hTERT associates with human telomeres and enhances genomic stability and DNA repair

Mitochondrial localization of telomerase as a determinant for hydrogen peroxide-induced mitochondrial DNA damage and apoptosis

We have previously shown that the protein subunit of telomerase, hTERT, has a bonafide N-terminal mitochondrial targeting sequence, and that ectopic hTERT expression in human cells correlated with increase in mtDNA damage after hydrogen peroxide treatment. In this study, we show, using a loxP hTERT construct, that this increase in mtDNA damage following hydrogen peroxide exposure is dependent on the presence of hTERT itself. Further experiments using a dominant negative hTERT mutant shows that telomerase must be catalytically active to mediate the increase in mtDNA damage. Etoposide, but not methylmethanesulfate, also promotes mtDNA lesions in cells expressing active hTERT, indicating genotoxic specificity in this response. Fibroblasts expressing hTERT not only show a approximately 2-fold increase in mtDNA damage after oxidative stress but also suffer a 10-30-fold increase in apoptotic cell death as assayed by Annexin-V staining, caspase-3 activation and PARP cleavage. Mutations to the N-terminal mitochondrial leader sequence causes a complete loss of mitochondrial targeting without affecting catalytic activity. Cells carrying this mutated hTERT not only have significantly reduced levels of mtDNA damage following hydrogen peroxide treatment, but strikingly also do not shown any loss of viability or cell growth. Thus, localization of hTERT to the mitochondria renders cells more susceptible to oxidative stress-induced mtDNA damage and subsequent cell death, whereas nuclear-targeted hTERT, in the absence of mitochondrial localization, is associated with diminished mtDNA damage, increased cell survival and protection against cellular senescence.

Mammalian Rad9 plays a role in telomere stability, S- and G2-phase-specific cell survival, and homologous recombinational repair

The protein products of several rad checkpoint genes of Schizosaccharomyces pombe (rad1+, rad3+, rad9+, rad17+, rad26+, and hus1+) play crucial roles in sensing changes in DNA structure, and several function in the maintenance of telomeres. When the mammalian homologue of S. pombe Rad9 was inactivated, increases in chromosome end-to-end associations and frequency of telomere loss were observed. This telomere instability correlated with enhanced S- and G2-phase-specific cell killing, delayed kinetics of gamma-H2AX focus appearance and disappearance, and reduced chromosomal repair after ionizing radiation (IR) exposure, suggesting that Rad9 plays a role in cell cycle phase-specific DNA damage repair. Furthermore, mammalian Rad9 interacted with Rad51, and inactivation of mammalian Rad9 also resulted in decreased homologous recombinational (HR) repair, which occurs predominantly in the S and G2 phases of the cell cycle. Together, these findings provide evidence of roles for mammalian Rad9 in telomere stability and HR repair as a mechanism for promoting cell survival after IR exposure.

SNEV overexpression extends the life span of human endothelial cells

In a recent screening for genes down regulated in replicatively senescent human umbilical vein endothelial cells (HUVECs), we have isolated the novel protein SNEV. Since then SNEV has proven as a multifaceted protein playing a role in pre-mRNA splicing, DNA repair, and the ubiquitin/proteosome system. Here, we report that SNEV mRNA decreases in various cell types during replicative senescence, and that it is increased in various immortalized cell lines, as well as in breast tumors, where SNEV transcript levels also correlate with the survival of breast cancer patients. Since these mRNA profiles suggested a role of SNEV in the regulation of cell proliferation, the effect of its overexpression was tested. Thereby, a significant extension of the cellular life span was observed, which was not caused by altered telomerase activity or telomere dynamics but rather by enhanced stress resistance. When SNEV overexpressing cells were treated with bleomycin or bleomycin combined with BSO, inducing DNA damage as well as reactive oxygen species, a significantly lower fraction of apoptotic cells was found in comparison to vector control cells. These data suggest that high levels of SNEV might extend the cellular life span by increasing the resistance to stress or by improving the DNA repair capacity of the cells.

