with an expression vector containing the full-length bacterial sphingomyelinase cDNA (b-SMase). The overexpression of b-SMase, but not exogenously applied purified b-SMase enzyme, resulted in significantly decreased telomerase activity compared with controls, showing that the increased endogenous ceramide is sufficient for telomerase inhibition. Moreover, treatment of A549 cells with daunorubicin at 1 microm for 6 h resulted in the inhibition of telomerase, which correlated with the elevation of endogenous ceramide levels and growth arrest. Finally, stable overexpression of human glucosylceramide synthase, which attenuates ceramide levels by converting ceramide to glucosylceramide, prevented the inhibitory effects of C(6)-ceramide and daunorubicin on telomerase. Therefore, these results provide novel data showing for the first time that ceramide is a candidate upstream regulator of telomerase.

Beta -amyloid-(1-42) impairs activity-dependent cAMP-response element-binding protein signaling in neurons at concentrations in which cell survival Is not compromised

Cognitive impairment is a major feature of Alzheimer's disease and is accompanied by beta-amyloid (Abeta) deposition. Transgenic animal models that overexpress Abeta exhibit learning and memory impairments, but neuronal degeneration is not a consistent characteristic. We report that levels of Abeta-(1-42), which do not compromise the survival of cortical neurons, may indeed interfere with functions critical for neuronal plasticity. Pretreatment with Abeta-(1-42), at sublethal concentrations, resulted in a suppression of cAMP-response element-binding protein (CREB) phosphorylation, induced by exposure to either 30 mm KCl or 10 microm N-methyl-d-aspartate. The effects of Abeta-(1-42) seem to involve mechanisms unrelated to degenerative changes, since Abeta-(25-35), a toxic fragment of Abeta, at sublethal concentrations did not interfere with activity-dependent CREB phosphorylation. Furthermore, caspase inhibitors failed to counteract the Abeta-(1-42)-evoked suppression of CREB activation. Abeta-(1-42) also interfered with events downstream of activated CREB. The Abeta-(1-42) treatment suppressed the activation of the cAMP response element-containing brain-derived neurotrophic factor (BDNF) exon III promoter and the expression of BDNF exon IIII mRNA induced by neuronal depolarization. In view of the critical role of CREB and BDNF in neuronal plasticity, including learning and memory, the observations indicate a novel pathway through which Abeta may interfere with neuronal functions and contribute to cognitive deficit in Alzheimer's disease before the stage of massive neuronal degeneration.

Differential regulation of telomerase activity and TERT expression during brain development in mice

Telomerase is an enzyme activity consisting of a reverse transcriptase called TERT and an RNA component that adds repeats of a DNA sequence (TTAGGG) to the ends of chromosomes, thereby preventing their shortening. Associations between telomerase activity and proliferation and differentiation of neural tumor cells and neural stem cells have been reported, but the role of telomerase in brain development is unknown. We now report analyses of telomerase activity, TERT mRNA levels and levels of mRNAs encoding the telomere-associated proteins TRF1 and TRF2 in three different brain regions (brainstem, hippocampus and cerebral cortex) and the eye of mice at increasing developmental time points. Telomerase activity is high in the brain at embryonic day 13 (E13), declines markedly between E13 and E18, remains at a low level until postnatal day 3 (P3) and becomes undetectable by P10. Surprisingly, the temporal pattern of change in telomerase activity is not paralleled by a decrease in levels of TERT mRNA that remain elevated from E13 to P5 (with fluctuations during this time window that vary among brain regions), and then decrease to a lower level that is maintained into adulthood. TRF1 and TRF2 mRNA levels are relatively constant throughout brain development. Our data are consistent with a role for telomerase activity in proliferation of neural progenitor cells, and further suggest that TERT may play roles in neuronal differentiation and survival. The dissociation between TERT expression and telomerase activity is a novel finding that suggests biological functions for TERT in addition to telomere maintenance. J. Neurosci. Res. 64:252-260, 2001. Published 2001 Wiley-Liss, Inc.

