Telomerase activity during the cell cycle and in gamma-irradiated hematopoietic cells

Role of Telomere Length in Radiation Response of Hematopoietic Stem & Progenitor Cells in Newborns

OBJECTIVE

Wide inter-individual variations in ionizing radiation (IR) responses of neonatal hematopoietic system calls for identifying reliable biomarkers to effectively estimate radiation exposure damages in neonates.

METHODS

Association between telomere length (TL) at birth and radiation sensitivity of cord blood hematopoietic stem cells (HSC) from 166 healthy newborns were investigated by assessing their clonogenic differentiation. TL was determined as terminal restriction fragment (TRF) by Southern blot method.

RESULTS

TL correlated with surviving fractions of total progenitor colony forming cell (CFC) content at 0.75 Gy (p < 0.05), granulo-macrophagic lineage colony forming units (CFU-GM) at 0.75 Gy (p < 0.05) and erythroid burst forming unit (BFU-E) at 0.75 Gy (p < 0.05) & at 3 Gy (p < 0.05) of newborns.

CONCLUSION

Our results indicate risks for HSC clonogenic survival in neonates with shorter telomeres after IR exposure. These observations might aid in considering TL at birth as an assessment factor for radiation related hematopoietic challenges in children.

The relationship between longer leukocyte telomeres and dNCR in non-cardiac surgery patients: a retrospective analysis

BACKGROUND

Cognitive decline following surgery is a common concern among elderly individuals. Leukocyte telomere length (LTL) can be assessed as a biological clock connected to an individual lifespan. However, the mechanisms causing this inference are still not fully understood. As a result of this, LTL has the potential to be useful as an aging-related biomarker for assessing delayed neurocognitive recovery (dNCR) and related diseases.

METHODS

For this study, 196 individuals over 60 who were scheduled due to major non-cardiac surgical operations attended neuropsychological testing before surgery, followed by additional testing one week later. The finding of dNCR was based on a measured Z-score ≤ -1.96 on two or more separate tests. The frequency of dNCR was presented as the primary outcome of the study. Secondly, we evaluated the association between dNCR and preoperative LTL.

RESULTS

Overall, 20.4% [40/196; 95% confidence interval (CI), 14.7-26.1%] of patients exhibited dNCR 1-week post-surgery. Longer LTL was identified as a predictor for the onset of early cognitive impairment resulting in postoperative cognitive decline [odds ratio (OR), 14.82; 95% CI, 4.01-54.84; P < 0.001], following adjustment of age (OR, 12.33; 95% CI, 3.29-46.24; P < 0.001). The dNCR incidence based on LTL values of these patients, the area under the receiver operating characteristic (ROC) curve was 0.79 (95% CI, 0.722-0.859; P < 0.001). At an optimal cut-off value of 0.959, LTL values offered respective specificity and sensitivity values of 64.7% and 87.5%.

CONCLUSIONS

In summary, the current study revealed that the incidence of dNCR was strongly associated with prolonged LTL. Furthermore, this biomarker could help identify high-risk patients and offer insight into the pathophysiology of dNCR.

Targeting telomeres: advances in telomere maintenance mechanism-specific cancer therapies

Cancer cells establish replicative immortality by activating a telomere-maintenance mechanism (TMM), be it telomerase or the alternative lengthening of telomeres (ALT) pathway. Targeting telomere maintenance represents an intriguing opportunity to treat the vast majority of all cancer types. Whilst telomerase inhibitors have historically been heralded as promising anticancer agents, the reality has been more challenging, and there are currently no therapeutic options for cancer types that use ALT despite their aggressive nature and poor prognosis. In this Review, we discuss the mechanistic differences between telomere maintenance by telomerase and ALT, the current methods used to detect each mechanism, the utility of these tests for clinical diagnosis, and recent developments in the therapeutic strategies being employed to target both telomerase and ALT. We present notable developments in repurposing established therapeutic agents and new avenues that are emerging to target cancer types according to which TMM they employ. These opportunities extend beyond inhibition of telomere maintenance, by finding and exploiting inherent weaknesses in the telomeres themselves to trigger rapid cellular effects that lead to cell death.

Mutagen-induced telomere instability in human cells

Telomere instability is one of the main sources of genome instability and may result from chromosome end loss (due to chromosome breakage at one or both ends) or, more frequently, telomere dysfunction. Dysfunctional telomeres arise when they lose their end-capping function or become critically short, which causes chromosomal termini to behave like a DNA double-strand break. Telomere instability may occur at the chromosomal or at the molecular level, giving rise, respectively, to telomere-related chromosomal aberrations or the loss or modification of any of the components of the telomere (telomere DNA, telomere-associated proteins, or telomere RNA). Since telomeres play a fundamental role in maintaining genome stability, the study of telomere instability in cells exposed to mutagens is of great importance to understand the telomere-driven genomic instability present in those cells. In the present review, we will focus on the current knowledge about telomere instability induced by physical, chemical, and biological mutagens in human cells.