Overexpression of telomerase confers growth advantage, stress resistance, and enhanced differentiation of ESCs toward the hematopoietic lineage

Embryonic stem cells (ESCs) are capable of extended self-renewal and maintenance of pluripotency even after many population doublings. This is supported by high levels of telomerase activity and enhanced antioxidant protection in ESCs, both of which are downregulated during differentiation. To examine the role of telomerase for ESC self-renewal and differentiation, we overexpressed the reverse transcriptase subunit (Tert) of murine telomerase in ESCs. Increased telomerase activity enhances the self-renewal ability of the Tert-overexpressing ESCs, improves their resistance to apoptosis, and increases their proliferation. The differentiated progeny of wild-type ESCs express little Tert and show shortening of telomeric overhangs. In contrast, the progeny of Tert-overexpressing ESCs maintain high telomerase activity, as well as the length of G-rich overhangs. In addition, these cells accumulate lower concentrations of peroxides than wild-type cells, implying greater resistance to oxidative stress. Finally, differentiation toward hematopoietic lineages is more efficient as a result of the continued expression of Tert. Microarray analysis revealed that overexpression of Tert altered expression of a variety of genes required for extended self-renewal and lifespan. Our results suggest that telomerase functions as a "survival enzyme" in ESCs and its differentiated progeny by protecting the telomere cap and by influencing the expression patterns of stress response and defense genes. This results in improved proliferation of ESCs and more efficient differentiation, and these results might have profound consequences for stem cell-replacement therapies.

Role of HSPs and telomerase in radiotherapy

The inability of radiotherapy to control tumour growth is still a daunting clinical problem leading to failure of the overall treatment regimens. The fundamental question is; could tumour cells be specifically sensitized to ionizing radiation (IR) by heat or factors exclusively expressed in tumour cells? One such factor, expressed in most tumours and silent in somatic cells, is telomerase. Biochemical and genetic studies have established an association between telomere maintenance and extended life span of human cells mediated through the expression of the catalytic sub-unit of telomerase (hTERT). Because of this, telomerase is an attractive target for inhibition in anti-cancer therapy. Telomeres are maintained by telomerase and hTERT interacts with heat shock protein (HSP) chaperones. This review will focus on the possible role of HSPs and telomerase in sensitizing tumour cells and, thus, enhancing the potential of targeted radiotherapy.

Microarray analyses in bladder cancer cells: Inhibition of hTERT expression down-regulates EGFR

The human telomerase reverse transcriptase (hTERT) contributes to the immortal phenotype of the majority of cancers. Targeting hTERT by transfection with antisense oligonucleotides (AS-ODNs) induced immediate growth inhibition in human bladder cancer (BCa) cells. The molecular basis of the antiproliferative capacity of hTERT AS-ODNs was investigated by oligonucleotide microarray analyses and was compared to effects caused by siRNA-mediated knock-down of hTERT in EJ28 BCa cells. Two different AS-ODNs -- both down-regulated the expression of hTERT -- changed the expression of different genes mainly involved in stress response (including EGR1, ATF3 and GDF15), but without an association to telomerase function. This indicates that the immediate growth inhibition was caused, at least in part, by off-target effects. In comparison to that the blockade of the expression of hTERT using 2 different siRNAs was accompanied by the down-regulation of the oncogenes FOS-like antigen 1 (FOSL1) and epidermal growth factor receptor (EGFR), known to be overexpressed in BCa. We show here for the first time that repression of the hTERT transcript number decreased the expression of EGFR both at the mRNA and protein levels, suggesting a potential new function of hTERT in the regulation of EGFR-stimulated proliferation. Furthermore, the suppression of hTERT by siRNAs caused an enhancement of the antiproliferative capacity of the chemotherapeutics mitomycin C and cisplatin. The results presented herein may support the hypothesis that hTERT promotes the growth of tumor cells by mechanisms independent from telomere lengthening. The detailed clarification of these processes will shed light on the question, whether telomerase inhibitors might constitute suitable anticancer tools.

Changes in telomerase activity after irradiation of human peripheral blood mononuclear cells (PBMC) in vitro

Telomerase activity (TA) has been shown to be up-regulated by ionising radiation in immortal hematopoietic cell lines. The purpose of the present experiments was to test whether ionising radiation may regulate TA in normal human peripheral blood mononuclear cells (PBMC). A real-time PCR assay was established to quantify TA detected by the telomeric repeat amplification protocol. TA of PBMCs isolated from young healthy donors was highly increased by stimulation with phytohemagglutinin (PHA) for 72 h. Irradiation of PHA-stimulated PBMCs with 2-10 Gy of X rays showed up-regulated TA 4 h after irradiation with an enhancement at least as strong as for the human TK6 lymphoblastoid cell line. The present results show that TA is up-regulated by irradiation not only in immortal cell lines but also in mitogen-stimulated PBMCs. This supports a possible role for telomerase in the cellular radiation response.