Hypoxia extends the life span of vascular smooth muscle cells through telomerase activation

Chronic hypoxia induces smooth muscle cell proliferation and vessel wall remodeling in the vasculature of the lung. One well-characterized component of the hypoxic response is transcriptional activation of genes encoding vascular smooth muscle cell (VSMC) mitogens. We report here that chronic hypoxia can also prolong the growth of human VSMC by inducing telomerase activity and telomere stabilization. We demonstrate that hypoxia induced phosphorylation of the telomerase catalytic component (TERT) and sustained high levels of TERT protein expression in VSMC compared to normoxia. Furthermore, inhibition of telomerase shortened cell life span in hypoxic cultures, whereas constitutive expression of TERT extended the life span of cells under normoxic conditions. Our data indicate that hypoxic induction of telomerase activity could be involved in long-term growth of VSMC and may thus contribute to human vascular disorders.

A quantitative assay of telomerase activity

PURPOSE

Telomerase is a ribonucleoprotein that extends telomeres at the ends of chromosome. Increased telomerase activity is associated with cellular immortality. The currently available assay for telomerase, i.e., telomeric repeat amplification protocol (TRAP), consists of 2 steps: (a) telomerase-mediated extension of an oligonucleotide primer by the enzyme-containing extracts of cells and tissues, and (b) amplification of the telomerase-extended primer products by polymerase chain reaction (PCR) and detection of the PCR products. It is generally accepted that the current TRAP assay lacks quantitative precision. The present study was to develop a quantitative telomerase assay with greater precision and sensitivity.

METHODS

This new method used the primer extension method as in TRAP, plus the following modifications: (a) used a lysis buffer that yielded complete lysis of nuclei; (b) removal of PCR inhibitors by phenol/chloroform extraction after primer extension; and (c) used primers for the internal standard that were designed to reduce their competition with the telomerase products for PCR.

RESULTS

The modified method showed a good correlation (r2 = 0.99, P < 0.001) between telomerase amount (expressed as total protein in cell lysate) and its activity (expressed as telomerase products). Compared to the conventional TRAP, the new method (a) was more sensitive (average of 5.5-fold in cultured cancer cells and >5.9-fold in patient tumors), (b) had a lower inter- and intra-day variability (>3fold), and (c) showed a 2 to 4-fold broader range of linearity in the standard curve. The higher assay sensitivity further enabled the use of a nonradioactive method, i.e., ethidium bromide staining of DNA, to detect the TRAP products, as opposed to the use of radioactive nucleotide and the more labor-intensive autoradiography mandated by the conventional TRAP.

CONCLUSION

We report here a quantitative assay for telomerase activity in cultured human cancer cells and patient tumors.

Telomeric protein Pin2/TRF1 induces mitotic entry and apoptosis in cells with short telomeres and is down-regulated in human breast tumors

Telomeres are essential for cell survival and have been implicated in the mitotic control. The telomeric protein Pin2/TRF1 controls telomere elongation and its expression is tightly regulated during cell cycle. We previously reported that overexpression of Pin2/TRF1 affects mitotic progression. However, the role of Pin2/TRF1 at the interface between cell division and cell survival remains to be determined. Here we show that overexpression of Pin2 induced apoptosis in cells containing short telomeres, but not in cells with long telomeres. Furthermore, before entering apoptosis, Pin2-expressing cells first accumulated in mitosis and strongly stained with the mitosis-specific MPM2 antibody. Moreover, Pin2-induced apoptosis is potentiated by arresting cells in mitosis, but suppressed by accumulating cells in G1. In addition, overexpression of Pin2 also resulted in activation of caspase-3, and its proapoptotic activity was significantly reduced by inhibition of caspase-3. These results indicate that up-regulation of Pin2/TRF1 can specifically induce entry into mitosis and apoptosis, likely via a mechanism related to activation of caspase-3. Significantly, we also found that, out of 51 human breast cancer tissues and 10 normal controls examined, protein levels of Pin2/TRF1 in tumors were significantly lower than in normal tissues, as detected by immunoblotting analysis and immunocytochemistry. Since down-regulation of Pin2/TRF1 allows cells to maintain long telomeres, these results suggest that down-regulation of Pin2/TRF1 may be important for cancer cells to extend their proliferative potential.