Evaluating biomarkers to model cancer risk post cosmic ray exposure

Robust predictive models are essential to manage the risk of radiation-induced carcinogenesis. Chronic exposure to cosmic rays in the context of the complex deep space environment may place astronauts at high cancer risk. To estimate this risk, it is critical to understand how radiation-induced cellular stress impacts cell fate decisions and how this in turn alters the risk of carcinogenesis. Exposure to the heavy ion component of cosmic rays triggers a multitude of cellular changes, depending on the rate of exposure, the type of damage incurred and individual susceptibility. Heterogeneity in dose, dose rate, radiation quality, energy and particle flux contribute to the complexity of risk assessment. To unravel the impact of each of these factors, it is critical to identify sensitive biomarkers that can serve as inputs for robust modeling of individual risk of cancer or other long-term health consequences of exposure. Limitations in sensitivity of biomarkers to dose and dose rate, and the complexity of longitudinal monitoring, are some of the factors that increase uncertainties in the output from risk prediction models. Here, we critically evaluate candidate early and late biomarkers of radiation exposure and discuss their usefulness in predicting cell fate decisions. Some of the biomarkers we have reviewed include complex clustered DNA damage, persistent DNA repair foci, reactive oxygen species, chromosome aberrations and inflammation. Other biomarkers discussed, often assayed for at longer points post exposure, include mutations, chromosome aberrations, reactive oxygen species and telomere length changes. We discuss the relationship of biomarkers to different potential cell fates, including proliferation, apoptosis, senescence, and loss of stemness, which can propagate genomic instability and alter tissue composition and the underlying mRNA signatures that contribute to cell fate decisions. Our goal is to highlight factors that are important in choosing biomarkers and to evaluate the potential for biomarkers to inform models of post exposure cancer risk. Because cellular stress response pathways to space radiation and environmental carcinogens share common nodes, biomarker-driven risk models may be broadly applicable for estimating risks for other carcinogens.

Telomeres and Telomerase in the Radiation Response: Implications for Instability, Reprograming, and Carcinogenesis

Telomeres are nucleoprotein complexes comprised of tandem arrays of repetitive DNA sequence that serve to protect chromosomal termini from inappropriate degradation, as well as to prevent these natural DNA ends from being recognized as broken DNA (double-strand breaks) and triggering of inappropriate DNA damage responses. Preservation of telomere length requires telomerase, the specialized reverse transcriptase capable of maintaining telomere length via template-mediated addition of telomeric repeats onto the ends of newly synthesized chromosomes. Loss of either end-capping function or telomere length maintenance has been associated with genomic instability or senescence in a variety of settings; therefore, telomeres and telomerase have well-established connections to cancer and aging. It has long been recognized that oxidative stress promotes shortening of telomeres, and that telomerase activity is a radiation-inducible function. However, the effects of ionizing radiation (IR) exposure on telomeres per se are much less well understood and appreciated. To gain a deeper understanding of the roles, telomeres and telomerase play in the response of human cells to IRs of different qualities, we tracked changes in telomeric end-capping function, telomere length, and telomerase activity in panels of mammary epithelial and hematopoietic cell lines exposed to low linear energy transfer (LET) gamma(γ)-rays or high LET, high charge, high energy (HZE) particles, delivered either acutely or at low dose rates. In addition to demonstrating that dysfunctional telomeres contribute to IR-induced mutation frequencies and genome instability, we reveal non-canonical roles for telomerase, in that telomerase activity was required for IR-induced enrichment of mammary epithelial putative stem/progenitor cell populations, a finding also suggestive of cellular reprograming. Taken together, the results reported here establish the critical importance of telomeres and telomerase in the radiation response and, as such, have compelling implications not only for accelerated tumor repopulation following radiation therapy but also for carcinogenic potential following low dose exposures as well, including those of relevance to spaceflight-associated galactic cosmic radiations.