Involvement of hTERT in apoptosis induced by interference with Bcl-2 expression and function

Here, we investigated the role of telomerase on Bcl-2-dependent apoptosis. To this end, the 4625 Bcl-2/Bcl-xL bispecific antisense oligonucleotide and the HA14-1 Bcl-2 inhibitor were used. We found that apoptosis induced by 4625 oligonucleotide was associated with decreased Bcl-2 protein expression and telomerase activity, while HA14-1 triggered apoptosis without affecting both Bcl-2 and telomerase levels. Interestingly, HA14-1 treatment resulted in a profound change from predominantly nuclear to a predominantly cytoplasmic localization of hTERT. Downregulation of endogenous hTERT protein by RNA interference markedly increased apoptosis induced by both 4625 and HA14-1, while overexpression of wild-type hTERT blocked Bcl-2-dependent apoptosis in a p53-independent manner. Catalytically and biologically inactive hTERT mutants showed a similar behavior as the wild-type form, indicating that hTERT inhibited the 4625 and HA14-1-induced apoptosis regardless of telomerase activity and its ability to lengthening telomeres. Finally, hTERT overexpression abrogated 4625 and HA14-1-induced mitochondrial dysfunction and nuclear translocation of hTERT. In conclusion, our results demonstrate that hTERT is involved in mitochondrial apoptosis induced by targeted inhibition of Bcl-2.

Efficient inhibition of human telomerase reverse transcriptase expression by RNA interference sensitizes cancer cells to ionizing radiation and chemotherapy

Telomerase activation plays critical roles in tumor growth and progression in part through the maintenance of telomere structure. Indeed, the ubiquitous expression of telomerase in human cancers makes telomerase a promising target for cancer therapy. Genetic, pharmacologic, and antisense methods to inhibit telomerase have been described; however, in most cases, cancer cell death was observed only after many cell divisions. Here, using retroviral delivery of small interfering RNAs (siRNAs) specific for the human telomerase reverse transcriptase (hTERT), we successfully inhibited telomerase activity in cervical cancer cell lines. Cells lacking hTERT expression exhibited significantly decreased telomerase activity and showed shortened telomeres and telomeric 3' overhangs with passage. These cells entered replicative senescence after a considerable number of cell divisions. Notably, the proliferative rate of these cells was significantly impaired, compared with control cells with telomerase activity, even in low-passage cells (population doubling 5). Likewise, colony-forming ability and tumorigenicity in mice were attenuated in low-passage cells lacking hTERT. We further examined the effects of chemotherapy and ionizing radiation on cells in which hTERT expression was suppressed. Cells lacking hTERT showed a significantly increased sensitivity, compared with control cells, to ionizing radiation or chemotherapeutic agents that induce DNA double- strand breaks, such as topoisomerase inhibitors or bleomycin. These findings suggest that an siRNA-based strategy can be applied to the development of novel telomerase inhibitors, the antitumor effects of which may be enhanced in combination with ionizing radiation and chemotherapy.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Transcriptional upregulation of DNA polymerase beta by TEIF

The overexpression of DNA polymerase beta (beta-pol) has been identified in lots of human cancers, but the mechanism has seldom been investigated. Telomerase transcriptional element-interacting factor (TEIF) can bind to hTERT promoter, stimulating its transcription and telomerase activities. Here, we report that TEIF could also enhance the expression of beta-pol at transcription level. TEIF could specifically activate transcription of beta-pol promoter, but not that of DNA polymerase alpha or delta promoter. The responsible sequences for binding of TEIF were revealed as GC-rich elements dispersing from +19 to -29 nt of beta-pol promoter, which due to mutations caused decreasing in binding of TEIF and apparent losing of transactivation activity. The in vivo interaction between TEIF and beta-pol promoter was identified by chromatin immunoprecipitation assay. Besides, ectopic expression of TEIF in HeLa cells could upregulate both levels of endogenous beta-pol mRNA and protein, and consequently increases resistance to the oxidative stress of H2O2. The data may provide new clue to the elucidation of beta-pol overexpression in cancers and also a functional link between beta-pol and telomerase.

The canine telomerase catalytic subunit (dogTERT): Characterisation of the gene promoter and identification of proximal core sequences necessary for specific transcriptional activity in canine telomerase positive cell lines

Telomerase biology is complicated by studies that show that telomere expression and telomere biology differs between species, and that existing animal models do not closely resemble the human situation. We have previously reported a description of telomere/telomerase biology in the dog and have suggested this as an alternative model system. To further elucidate telomerase biology in this species we have cloned and characterised the canine reverse transcriptase (dogTERT) promoter. We demonstrate that core promoter activity is contained within a region extending approximately 300 bp upstream of the ATG codon. Transient transfections in telomerase-positive canine cell lines and telomerase negative fibroblasts showed that the promoter is only active in telomerase positive cell lines. Sequence analysis demonstrated that the 5' regulatory region is GC-rich and contains no TATA or CAAT box, similar to the human TERT promoter. Motif searches revealed the presence of multiple transcription factor binding sites common to both the human and canine TERT promoters, including a single E-box, Sp1, AP1, MZF-2 and ER/Sp1 binding sites. These findings suggest that the dogTERT gene shares similar transcriptional control to hTERT. Identification of the core promoter necessary for activity may allow the use of naturally occurring cancers in dogs as a preclinical testing ground for telomerase targeted therapies in human cancer patients.