Telomerase, immortality and cancer

Replication of eukaryotic linear chromosomes is incomplete and leaves terminal gaps. The evolutionary widely distributed solution to this "end replication" is twofold: chromosome ends are capped with telomeres, bearing multiple copies of redundant telomeric sequences, and the telomerase enzyme can add (lost) telomeric repeats. Telomerase in humans, as in all mammals, is ubiquitous in all embryonic tissues. In adults, telomerase remains active in germs cells, and, although down-regulated in most somatic tissues, telomerase is active in regenerative tissues and notably, in tumor cells. Telomerase activity is linked to cellular proliferation, and its activation seems to be a mandatory step in carcinogenesis. In contrast to mammals, indeterminately growing multicellular organisms, like fish and crustaceae, maintain unlimited growth potential or 'immortality' in all somatic tissues throughout their entire life. Also this cell immortalization is brought about by maintaining telomerase expression. Disease prognosis for human tumors includes evaluation of cell proliferation, based on the detection of proliferation markers with monoclonal antibodies. The significance of the classical marker Ki-67, and of a novel marker repp-86 are compared with semiquantitative telomerase assays. For tumor therapy, telomerase inhibitors are attractive tools. Results with telomerase knock-out mice have revealed promise, but also risk of this approach. On the other side, telomerase stimulation is attractive for expanding the potential of cellular proliferation in vitro, with possible applications for transplantation of in vitro expanded human cells, for immortalizing primary human cells as improved tissue models, and for the isolation of otherwise intractable products, like genuine human monoclonal antibodies.

Telomerase activity of cultured human pancreatic carcinoma cell lines correlates with their potential for migration and invasion

BACKGROUND

Despite the recent clinical finding that high telomerase activity is an unfavorable prognostic marker for various human malignant tumors, there has been no experimental evidence supporting the link between telomerase and tumor aggressiveness. In the current investigation, the authors examined the relation between telomerase activity and potential for biologic aggressiveness in human pancreatic carcinoma cells.

METHODS

Telomerase activity was measured in a poorly metastatic cell line HPC-3 and its highly metastatic variant HPC-3H4, as well as in many pancreatic carcinoma cell lines. Aggressive behavior of cancer cells was assessed by in vitro migration and invasion assay.

RESULTS

Compared with parental HPC-3, HPC-3H4 displayed higher telomerase activity, which was associated with a scattered phenotype and enhanced migration activity. Furthermore, the authors found that relative telomerase levels correlated well with both motility (P = 0.0041) and invasion (P = 0.0114) in 13 pancreatic carcinoma cell lines. There was, however, no significant association between telomerase activity and cell proliferation. When telomerase activity of KP-1N cells was inhibited by transfection with antisense oligonucleotides, their motility and invasion rates were significantly decreased.

CONCLUSIONS

The authors concluded that the magnitude of telomerase activation may reflect the potential for aggressive behavior within cancer cells. These findings support the clinical utility of telomerase activity as a prognostic indicator. Their results also suggest a therapeutic potential for telomerase inhibitors to prevent tumor invasion and possibly metastasis.

Expression of telomerase inhibits hydroxyl radical-induced apoptosis in normal telomerase negative human lung fibroblasts

In tumor cells telomerase activity is associated with resistance to apoptosis and the introduction of the human telomerase reverse transcriptase (hTERT) subunit into normal human cells is associated with life span extension of the cells. To determine the role of telomerase in regulating apoptosis, telomerase negative human embryo lung fibroblasts were transfected with the hTERT gene. Unlike the control fibroblasts, the telomerase-expressing cells had elongated telomeres and were resistant to apoptosis induced by hydroxyl radicals. The results indicate that expression of telomerase and, thus, the maintenance of telomere length in normal human somatic cells caused resistance to not only cellular senescence but also apoptosis. Moreover, we found that hydroxyl radical-induced apoptosis in telomerase-expressing and control fibroblasts was caspase-3 independent. These findings have revealed a new type of interrelation between telomerase and caspase-3, which may indicate that in this case the expressed telomerase may inhibit apoptosis at a site not related to the caspase-3 cascade.