Changes in telomere length distribution in low-dose X-ray-irradiated human umbilical vein endothelial cells

Ionizing radiation (IR) is known to be a cause of telomere dysfunction in tumor cells; however, very few studies have investigated X-ray-related changes in telomere length and the telomerase activity in normal human cells, such as umbilical vein endothelial cells (HUVECs). The loss of a few hundred base pairs from a shortened telomere has been shown to be important with respect to cellular senescence, although it may not be detected according to traditional mean telomere length [assessed as the terminal restriction fragment (TRF)] analyses. In the present study, a continuous time window from irradiation was selected to examine changes in the telomere length, including the mean TRF length, percentage of the telomere length, telomerase activity, apoptotic rate, and survival rate in HUVECs from the first day to the fourth day after the administration of a 0.5-Gy dose of irradiation. The mean TRF length in the irradiated HUVECs showed shorter telomere length in first 3 days, but they were not statistically significant. On the other hand, according to the percentage analysis of the telomere length, a decreasing tendency was noted in the longer telomere lengths (9.4-4.4 kb), with a significant increase in the shortest telomeres (4.4-2.3 kb) among the irradiated cells versus the controls from the first day to the third after irradiation; no significant differences were noted on the fourth day. These results suggest that the shortest telomeres are sensitive to the late stage of radiation damage. The proliferation of irradiated cells was suppressed after IR in contrast to the non-irradiated cells. The apoptotic rate was elevated initially both in IR- and non-IR-cells, but that of IR-cells was maintained at an elevated level thereafter in contrast to that of non-IR-cells decreasing promptly. Therefore, a 0.5-Gy dose of IR induces persistent apoptosis leading to an apparent growth arrest of the normal HUVECs.

Crosstalk between telomere maintenance and radiation effects: A key player in the process of radiation-induced carcinogenesis

It is well established that ionizing radiation induces chromosomal damage, both following direct radiation exposure and via non-targeted (bystander) effects, activating DNA damage repair pathways, of which the proteins are closely linked to telomeric proteins and telomere maintenance. Long-term propagation of this radiation-induced chromosomal damage during cell proliferation results in chromosomal instability. Many studies have shown the link between radiation exposure and radiation-induced changes in oxidative stress and DNA damage repair in both targeted and non-targeted cells. However, the effect of these factors on telomeres, long established as guardians of the genome, still remains to be clarified. In this review, we will focus on what is known about how telomeres are affected by exposure to low- and high-LET ionizing radiation and during proliferation, and will discuss how telomeres may be a key player in the process of radiation-induced carcinogenesis.

Inhibition of UBE2D3 expression attenuates radiosensitivity of MCF-7 human breast cancer cells by increasing hTERT expression and activity

The known functions of telomerase in tumor cells include replenishing telomeric DNA and maintaining cell immortality. We have previously shown the existence of a negative correlation between human telomerase reverse transcriptase (hTERT) and radiosensitivity in tumor cells. Here we set out to elucidate the molecular mechanisms underlying regulation by telomerase of radiosensitivity in MCF-7 cells. Toward this aim, yeast two-hybrid (Y2H) screening of a human laryngeal squamous cell carcinoma radioresistant (Hep2R) cDNA library was first performed to search for potential hTERT interacting proteins. We identified ubiquitin-conjugating enzyme E2D3 (UBE2D3) as a principle hTERT-interacting protein and validated this association biochemically. ShRNA-mediated inhibition of UBE2D3 expression attenuated MCF-7 radiosensitivity, and induced the accumulation of hTERT and cyclin D1 in these cells. Moreover, down-regulation of UBE2D3 increased hTERT activity and cell proliferation, accelerating G1 to S phase transition in MCF-7 cells. Collectively these findings suggest that UBE2D3 participates in the process of hTERT-mediated radiosensitivity in human breast cancer MCF-7 cells by regulating hTERT and cyclin D1.

Radiation-induced upregulation of telomerase activity escapes PI3-kinase inhibition in two malignant glioma cell lines

Tumor relapse after radiotherapy is a great concern in the treatment of high-grade gliomas. Inhibition of the PI3-kinase/AKT pathway is known to radiosensitize cancer cells and to delay their DNA repair after irradiation. In this study, we show that the radiosensitization of CB193 and T98G, two high-grade glioma cell lines, by the PI3K inhibitor LY294002, correlates with the induction of G1 and G2/M arrest, but is inconsistently linked to a delayed DNA double-strand break (DSBs) repair. The PI3K/AKT pathway has been shown to activate radioprotective factors such as telomerase, whose inhibition may contribute to the radiosensitization of cancer cells. However, we show that radiation upregulates telomerase activity in LY-294002-treated glioma cells as well as untreated controls, demonstrating a PI3K/AKT-independent pathway of telomerase activation. Our study suggests that radiosensitizing strategies based on PI3-kinase inhibition in high-grade gliomas may be optimized by additional treatments targeting either telomerase activity or telomere maintenance.