Involvement of human MOF in ATM function

We have determined that hMOF, the human ortholog of the Drosophila MOF gene (males absent on the first), encoding a protein with histone acetyltransferase activity, interacts with the ATM (ataxia-telangiectasia-mutated) protein. Cellular exposure to ionizing radiation (IR) enhances hMOF-dependent acetylation of its target substrate, lysine 16 (K16) of histone H4 independently of ATM function. Blocking the IR-induced increase in acetylation of histone H4 at K16, either by the expression of a dominant negative mutant DeltahMOF or by RNA interference-mediated hMOF knockdown, resulted in decreased ATM autophosphorylation, ATM kinase activity, and the phosphorylation of downstream effectors of ATM and DNA repair while increasing cell killing. In addition, decreased hMOF activity was associated with loss of the cell cycle checkpoint response to DNA double-strand breaks. The overexpression of wild-type hMOF yielded the opposite results, i.e., a modest increase in cell survival and enhanced DNA repair after IR exposure. These results suggest that hMOF influences the function of ATM.

Telomerase: a potential therapeutic target for cancer

Telomeres are complex structures which serve to protect chromosome ends. Telomere shortening occurs in normal somatic cells reaching a point in which cells senesce. Senescence can be counteracted by activating telomerase. Telomerase activity is present in a majority of cancer cells and requires the upregulation of the reverse transcriptase component called hTERT. Because telomerase activity is essential for proliferation of most cancer cells, therapeutic strategies have been developed to inhibit its activity. These strategies centre on targeting the active site, hTERT and hTERC expression, core enzyme stability and telomeric DNA. Successful approaches involve a combination of traditional drugs with telomerase inhibitors. Disrupting the functional expression of hTERT is particularly effective in agreement with evidence that hTERT is an antiapoptotic factor in some cancer cells. In addition, approaches that stabilise DNA secondary structures may disrupt telomere maintenance through a variety of routes making them, potentially, very potent in attack-ing cancer cells.

Manganese Superoxide Dismutase Protects the Proliferative Capacity of Confluent Normal Human Fibroblasts

We tested the hypothesis that manganese superoxide dismutase (MnSOD), an antioxidant enzyme, regulates the proliferative potential of confluent human fibroblasts. Normal human skin (AG01522) and lung (WI38, CCL-75) fibroblasts kept in confluence (>95% G(0)/G(1)) showed a significant decrease in their capacity to re-enter the proliferation cycle after 40-60 days. The inhibition of re-entry was accompanied with the age-dependent increase of p16 protein levels in the confluent culture. Adenoviral mediated overexpression of MnSOD during confluent growth suppressed p16, enhanced p21 protein accumulation, and protected fibroblasts against the loss of proliferation potential. Increases in p21 protein levels in MnSOD overexpressing confluent fibroblasts were independent of p53 protein levels. p53 protein levels did not change in control, replication-defective adenovirus containing an insertless vector (AdBgl II), or AdMnSOD-infected confluent cells cultured for 20 and 60 days. In addition, MnSOD-induced protection of the proliferation capacity of confluent fibroblasts was independent of their telomerase activity. However, telomerase-transformed fibroblasts showed increased MnSOD expression in confluent growth, maintaining their capacity to re-enter the proliferation cycle. Although inactivation of the retinoblastoma protein in cells subcultured from the 60-day confluent control, AdBgl II-, and AdMnSOD-infected fibroblasts was identical, only MnSOD-overexpressing cells showed a higher percentage of S-phase. These results support the hypothesis that a redox-sensitive checkpoint regulated the progression of fibroblasts from G(0)/G(1) to S-phase.

Telomere dysfunction in aging and cancer

Telomeres are unique DNA-protein structures that contain noncoding TTAGGG repeats and telomere-associated proteins. These specialized structures are essential for maintaining genomic integrity. Alterations that lead to the disruption of telomere maintenance result in chromosome end-to-end fusions and/or ends being recognized as double-strand breaks. A large body of evidence suggests that the cell responds to dysfunctional telomeres by undergoing senescence, apoptosis, or genomic instability. In conjunction with other predisposing mechanisms, the genomic instability encountered in preimmortal cells due to dysfunctional or uncapped telomeres might lead to cancer. Furthermore, telomere dysfunction has been proposed to play critical roles in aging as well as cancer progression. Conversely, recent evidence has shown that targeting telomere maintenance mechanisms and inducing telomere dysfunction in cancer cells by inhibiting telomerase can lead to catastrophic events including rapid cell death and increased sensitivity to other cancer therapeutics. Thus, given the major role telomeres play during development, it is important to continue our understanding telomere structure, function and maintenance. Herein, we provide an overview of the emerging knowledge of telomere dysfunction and how it relates to possible links between aging and cancer.