Hydroxyl radical-induced apoptosis in human tumor cells is associated with telomere shortening but not telomerase inhibition and caspase activation

Reactive oxygen species (ROS) have been found to trigger apoptosis in tumor cells. At the same time, telomerase is found to be associated with malignancy and reduced apoptosis. However little is known about the linkage between ROS such as *OH and telomerase/telomere. To address the interrelations between *OH and telomerase/telomere in tumor cell killing, HeLa, 293 and MW451 cells were induced to undergo apoptosis with *OH radicals generated via Fe(2+)-mediated Fenton reactions (0.1 mM FeSO(4) plus 0.3-0.9 mM H2O2) and telomerase activity, telomere length were measured during apoptosis. We found that during *OH-induced apoptosis, telomere shortening took place while no changes in telomerase activity were observed. Our results suggest that *OH-induced telomere shortening is not through telomerase inhibition but possibly a direct effect of *OH on telomeres themselves indicating that telomere shortening but not telomerase inhibition is the primary event during *OH-induced apoptosis. Strikingly, we also found that *OH-induced apoptosis in HeLa cells is caspase-3-independent but is associated with reduction of mitochondrial transmembrane potential. Our results indicate that *OH triggers apoptotic tumor cell death through a telomere-related, caspase-independent pathway.

Emerging roles for telomerase in neuronal development and apoptosis

Telomerase is an enzyme consisting of a reverse transcriptase called TERT and an RNA component that adds repeats of a DNA sequence (TTAGGG) to the ends of chromosomes, thereby preventing their shortening and cell cycle arrest. Telomerase levels are high in neural progenitor cells and neurons during early development, and decrease in association with cell differentiation. A role for TERT in regulation of developmental death of neurons is suggested by a decrease in TERT expression that coincides with the period of neuronal death and by data showing that TERT promotes survival of developing brain neurons. Suppression of telomerase activity and TERT expression promotes apoptosis, whereas overexpression of TERT prevents apoptosis by suppressing cell death at a premitochondrial step in the death cascade Moreover, neurotrophic factors known to play important roles in brain development can regulate telomerase activity and TERT expression in cultured neural cells. A better understanding of the functions of telomerase and TERT in neuronal differentiation and survival may lead to novel approaches for preventing neuronal death and promoting recovery in various neurodegenerative conditions. J. Neurosci. Res. 63:1-9, 2001. Published 2001 Wiley-Liss, Inc.

Development of a condemned mucosa syndrome and pathogenesis of human papillomavirus-associated upper aerodigestive tract and uterine cervical tumors

High-risk human papillomaviruses (HPVs) have been shown to be involved in the pathogenesis of many squamous carcinomas, particularly those of the uterine cervix. A number of random studies have also reported association of high-risk HPV subtypes with cancers of the oral cavity, larynx, hypopharynx, and esophagus. The roles of other molecular factors involved during HPV infection in these tumors still remain unclear. Recent findings from our laboratories have suggested possible mechanisms associated with HPV-mediated carcinogenesis. Both p53 mutation-dependent and mutation-independent pathways may be associated with HPV-mediated carcinogenesis, the former mainly in upper aerodigestive tract tumors (UADT) and the latter in cervical tumors. In cervical tumors, inactivation of the p53 tumor suppressor protein by the E6 gene product of high-risk HPVs and mutation of the p53 gene in UADT is associated with alterations in the apoptotic regulatory bcl-2 and bax genes, leading to downregulation of programmed cell death (PCD) and increased cell proliferation. HPV infection is also associated with increased tissue angiogenesis and activation of telomerase. Altered kinetics of telomere fragments is evident in HPV-infected tissue. We therefore believe that the combined manifestations of all these factors may contribute to development of a "condemned mucosa syndrome" facilitating development UADT and cervical cancers. A distinct step in the pathogenesis of both types of tumors may only be in the mode of p53 inactivation, whereas all other events appear to be strongly correlated to the presence of HPV. The development and validation of such a molecular model has significant clinical priority. It can be used to identify target populations or individuals for intervention, to monitor effects of intervention, and to determine which individuals or groups are at increased risk of developing cancer.

hTERT can function with rabbit telomerase RNA: regulation of gene expression and attenuation of apoptosis

Telomerase is a specialized DNA polymerase that adds telomeric sequences onto chromosome ends. The functional telomerase complex contains a telomerase reverse transcriptase (TERT) and also a telomerase RNA (TR). Although it is well established that the human telomerase reverse transcriptase (hTERT) can function well in different human cell lines, it has not been shown whether it is compatible with telomerase template RNA from other species. Here we report that the expressed hTERT is functionally compatible with rabbit telomerase template RNA (rTR) as demonstrated by TRAP assay. The direct interaction between hTERT and rTR is further confirmed by immunoprecipitation-linked RT-PCR in which rTR is detected from the complex immunoprecipitated by an anti-hTERT antibody. The hTERT expressed in rabbit lens epithelial cells demonstrates two major functions: modulation of expression of other genes and attenuation of apoptosis. Thus, telomerase has a variety of functions besides telomere synthesis, and the template RNA is functionally conserved between human and rabbit.