Telomere loss, not average telomere length, confers radiosensitivity to TK6-irradiated cells

Many and varied are the proposed mechanisms that lead to resistance to ionizing radiation treatment. Among them, an inverse relationship between telomere length and radioresistance has been recently advanced. Investigating such a relationship in TK6 lymphoblasts, we found that clones originating from cells survived to 4Gy of X-rays showed a significantly higher telomere length when compared with clones grown from untreated cells. The lengthening observed was not attributable to a radiation-induced increase in telomerase activity, as demonstrated by TRAP assay performed in the dose range of 1-10Gy. Given the evidence that TK6 whole population was characterized by heterogeneity in cellular mean telomere length and telomere loss, we tested the hypothesis that a process of selection may favour cells with longer telomeres (more radioresistant cells) following exposure to irradiation. In order to do this 15 independent TK6 clones were selected and characterized for telomere length and loss on the basis of q-FISH and flow-FISH analysis. Among the screened clones four characterized by long telomeres and four characterized by short telomeres were tested for their radiosensitivity by means of clonogenic assay. The results obtained showed that, in our experimental conditions (cellular model, radiation doses) no significant correlation was observed between radiosensitivity and mean telomere lengths, whereas a positive correlation was observed with respect to telomere loss. Overall, these results indicate that telomere loss and not mean telomere length plays a critical role in the phenomenon of radiosensitivity/radioresistance.

Telomere profiling: toward glioblastoma personalized medicine

Despite a standard of care combining surgery, radiotherapy (RT), and temozolomide chemotherapy, the average overall survival (OS) of glioblastoma patients is only 15 months, and even far lower when the patient cannot benefit from this combination. Therefore, there is a strong need for new treatments, such as new irradiation techniques. Against this background, carbon ion hadrontherapy, a new kind of irradiation, leads to a greater biological response of the tumor, while minimizing adverse effects on healthy tissues in comparison with RT. As carbon ion hadrontherapy is restricted to RT-resistant patients, photon irradiation resistance biomarkers are needed. Long telomeres and high telomerase activity have been widely associated with photon radioresistance in other cancers. Moreover, telomere protection, telomere function, and telomere length (TL) also depend on the shelterin protein complex (TRF1, TRF2, TPP1, POT1, TIN2, and hRAP1). We thus decided to evaluate an enlarged telomeric status (TL, telomerase catalytic subunit, and the shelterin component expression level) as a potential radioresistance biomarker in vitro using cellular models and ex vivo using patient tumor biopsies. In addition, nothing was known about the role of telomeres in carbon ion response. We thus evaluated telomeric status after both types of irradiation. We report here a significant correlation between TL and the basal POT1 expression level and photon radioresistance, in vitro, and a significant increase in the OS of patients with long telomeres or a high POT1 level, in vivo. POT1 expression was predictive of patient response irrespective of the TL. Strikingly, these correlations were lost, in vitro, when considering carbon irradiation. We thus propose (1) a model of the implications of telomeric damage in the cell response to both types of irradiation and (2) assessment of the POT1 expression level and TL using patient tumor biopsies to identify radioresistant patients who could benefit from carbon hadrontherapy.

Transient activation of the ALT pathway in human primary fibroblasts exposed to high-LET radiation

It is well established that high-LET radiations efficiently induce chromosome aberrations. However, data on the effect of protons on telomere maintenance, as involved in genomic stability, are scarce and contradictory. Here we demonstrate that high-LET protons induce telomere lengthening in human primary fibroblasts and that this elongation does not involve the telomerase enzyme, supporting the hypothesis that high-LET radiations are able to activate a telomerase-independent mechanism. In tumor cells that lack telomerase, one or more non-telomerase mechanisms for telomere maintenance are present, which are termed alternative lengthening of telomeres (ALT). Since ALT cells are characterized by recombinational events at telomeres, known as telomeric-sister chromatid exchanges (T-SCE), and colocalization of telomeres and premyelocytic leukemia protein (PML), we analyzed both T-SCE and PML. Our results show that high-LET protons induce a 2.5-fold increase of T-SCE and a colocalization of PML protein and telomeric DNA. Furthermore, our data show that the ALT pathway can be activated in human primary cells after induction of severe DNA damage. Thus, since telomeres are known to be involved in chromosome maintenance, the present work may contribute in the elucidation of the mechanism by which ionizing radiation induces genomic instability.

Irradiation-induced telomerase activity and gastric cancer risk: a case-control analysis in a Chinese Han population

BACKGROUND

Telomerase expression is one of the characteristics of gastric cancer (GC) cells and telomerase activity is frequently up-regulated by a variety of mechanisms during GC development. Therefore, we hypothesized that elevated levels of activated telomerase might enhance GC risk due to increased propagation of cells with DNA damage, such as induced by gamma-radiation.

METHODS

To explore this hypothesis, 246 GC cases and 246 matched controls were recruited in our case-control study. TRAP-ELISA was used to assess the levels of telomerase activity at baseline and after gamma-radiation and the gamma-radiation-induced telomerase activity (defined as after gamma-irradiation/baseline) in cultured peripheral blood lymphocytes (PBLs).