Telomeres, telomerase, and apoptosis

Telomeres are specialized high-order chromatin structures that cap the ends of eukaryotic chromosomes. In vertebrates, telomeric DNA is composed of repetitions of the TTAGGG hexanucleotide, is bound to a set of specific proteins, and is elongated by the reverse transcriptase enzyme telomerase. Telomerase activity is promptly detected in cells with an indefinite replicative potential, such as cancer cells, while is almost undetectable in normal cells, which are characterized by a limited life span. Mounting evidence indicates that the maintenance of telomere integrity and telomerase protect cells from apoptosis. Disruption of the telomere capping function and (or) telomerase inhibition elicit an apoptotic response in cancer cells, while restoration of telomerase activity in somatic cells confers resistance to apoptosis. The possible mechanisms linking telomeres, telomerase and apoptosis are discussed in this review, together with the impact of this field in anticancer research.

Antisense oligonucleotide-mediated inhibition of hTERT, but not hTERC, induces rapid cell growth decline and apoptosis in the absence of telomere shortening in human prostate cancer cells

Recent evidence points to a novel function of human telomerase reverse transcriptase (hTERT) in promoting tumour cell survival, which might be independent of the telomere-elongating activity of the enzyme. To test this hypothesis, we evaluated comparatively the effects of telomerase inhibition, accomplished through antisense oligonucleotide-mediated interference with hTERT or human telomerase RNA component (hTERC), on the proliferative potential of DU145 human prostate cancer cells. Exposure of cells to a 2'-O-methyl-RNA phosphorothioate oligonucleotide targeting a splicing site within hTERT pre-mRNA induced almost complete inhibition of telomerase activity as a consequence of a marked reduction of the hTERT mRNA expression level, an early decline of DU145 cell growth and apoptotic cell death without any appreciable telomere shortening. Conversely, exposure of DU145 cells to a 2'-O-methyl-RNA phosphorothioate oligonucleotide targeting the template region of hTERC failed to interfere with cell proliferation in spite of the almost complete abrogation of telomerase activity. These results extend and corroborate earlier evidence in favour of an enzymatic activity-independent mechanism by which hTERT maintains tumour cell survival and proliferation.

Chemosensitization of bladder cancer cell lines by human telomerase reverse transcriptase antisense treatment

PURPOSE

Responses of transitional cell carcinoma of the bladder (TCC) to commonly used chemotherapy agents such as mitomycin C (MMC), cisplatin and gemcitabine are often disappointing. Since human telomerase reverse transcriptase (hTERT) is tumor specifically expressed and contributes to the immortality and malignancy of the majority of tumors, it is regarded as a suitable antitumor target. We investigated whether combinations of hTERT antisense (AS)-oligonucleotides (ODNs) with common chemotherapy (CT) schedules may improve drug mediated antitumor effects.

MATERIALS AND METHODS

Initial screening for enhancement of the inhibitory effects of MMC, cisplatin and gemcitabine on viability by treatment with the 2 hTERT AS-ODNs ASt2206 and ASt2331 was performed in 4 TCC cell lines prior to CT. Apoptosis was assessed by annexin V staining and detection of activated caspase-3 using Western blot analysis. Nonsense (NS)-ODN served as a control in all experiments.

RESULTS

All cell lines responded to the anticancer agents tested. Treatment with AS plus CT resulted in a significantly stronger inhibition of viability than the NS plus CT control in the majority of combinations, indicating an AS specific enhancement effect. For example, ASt2331 plus MMC decreased viability to 17% in contrast to NS plus MMC (58%) in EJ28 cells. All ASt2331 plus CT combinations specifically increased the rate of apoptosis 1.3 to 3.0-fold compared with NS plus CT. Apoptosis induction was associated with caspase 3 activation.

CONCLUSIONS

Enhancement of cytotoxic drug effects on the growth of TCC cells by hTERT AS-ODNs presented herein allows a dose decrease in chemotherapy and confirms the suitability of hTERT as a target in a specific therapy approach.