Apoptosis in neurodegenerative disorders

Neuronal death underlies the symptoms of many human neurological disorders, including Alzheimer's, Parkinson's and Huntington's diseases, stroke, and amyotrophic lateral sclerosis. The identification of specific genetic and environmental factors responsible for these diseases has bolstered evidence for a shared pathway of neuronal death--apoptosis--involving oxidative stress, perturbed calcium homeostasis, mitochondrial dysfunction and activation of cysteine proteases called caspases. These death cascades are counteracted by survival signals, which suppress oxyradicals and stabilize calcium homeostasis and mitochondrial function. With the identification of mechanisms that either promote or prevent neuronal apoptosis come new approaches for preventing and treating neurodegenerative disorders.

Attenuation of telomerase activity does not increase sensitivity of human melanoma cells to anticancer agents

In tumour cells, replicative immortality is attained through stabilisation of telomeres by telomerase. Recent evidence suggests that telomerase plays an anti-apoptotic role. Since apoptosis is the primary mode of cell death induced by several drugs, telomerase could be involved in determining the chemosensitivity profile of tumour cells. We investigated whether inhibition of telomerase activity through a hammerhead ribozyme targeting the RNA template of telomerase influences the susceptibility of human melanoma cells to a variety of anticancer agents (platinum compounds, taxanes, topoisomerase I inhibitors). The ribozyme sequence was inserted into an expression vector and the JR8 human melanoma cell line was transfected with it. The cell clones obtained showed a reduced telomerase activity. Growth inhibition curves generated after exposure of ribozyme-transfectant clones to individual drugs were superimposable to those obtained from parental cells. Moreover, telomerase inhibition did not promote apoptosis as a cellular response to drug treatment. Overall, our results indicate that downregulation of telomerase activity does not increase the sensitivity of melanoma cells to anticancer drugs.

Comparison of telomerase levels before and after differentiation of two cell lines of human neuroblastoma

BACKGROUND

Telomerase is the enzyme that is responsible for maintaining telomere length in human germ cells, tumor cells, and immortalized cells. Its specific role in the immortilization process is unknown. This study was performed to determined whether the level of telomerase activity in human neuroblastoma cell lines correlated with their level of differentiation. We proposed that as neuroblastoma cells differentiated into more mature or benign cells, the levels of telomerase expression would decrease.

MATERIALS AND METHODS

Two human neuroblastoma cells lines, SK-N-AS and SK-N-DZ, were differentiated using retinoic acid. These cells were assayed for telomerase activity by the telomere repeat amplification protocol (TRAP) before, during, and after treatment with retinoic acid for 8 days. Untreated cells were used for control and were compared to the retinoic acid-treated cells. Differentiation of the cell lines was confirmed by assaying expression of ret mRNA using the reverse-transcriptase polymerase chain reaction (RT-PCR) and gel electrophoresis of the radiolabeled products.

RESULTS

No statistical difference in telomerase activity was noted between control and treated groups.

CONCLUSIONS

While telomerase activity has been shown by others to correlate with tumor aggressiveness in human neuroblastoma cells, the mechanism that is involved appears to be separate from cellular differentiation.

Lineage-specific telomere shortening and unaltered capacity for telomerase expression in human T and B lymphocytes with age

Age effects on telomere length and telomerase expression in peripheral blood lymphocytes were analyzed from 121 normal individuals age newborn to 94 years and revealed several new findings. 1) Telomere shortening was observed in CD4+ and CD8+ T and B cells with age. However, the rate of telomere loss was significantly different in these populations, 35 +/- 8, 26 +/- 7, and 19 +/- 7 bp/year for CD4+ and CD8+ T and B cells, respectively. In addition, CD4+ T cells had the longest average telomeres at all ages, followed by B cells, with CD8+ T cell telomeres the shortest, suggesting that these lymphocyte populations may have different replicative histories in vivo. 2) Telomerase activity in freshly isolated T and B cells was indistinguishably low to undetectable at all ages but was markedly increased after Ag and costimulatory receptors mediated stimulation in vitro. Furthermore, age did not alter the magnitude of telomerase activity induced after stimulation of T or B lymphocytes through Ag and costimulatory receptors or in response to PMA plus ionomycin treatment. 3) The levels of telomerase activity induced by in vitro stimulation varied among individual donors but were highly correlated with the outcome of telomere length change in CD4+ T cells after Ag receptor-mediated activation. Together, these results indicate that rates of age-associated loss of telomere length in vivo in peripheral blood lymphocytes is specific to T and B cell subsets and that age does not significantly alter the capacity for telomerase induction in lymphocytes.