RESULTS

Our data showed that there was no significant difference for the baseline telomerase activity between GC cases and controls (10.17 +/- 7.21 vs. 11.02 +/- 8.03, p = 0.168). However, after gamma-radiation treatment, gamma-radiation-induced telomerase activity was significantly higher in the cases than in the controls (1.51 +/- 0.93 vs. 1.22 +/- 0.66, p < 0.001). Using the median value of gamma-radiation-induced telomerase activity in the controls as a cutoff point, we observed that high gamma-radiation-induced telomerase activity was associated with a significantly increased GC risk (adjusted odds ratio, 2.45; 95% confidence interval, 1.83-3.18). Moreover, a dose response association was noted between gamma-radiation-induced telomerase activity and GC risk. Age, but not sex, smoking and drinking status seem to have a modulating effect on the gamma-radiation-induced telomerase activities in both cases and controls.

CONCLUSION

Overall, our findings for the first time suggest that the increased gamma-radiation-induced telomerase activity in PBLs might be associated with elevated GC risk. Further confirmation of this association using a prospective study design is warranted.

Ionizing radiation up-regulates telomerase activity in cancer cell lines by post-translational mechanism via ras/phosphatidylinositol 3-kinase/Akt pathway

PURPOSE

Telomerase is considered currently as a hallmark of cancer, and its inhibition is expected to become an important anticancer modality. In contrast to abundant data concerning the effect of cytotoxic drugs on telomerase activity (TA), there is scant information on the effect of radiation on telomerase. The mechanism of telomerase regulation by irradiation has never been evaluated in detail. In the present study, we investigated the effect of radiation on TA and its regulation in cancer cells.

EXPERIMENTAL DESIGN

The effect of various radiation doses on TA in several malignant and nonmalignant cell lines was evaluated. All malignant cells exhibited similar telomerase response to radiation and its regulation was assessed at transcriptional and post-translational levels in K562 cells. Next step was the evaluation of the upstream signaling pathways leading to changes in TA using kinetics and specific inhibitors.

RESULTS

Radiation up-regulated TA in dose-dependent manner only in cancer cells. Telomerase was activated by phosphorylation by Akt and by cytoplasmic-nuclear shift. Transcriptional processes were not involved in TA. This telomerase regulation is mediated by Ras/phosphatidylinositol 3-kinase/Akt pathway. The canonical membrane effectors of irradiation (epidermal growth factor receptor, insulin-like growth factor-I receptor, and Ca2+ influx) were not involved in this process.

CONCLUSIONS

Radiation up-regulates telomerase activity specifically in cancer cells. This study adds to accumulating evidence pointing to post-translational level as important mode of telomerase regulation. Telomerase activation due to radiation may be detrimental in treatment of cancer. Data described in this study may add to future interventions aiming at inhibition of telomerase activation during irradiation.

Pharmacological inhibition of DNA repair enzymes differentially modulates telomerase activity and apoptosis in two human leukaemia cell lines

PURPOSE

To investigate the effect of wortmannin and 3-aminobenzamide (3-AB) on telomerase activity and apoptosis in two human leukaemia cells.

MATERIALS AND METHODS

MOLT-4 (p53-wild type) and KG1a (p53-null) cells were irradiated with gamma-rays (3 Gy at 1.57 Gy min(-1)) and the effects of wortmannin and 3-AB were evaluated. Telomerase activity was measured by polymerase chain reaction and the expression of human telomerase reverse transcriptase, human telomerase RNA and telomerase-associated protein 1 was assessed by reverse transcriptase-polymerase chain reaction. Apoptosis was evaluated by fluorescence microscopy and flow cytometry.

RESULTS

A radiation-induced up-regulation of telomerase activity was observed from 4 h post-irradiation in both cell lines. This up-regulation was abrogated by wortmannin and 3-AB. Telomerase activity was maximal 24 h post-irradiation, coinciding with an accumulation of human telomerase reverse transcriptase mRNA. Apoptosis and G2/M arrest were evident from 4 h post-irradiation in MOLT-4 cells. KG1a cells exhibited a G2/M block at 24 h post-irradiation and apoptosis increased between 24 and 48 h post-irradiation. 3-AB abolished G2/M blockage and enhanced radiation-induced apoptosis in both cell lines, while wortmannin increased apoptosis only in MOLT-4 cells.

CONCLUSIONS

3-AB inhibits the radiation-associated telomerase activity increase and enhances apoptosis in MOLT-4 and KG1a cells. Wortmannin, which also inhibits the radiation-associated telomerase activity increase in both cell lines, does not modify radiation-induced apoptosis in KG1a cells. DNA repair enzymes might be selective targets for enhancing radiosensitivity in certain tumour cells.