Tankyrase 1 as a target for telomere-directed molecular cancer therapeutics

Telomere elongation by telomerase is repressed in cis by the telomeric protein TRF1. Tankyrase 1 poly(ADP-ribosyl)ates TRF1 and releases it from telomeres, allowing access of telomerase to telomeres. Here we demonstrate that tankyrase 1 inhibition in human cancer cells enhances telomere shortening by a telomerase inhibitor and hastens cell death. Conversely, either tankyrase 1 upregulation or telomere shortening, each of which decreased TRF1 loading on a chromosome end, attenuated the impact of telomerase inhibition. These results are consistent with the idea that telomeres having fewer TRF1s increase the efficiency of their elongation by telomerase. This study implies that both enzyme activity and accessibility to telomeres can be targets for telomerase inhibition.

Introduction of human telomerase reverse transcriptase to normal human fibroblasts enhances DNA repair capacity

PURPOSE

From numerous reports on proteins involved in DNA repair and telomere maintenance that physically associate with human telomerase reverse transcriptase (hTERT), we inferred that hTERT/telomerase might play a role in DNA repair. We investigated this possibility in normal human oral fibroblasts (NHOF) with and without ectopic expression of hTERT/telomerase.

EXPERIMENTAL DESIGN

To study the effect of hTERT/telomerase on DNA repair, we examined the mutation frequency rate, host cell reactivation rate, nucleotide excision repair capacity, and DNA end-joining activity of NHOF and NHOF capable of expressing hTERT/telomerase (NHOF-T). NHOF-T was obtained by transfecting NHOF with hTERT plasmid.

RESULTS

Compared with parental NHOF and NHOF transfected with empty vector (NHOF-EV), we found that (a) the N-methyl-N'-nitro-N-nitrosoguanidine-induced mutation frequency of an exogenous shuttle vector was reduced in NHOF-T, (b) the host cell reactivation rate of N-methyl-N'-nitro-N-nitrosoguanidine-damaged plasmids was significantly faster in NHOF-T; (c) the nucleotide excision repair of UV-damaged DNA in NHOF-T was faster, and (d) the DNA end-joining capacity in NHOF-T was enhanced. We also found that the above enhanced DNA repair activities in NHOF-T disappeared when the cells lost the capacity to express hTERT/telomerase.

CONCLUSIONS

These results indicated that hTERT/telomerase enhances DNA repair activities in NHOF. We hypothesize that hTERT/telomerase accelerates DNA repair by recruiting DNA repair proteins to the damaged DNA sites.

Repair of DNA broken ends is similar in embryonic fibroblasts with and without telomerase

Telomeres cap the ends of chromosomes, preventing end-to-end fusions and subsequent chromosome instability. Here we used a telomerase knockout model to investigate whether telomerase participates in the processes of DNA break repair by de novo synthesis of telomere repeats at broken chromosome ends (chromosome healing). Chromosome healing giving rise to new detectable telomeric signals has not been observed in embryonic fibroblasts of telomerase-proficient mice exposed to ionizing radiation. Since the synthesis of telomeric sequences to broken DNA ends would make them refractory to rejoining events, the efficiency of rejoining of broken chromosomes in cell environments with and without telomerase has also been investigated. We conclude that the efficiency of rejoining broken chromosomes is not significantly different in the two cell environments. All together, our results indicate that there is no significant involvement of telomerase in the healing of broken DNA ends by synthesizing new telomeres in mouse embryo fibroblasts after exposure to ionizing radiation.

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.

Cre-mediated reversible immortalization of human renal proximal tubular epithelial cells

Primary human renal proximal tubule epithelial cells (RPTECs) are of limited use for basic research and for clinical applications due to their limited lifespan in culture. Here we used two lentivirus vectors carrying the human telomerase (hTERT) and the SV40T antigen (Tag) flanked by loxP sites to reversibly immortalize RPTECs. Transduced RPTEC clones continued to proliferate while retaining biochemical and functional characteristics of primary cells. The clones exhibited contact-inhibited, anchorage- and growth factor-dependent growth and did not form tumors in nude mice, suggesting that the cells were not transformed. Transient Cre expression in these cells led to efficient proviral deletion, upregulation of some renal specific activities, and decreased growth rates. Ultimately, the cells underwent replicative senescence, indicating intact cell cycle control. Thus, reversible immortalization allows the expansion of human RPTECs, leading to large production of RPTECs that retain most tissue-specific properties.