The role of telomerase expression and telomere length maintenance in human and mouse

The molecular regulation of telomere length has been well elucidated by a series of elegant studies over the past decade. More recently, experimental evidence has accrued that addresses the challenging question of if and how telomere length regulation may contribute to normal human aging or to human disease. Recent studies in mice have provided a mammalian precedent indicating that telomerase deficiency can lead to in vivo dysfunction, most probably as a consequence of progressive telomere shortening. In humans, the evidence that telomere shortening might lead to in vivo dysfunction is far less direct, although the recent description of telomerase deficiency and telomere shortening associated with the DKC syndrome is suggestive of such a link. Methodologies exist and continue to be developed that are increasingly capable of manipulating telomerase activity and telomere length in human cells. It remains to be determined whether scientifically rigorous and (equally important) medically ethical approaches will emerge to directly assess the ability of telomere length modulation to correct functional disorders of human cellular function ex vivo or more challenging still, in vivo.

The catalytic subunit of telomerase protects neurons against amyloid beta-peptide-induced apoptosis

The catalytic subunit of telomerase (TERT) is a specialized reverse transcriptase that has been associated with cell immortalization and cancer. It was reported recently that TERT is expressed in neurons throughout the brain in embryonic and early postnatal development, but is absent from neurons in the adult brain. We now report that suppression of TERT levels and function in embryonic mouse hippocampal neurons in culture using antisense technology and the telomerase inhibitor 3' -azido-2' 3' -dideoxythymidine significantly increases their vulnerability to cell death induced by amyloid beta-peptide, a neurotoxic protein believed to promote neuronal degeneration in Alzheimer's disease. Neurons in which TERT levels were reduced exhibited increased levels of oxidative stress and mitochondrial dysfunction following exposure to amyloid beta-peptide. Overexpression of TERT in pheochromocytoma cells resulted in decreased vulnerability to amyloid beta-peptide-induced apoptosis. Our findings demonstrate a neuroprotective function of TERT in an experimental model relevant to Alzheimer's disease, and suggest the possibility that restoration of TERT expression in neurons in the adult brain may protect against age-related neurodegeneration.

Fibroblast growth factor 2 up regulates telomerase activity in neural precursor cells

During brain development, neuronal and glial cells are generated from neural precursors on a precise schedule involving steps of proliferation, fate commitment and differentiation. We report that telomerase activity is highly expressed during embryonic murine cortical neurogenesis and early steps of gliogenesis and progressively decreases thereafter during cortex maturation to be undetectable in the normal adult brain. We evidenced neural precursor cells (NPC) as the principal telomerase-expressing cells in primary cultures from E15 mouse embryo cortices. Their differentiation either in neurons or in glial cells lead to a down regulation of telomerase activity that was directly correlated to the decrease of telomerase core protein (mTERT) mRNA synthesis. Furthermore, we show that FGF2 (fibroblast growth factor 2), one of the main regulators of CNS development, induces a dose-dependant increase of both the proliferation of NPC and telomerase activity in primary cortical cultures without affecting the mTERT mRNA synthesis compared to that of glyceraldehyde-3-phosphate dehydrogenase (mGAPDH). Finally, we evidenced that AZT (3'-azido-2', 3'-dideoxythymidine), known to inhibit telomerase activity, blocks in a dose dependant manner the FGF2-induced proliferation of NPC. Altogether, our results are in favor of an important role of telomerase activity during brain organogenesis. Oncogene (2000).