Radiation-induced effects on telomerase in gynecological cancer cell lines with different radiosensitivity and repair capacity

PURPOSE

Telomerase activation in response to irradiation might enhance the radioresistance of cells. Thus, we have investigated radiation-induced effects on telomerase in six gynecological cancer cell lines, with different intrinsic radiosensitivity and capacity for sublethal damage repair (SLDR).

MATERIALS AND METHODS

Three endometrial adenocarcinoma (UM-EC-1, UT-EC-2B and UT-EC-3) and three vulvar squamous cell carcinoma (A431, UM-SCV-2 and UM-SCV-7) cell lines were irradiated with doses of 5, 10 and 25 Gy and the effects on telomerase were evaluated at 0.5, 6, 24 and 48 h post-irradiation. Telomerase activity was quantitatively measured by SYBR Green real-time telomeric repeat amplification protocol.

RESULTS

The most radioresistant cell line A431 had the strongest stimulatory effects (approximately 2.0 - 2.5-fold) on telomerase activity 24 and 48 h post-irradiation with the highest radiation doses. In contrast to that, telomerase activities in the highly radiosensitive cell line UT-EC-2B remained below the basal level throughout the 48-h period of post-irradiation with the highest doses, and even a decline to approximately 50% of the basal level was found 24 h after exposure. In other cell lines being either moderately or highly radiation resistant, telomerase activity levels in response to irradiation remained mainly at the basal level or gradually increased.

CONCLUSIONS

The present findings indicate that there might be a connection between the radiation-induced telomerase response and radiosensitivity. However, no correlation was found between the radiation-induced effects on telomerase and the sublethal damage repair capacity of the cells.

Activation of telomerase by ionizing radiation: differential response to the inhibition of DNA double-strand break repair by abrogation of poly (ADP-ribosyl)ation, by LY294002, or by Wortmannin

PURPOSE

Telomerase activity represents a radiation-inducible function, which may be targeted by a double-strand break (DSB)-activated signal transduction pathway. Therefore, the effects of DNA-PK inhibitors (Wortmannin and LY294002) on telomerase upregulation after irradiation were studied. In addition, the role of trans-dominant inhibition of poly(ADP-ribosyl)ation, which strongly reduces DSB rejoining, was assessed in comparison with 3-aminobenzamide.

METHODS AND MATERIALS

COM3 rodent cells carry a construct for the dexamethasone-inducible overexpression of the DNA-binding domain of PARP1 and exhibit greatly impaired DSB rejoining after irradiation. Telomerase activity was measured using polymerase chain reaction ELISA 1 h after irradiation with doses up to 10 Gy. Phosphorylation status of PKB/Akt and of PKCalpha/beta(II) was assessed by western blotting.

RESULTS

No telomerase upregulation was detectable for irradiated cells with undisturbed DSB rejoining. In contrast, incubation with LY294002 or dexamethasone yielded pronounced radiation induction of telomerase activity that could be suppressed by Wortmannin. 3-Aminobenzamide not only was unable to induce telomerase activity but also suppressed telomerase upregulation upon incubation with LY294002 or dexamethasone. Phospho-PKB was detectable independent of irradiation or dexamethasone pretreatment, but was undetectable upon incubations with LY294002 or Wortmannin, whereas phospho-PKC rested detectable.

CONCLUSIONS

Telomerase activation postirradiation was triggered by different treatments that interfere with DNA DSB processing. This telomerase upregulation, however, was not reflected by the phosporylation status of the putative mediators of TERT activation, PKB and PKC. Although an involvement of PKB in TERT activation is not supported by the present findings, a respective role of PKC isoforms other than alpha/beta(II) cannot be ruled out.

Low oxygen delays fibroblast senescence despite shorter telomeres

It has been widely accepted that telomere shortening acts as a cell division counting mechanism that beyond a set critical length signals cells to enter replicative senescence. In this study, we demonstrate that by simply lowering the oxygen content of the cell culture environment 10-fold (20-2%) extends the replicative lifespan of fetal bovine fibroblasts at least five-times (30-150 days). Although, low oxygen fibroblasts display a slightly slower rate (P > 0.05) of telomere attrition than their high oxygen counterparts (171 bp versus 182 bp/PD), late passage fibroblasts (>50 PD) that have extended their replicative capacity under low oxygen conditions exhibited significantly (P < 0.05) shorter telomere lengths (11,135 +/- 467 bp) compared to senescent cells (25-34 PD) cultured under high oxygen conditions (14,827 +/- 1173 bp). There was a significant increase (P < 0.05) in chromosomal abnormalities with continual cell division under both high and low oxygen environments, however, fibroblasts displayed a significant reduction (P < 0.001) in chromosomal abnormalities at low oxygen tensions compared to those under 20% oxygen. These apparent protective effects on telomere shortening, delayed senescence and reduced chromosomal aberrations may be attributed to the up-regulation of telomerase activity observed for fibroblasts cultured under low oxygen. These results are consistent with the idea that a critically short telomere length may not be the sole trigger of replicative senescence, but may be regulated by the integrity of telomere structure itself and/or the amount of oxidative DNA damage in the cell.