The role of the DNA double-strand break response network in meiosis

Organisms with sexual reproduction have two homologous copies of each chromosome. Meiosis is characterized by two successive cell divisions that result in four haploid sperms or eggs, each carrying a single copy of homologous chromosome. This process requires a coordinated reorganization of chromatin and a complex network of meiotic-specific signaling cascades. At the beginning of meiosis, each chromosome must recognize its homolog, then the two become intimately aligned along their entire lengths which allows the exchange of DNA strands between homologous sequences to generate genetic diversity. DNA double-strand breaks (DSBs) initiate meiotic recombination in a variety of organisms. Numerous studies have identified both the genomic loci of the initiating DSBs and the proteins involved in their formation. This review will summarize the activation and signaling networks required for the DSB response in meiosis.

Human fibroblasts expressing hTERT show remarkable karyotype stability even after exposure to ionizing radiation

Ectopic expression of telomerase results in an immortal phenotype in various types of normal cells, including primary human fibroblasts. In addition to its role in telomere lengthening, telomerase has now been found to have various functions, including the control of DNA repair, chromatin modification, and the control of expression of genes involved in cell cycle regulation. The investigations on the long-term effects of telomerase expression in normal human fibroblast highlighted that these cells show low frequencies of chromosomal aberrations. In this paper, we describe the karyotypic stability of human fibroblasts immortalized by expression of hTERT. The ectopic overexpression of telomerase is associated with unusual spontaneous as well as radiation-induced chromosome stability. In addition, we found that irradiation did not enhance plasmid integration in cells expressing hTERT, as has been reported for other cell types. Long-term studies illustrated that human fibroblasts immortalized by telomerase show an unusual stability for chromosomes and for plasmid integration sites, both with and without exposure to ionizing radiation. These results confirm a role for telomerase in genome stabilisation by a telomere-independent mechanism and point to the possibility for utilizing hTERT-immortalized normal human cells for the study of gene targeting.

Degradation of p53, not telomerase activation, by E6 is required for bypass of crisis and immortalization by human papillomavirus type 16 E6/E7

Bypass of two arrest points is essential in the process of cellular immortalization, one of the components of the transformation process. Expression of human papillomavirus type 16 E6 and E7 together can escape both senescence and crisis, processes which normally limit the proliferative capacity of primary human keratinocytes. Crisis is thought to be mediated by telomere shortening. Because E6 stimulates telomerase activity and exogenous expression of the TERT gene with E7 can immortalize keratinocytes, this function is thought to be important for E6 to cooperate with E7 to bypass crisis. However, it has also been reported that E6 dissociates increased telomerase activity from maintenance of telomere length and that a dominant-negative p53 molecule can substitute for E6 in cooperative immortalization of keratinocytes with E7. Thus, to determine which functions of E6 are required to allow bypass of crisis and immortalization of keratinocytes with E7, immortalization assays were performed using specific mutants of E6, in tandem with E7. In these experiments, every clone expressing an E6 mutant capable of degrading p53 was able to bypass crisis and immortalize, regardless of telomerase induction. All clones containing E6 mutants incapable of degrading p53 died at crisis. These results suggest that the ability of E6 to induce degradation of p53 compensates for continued telomere shortening in E6/E7 cells and demonstrate that degradation of p53 is required for immortalization by E6/E7, while increased telomerase activity is dispensable.

Constitutive expression of tert in thymocytes leads to increased incidence and dissemination of T-cell lymphoma in Lck-Tert mice

Here we describe a new mouse model with constitutive expression of the catalytic subunit of telomerase (Tert) targeted to thymocytes and peripheral T cells (Lck-Tert mice). Two independent Lck-Tert mouse lines showed higher incidences of spontaneous T-cell lymphoma than the corresponding age-matched wild-type controls, indicating that constitutive expression of Tert promotes lymphoma. Interestingly, T-cell lymphomas in Lck-Tert mice were more disseminated than those in wild-type controls and affected both lymphoid and nonlymphoid tissues, while nonlymphoid tissues were never affected with lymphoma in age-matched wild-type controls. Importantly, these roles of Tert constitutive expression in promoting tumor progression and dissemination were independent of the role of telomerase in telomere length maintenance, since telomere length distributions on a single-cell basis were identical in Lck-Tert and wild-type thymocytes. Finally, Tert constitutive expression did not interfere with telomere capping in Lck-Tert primary thymocytes, although it resulted in greater chromosomal instability upon gamma irradiation in Lck-Tert primary lymphocytes than in controls, suggesting that Tert overexpression may interfere with the cellular response to DNA damage.