Up-regulation of telomerase activity in human pancreatic cancer cells after exposure to etoposide

Telomerase plays a critical role in the development of cellular immortality and oncogenesis. Activation of telomerase occurs in a majority of human malignant tumours, and the relation between telomerase and vulnerability to drug-mediated apoptosis remains unclear. In this study, we demonstrate, for the first time, up-regulation of telomerase activity in human pancreatic cancer cells treated with etoposide, a topoisomerase II inhibitor. Exposure of MIA PaCa-2 cells to etoposide at various concentrations (1-30 microM) resulted in two- to threefold increases in telomerase activity. Up-regulation was detectable 24 h after drug exposure and was accompanied by enhanced expression of mRNA of the human telomerase reverse transcriptase. Telomerase activation was also observed in AsPC-1 and PANC-1 cells but not in KP-3 and KP-1N cells. Furthermore, we found a negative correlation between increased telomerase activity and the percentage of dead cells after etoposide treatment. These findings suggest the existence of an anti-apoptotic pathway through which telomerase is up-regulated in response to DNA damage. This telomerase activation pathway may be one of the mechanisms responsible for the development of etoposide resistance in certain pancreatic cancer cells.

Telomerase activity in rat cardiac myocytes is age and gender dependent

Telomerase replaces telomeric repeat DNA lost during the cell cycle, restoring telomere length. This enzyme is present only during cell replication and its activity reflects the extent of proliferation. Whether cardiac myocytes are terminally differentiated cells is still a highly controversial issue, and the possibility of myocyte division is frequently rejected. On this basis, telomerase was measured in pure preparations of myocytes, isolated from rats throughout their lifespan. Fetal and neonatal rat myocytes were used as positive control cells. Contrary to expectation, the authors report that telomerase activity was detectable in pure preparations of young adult, fully mature adult, and senescent ventricular myocytes, defeating the dogma that this cell population is permanent and irreplaceable. Aging decreased 31% telomerase activity in male myocytes. An opposite effect occurred in female myocytes in which this enzyme increased 72%. This differential adaptation between the two genders in the rat model may be relevant to observations in humans; myocyte loss occurs in men as a function of age, whereas myocyte number is preserved in women. The greater growth potential of female myocytes may be critical for the longer lifespan and decreased incidence of heart failure in women.

Advances in the management of adrenal tumors

Adrenal tumors are very common, with the majority being nonhypersecretory and benign and less than 1% being malignant. Most primary adrenal tumors are sporadic, but may be associated with other endocrine and familial disorders, especially pheochromocytoma. All patients with "sporadic" pheochromocytoma should be screened for MEN-2 and Von Hippel-Lindau disease. As in many endocrine tumors, there are no uniform definitive histologic criteria to distinguish malignancy, which is dependent on the clinical behavior of the tumor and is accurately diagnosed in the presence of adjacent organ invasion, recurrence, or distant metastasis. Surgery remains the cornerstone and the treatment of choice for functional and primary malignant adrenal tumors, both for cure and palliation, with low morbidity and mortality.

Studies of the molecular mechanisms in the regulation of telomerase activity

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

Human endothelial cell life extension by telomerase expression

Normal human endothelial cells, like other somatic cells in culture, divide a limited number of times before entering a nondividing state called replicative senescence. Expression of the catalytic component of human telomerase, human telomerase reverse transcriptase (hTERT), extends the life span of human fibroblasts and retinal pigment epithelial cells beyond senescence without causing neoplastic transformation (Bodnar, A. G., Ouellette, M., Frolkis, M., Holt, S. E., Chiu, C. P., Morin, G. B., Harley, C. B., Shay, J. W., Lichtsteiner, S., and Wright, W. E. (1998) Science 279, 349-352; Jiang, X., Jimenez, G., Chang, E., Frolkis, M., Kusler, B., Sage, M., Beeche, M., Bodnar, A., Wahl, G., Tlsty, T., and Chiu, C.-P. (1999) Nat. Genet. 21, 111-114). Here, we show that both human large vessel and microvascular endothelial cells also bypass replicative senescence after introduction of hTERT. For the first time, we report that hTERT expression in these life-extended vascular cells does not affect their differentiated and functional phenotype and that these cells maintain their angiogenic potential in vitro. Furthermore, hTERT(+) microvascular endothelial cells have normal karyotype, and hTERT(+) endothelial cell strains do not exhibit a transformed phenotype. Relative to parental cells at senescence, hTERT-expressing endothelial cells exhibit resistance to induction of apoptosis by a variety of different conditions. Such characteristics are highly desirable for designing vascular transplantation and gene therapy delivery systems in vivo.