Telomeres: hallmarks of radiosensitivity

Telomeres are the very ends of the chromosomes. They can be seen as natural double-strand breaks (DSB), specialized structures which prevent DSB repair and activation of DNA damage checkpoints. In somatic cells, attrition of telomeres occurs after each cell division until replicative senescence. In the absence of telomerase, telomeres shorten due to incomplete replication of the lagging strand at the very end of chromosome termini. Moreover, oxidative stress and accumulating reactive oxygen species (ROS) lead to an increased telomere shortening due to a less efficient repair of SSB in telomeres. The specialized structures at telomeres include proteins involved in both telomere maintenance and DNA repair. However when a telomere is damaged and has to be repaired, those proteins might fail to perform an accurate repair of the damage. This is the starting point of this article in which we first summarize the well-established relationships between DNA repair processes and maintenance of functional telomeres. We then examine how damaged telomeres would be processed, and show that irradiation alters telomere maintenance leading to possibly dramatic consequences. Our point is to suggest that those consequences are not restricted to the short term effects such as increased radiation-induced cell death. On the contrary, we postulate that the major impact of the loss of telomere integrity might occur in the long term, during multistep carcinogenesis. Its major role would be to act as an amplificator event unmasking in one single step recessive radiation-induced mutations among thousands of genes and providing cellular proliferative advantage. Moreover, the chromosomal instability generated by damaged telomeres will favour each step of the transformation from normal to fully transformed cells.

Irradiation-induced telomerase activity and the risk of lung cancer: a pilot case-control study

BACKGROUND

Telomerase activity is undetectable in most normal somatic cells, but is up-regulated by various mechanisms during tumorigenesis. Telomerase activation enables cells to overcome replicative senescence and maintain telomere stability during cell proliferation. The aim of the study was to evaluate the association between irradiation-induced telomerase activity and the risk of lung cancer.

METHODS

A case-control design was used that measured the baseline and gamma-radiation-induced telomerase activity in cultured peripheral blood lymphocytes from 44 lung cancer patients and 44 healthy controls. The associations between gamma-radiation-inducible telomerase activity and the risk of lung cancer were then analyzed.

RESULTS

The baseline telomerase activity was lower in cases than in controls (0.956 vs 1.222, P = .126). After gamma-radiation the telomerase activity in cases experienced a significant increase over baseline (1.480 vs 0.956, P < .001); the telomerase activity in controls also increased, but on a smaller scale (1.485 vs 1.222, P = .0025). The relative gamma-radiation-induced telomerase activity, defined as the ratio of the net increase of telomerase activity (gamma-radiation induced minus baseline) to the baseline telomerase activity, was significantly higher in cases than in controls (0.730 vs 0.224, P = .0003). When dichotomized, the subjects at the 75th percentile of the relative gamma-radiation-induced telomerase activity in controls, a higher ratio was associated with a significantly increased lung cancer risk (odds ratio [OR], 4.71, 95% confidence interval [CI]: 1.37, 16.21). Moreover, a dose response was observed between the relative gamma-radiation-induced telomerase activity and lung cancer risk. Compared with individuals with the lowest tertile of the relative gamma-radiation-induced telomerase activity, individuals with the second and the highest tertiles of the relative telomerase activity exhibited significantly elevated risks of lung cancer, with adjusted ORs of 12.58 (95% CI: 1.08, 146.86) and 31.08 (95% CI: 2.71, 356.81), respectively (P for trend <.001).

CONCLUSIONS

The pilot-case control study suggested that the gamma-radiation-induced telomerase activation is associated with a significantly increased risk of developing lung cancer. Larger case-control studies and prospective studies are needed to confirm the findings.

Differential induction in telomerase activity among bladder cancer patients and controls on gamma-radiation