Genomic instability and enhanced radiosensitivity in Hsp70.1- and Hsp70.3-deficient mice

Heat shock proteins (HSPs) are highly conserved among all organisms from prokaryotes to eukaryotes. In mice, the HSP genes Hsp70.1 and Hsp70.3 are induced by both endogenous and exogenous stressors, such as heat and toxicants. In order to determine whether such proteins specifically influence genomic instability, mice deficient for Hsp70.1 and Hsp70.3 (Hsp70.1/3(-/-) mice) were generated by gene targeting. Mouse embryonic fibroblasts (MEFs) prepared from Hsp70.1/3(-/-) mice did not synthesize Hsp70.1 or Hsp70.3 after heat-induced stress. While the Hsp70.1/3(-/-) mutant mice were fertile, their cells displayed genomic instability that was enhanced by heat treatment. Cells from Hsp70.1/3(-/-) mice also display a higher frequency of chromosome end-to-end associations than do control Hsp70.1/3(+/+) cells. To determine whether observed genomic instability was related to defective chromosome repair, Hsp70.1/3(-/-) and Hsp70.1/3(+/+) fibroblasts were treated with ionizing radiation (IR) alone or heat and IR. Exposure to IR led to more residual chromosome aberrations, radioresistant DNA synthesis (a hallmark of genomic instability), increased cell killing, and enhanced IR-induced oncogenic transformation in Hsp70.1/3(-/-) cells. Heat treatment prior to IR exposure enhanced cell killing, S-phase-specific chromosome damage, and the frequency of transformants in Hsp70.1/3(-/-) cells in comparison to Hsp70.1/3(+/+) cells. Both in vivo and in vitro studies demonstrate for the first time that Hsp70.1 and Hsp70.3 have an essential role in maintaining genomic stability under stress conditions.

Human heterochromatin protein 1 isoforms HP1(Hsalpha) and HP1(Hsbeta) interfere with hTERT-telomere interactions and correlate with changes in cell growth and response to ionizing radiation

Telomeres are associated with the nuclear matrix and are thought to be heterochromatic. We show here that in human cells the overexpression of green fluorescent protein-tagged heterochromatin protein 1 (GFP-HP1) or nontagged HP1 isoforms HP1(Hsalpha) or HP1(Hsbeta), but not HP1(Hsgamma), results in decreased association of a catalytic unit of telomerase (hTERT) with telomeres. However, reduction of the G overhangs and overall telomere sizes was found in cells overexpressing any of these three proteins. Cells overexpressing HP1(Hsalpha) or HP1(Hsbeta) also display a higher frequency of chromosome end-to-end associations and spontaneous chromosomal damage than the parental cells. None of these effects were observed in cells expressing mutants of GFP-DeltaHP1(Hsalpha), GFP-DeltaHP1(Hsbeta), or GFP-DeltaHP1(Hsgamma) that had their chromodomains deleted. An increase in the cell population doubling time and higher sensitivity to cell killing by ionizing radiation (IR) treatment was also observed for cells overexpressing HP1(Hsalpha) or HP1(Hsbeta). In contrast, cells expressing mutant GFP-DeltaHP1(Hsalpha) or GFP-DeltaHP1(Hsbeta) showed a decrease in population doubling time and decreased sensitivity to IR compared to the parental cells. The effects on cell doubling times were paralleled by effects on tumorigenicity in mice: overexpression of HP1(Hsalpha) or HP1(Hsbeta) suppressed tumorigenicity, whereas expression of mutant HP1(Hsalpha) or HP1(Hsbeta) did not. Collectively, the results show that human cells are exquisitely sensitive to the amount of HP1(Hsalpha) or HP1(Hsbeta) present, as their overexpression influences telomere stability, population doubling time, radioresistance, and tumorigenicity in a mouse xenograft model. In addition, the isoform-specific effects on telomeres reinforce the notion that telomeres are in a heterochromatinized state.

Ku interacts with telomerase RNA to promote telomere addition at native and broken chromosome ends

Ku is a conserved DNA end-binding protein that plays various roles at different kinds of DNA ends. At telomeres, Ku is part of the structure that protects the chromosome end, whereas at broken DNA ends, Ku promotes DNA repair as part of the nonhomologous end-joining (NHEJ) pathway. Here, we present evidence of a new role for Ku that impacts both telomere-length maintenance and DNA repair in Saccharomyces cerevisiae. We show that Ku binds TLC1, the RNA component of telomerase. We also describe a novel separation-of-function allele of Ku that is specifically defective in TLC1 binding. In this mutant, telomeres are short and the kinetics of telomere addition are slow, but other Ku-dependent activities, such as chromosome end protection and NHEJ, are unaffected. At low frequency, yeast will use telomerase to heal DNA damage by capping the broken chromosome with telomeric DNA sequences. We show that when Ku's ability to bind TLC1 is disrupted, DNA repair via telomere healing is reduced 10- to 100-fold, and the spectrum of sequences that can acquire a telomere changes. Thus, the interaction between Ku and TLC1 RNA enables telomerase to act at both broken and normal chromosome ends.