Expression of telomerase is one of the hallmarks of tumor cells and has been used as a diagnostic biomarker and a therapeutic target in cancer. Novel findings have shown that telomerase activation in normal human epithelial cells may affect expression of several cancer-related genes, such as growth-related genes and c-myc gene, suggesting a possible role of telomerase in tumor initiation. Therefore, we hypothesized that individuals who are sensitive to mutagen challenge in terms of induced telomerase activity might have increased cancer risk. We tested this hypothesis in a bladder cancer case-control study (51 cases and 51 matched controls) by measuring baseline and gamma-radiation-induced telomerase activities in peripheral blood lymphocytes. We found a significantly higher gamma-radiation-induced telomerase activity in bladder cancer cases compared with the controls (1.34 versus 1.23; P = 0.044). A similar finding was also observed using the normalized telomerase activity (ratio of gamma-radiation induced versus baseline; 1.49 versus 1.19; P < 0.001). In further categorizing the telomerase activity using 75% of the normalized value in the controls as a cutoff point, we found a significantly increased risk for bladder cancer associated with higher induced telomerase activity (adjusted odds ratio, 3.62; 95% confidence interval, 1.38-9.51). In quartile analysis, a dose-response association was noted between the induced telomerase activity and increased bladder cancer risk (P(trend) = 0.005). Our findings provide the first evidence linking the mutagen-induced telomerase activity in peripheral blood lymphocytes to the risk of bladder cancer, which warrants further investigation in large-sized studies and other cancer types.

The role of DNA damage response proteins at telomeres—an “integrative” model

Telomeres are specialized structures at chromosome ends which play the key role in chromosomal end protection. There is increasing evidence that many DNA damage response proteins are involved in telomere maintenance. For example, cells defective in DNA double strand break repair proteins including Ku, DNA-PKcs, RAD51D and the MRN (MRE11/RAD51/NBS1) complex show loss of telomere capping function. Similarly, mouse and human cells defective in ataxia telangiectasia mutated (ATM) have defective telomeres. A total of 14 mammalian DNA damage response proteins have, so far, been implicated in telomere maintenance. Recent studies indicate that three more proteins, namely BRCA1, hRad9 and PARP1 are involved in telomere maintenance. The involvement of a wide range of DNA damage response proteins at telomeres raises an important question: do telomere maintenance mechanisms constitute an integral part of DNA damage response machinery? A model termed the "integrative" model is proposed here to argue in favour of telomere maintenance being an integral part of DNA damage response. The "integrative" model is supported by the observation that a telomeric protein, TRF2, is not confined to its local telomeric environment but it migrates to the sites of DNA breakage following exposure of cells to ionizing radiation. Furthermore, even if telomeres are maintained in a non-canonical way, as in the case of Drosophila, DNA damage response proteins are still involved in telomere maintenance suggesting integration of telomere maintenance mechanisms into the DNA damage response network.

Ultraviolet irradiation up-regulates telomerase transcription and activity in lens epithelial cells

PURPOSE

Ultraviolet irradiation (UVR) increases telomerase activity in various cell types including skin, a sun-exposed organ. The lens is also continually exposed to UVR and we hypothesized that lenses exposed to UVR would have increased telomerase activity, with up-regulated TERT and TR, the two main components of the telomerase holoenzyme. To evaluate whether the cornea would protect lenses from such changes, whole globes, as well as isolated lenses, were exposed to UVR, and lenses were evaluated for changes in telomerase activity.

METHODS

There were three parts to this project. The first part of this experiment evaluated freshly harvested normal adult canine lenses exposed to 0, 300, 600, or 1200 J/m(2) UVR, and then allowed to recover for 1, 2, 3 and 4 h. Since only 600 J/m(2) UVR increased telomerase activity, four more postexposure recovery time-points for this UVR dose were evaluated: 10 min, 30 min, 8 h and 24 h. The second part of this experiment used freshly enucleated whole canine globes exposed to 0, 50, 100, 150, 300, 600 or 1200 J/m(2) and incubated overnight; lens epithelial cells (LEC) were evaluated for telomerase activity. The third part evaluated lenses that were exposed to 0 or 600 J/m(2) UVR, and then allowed to recover for 8 and 24 h, before TERT and TR mRNA levels were measured.

RESULTS

Isolated lenses exposed to 600 J/m(2) UVR had significantly higher telomerase activity than unexposed controls and other UVR doses, at all time-points except 24 h postexposure. Lenses from whole globes exposed to UVR showed a dose-dependent increase in telomerase activity except at 50 J/m(2) and 1200 J/m(2). Isolated lenses exposed to 600 J/m(2) UVR and then allowed to recover for 8 and 24 h significantly up-regulated TERT and TR mRNAs compared to unexposed control lenses.

CONCLUSIONS

Telomerase activity is regulated at both the transcriptional and post-translational levels in canine LEC. Previous work in our laboratory showed dose, time, and age-dependent changes in telomerase activity in the lens. The present study showed that TERT and TR mRNA transcription was increased for up to 24 h following an acute dose of UVR. Both telomerase activity and TERT and TR mRNA levels were elevated until 24 h post-UVR exposure, TERT in combination with TR functions in proliferation-related telomerase activity, but TERT alone has an anti-apoptotic function and its up-regulation may protect LEC from the acute effects of UVR. We are continuing to evaluate the mechanisms by which telomerase is regulated in normal and cataractous LEC.

Correlation between radiation-induced telomerase activity and human telomerase reverse transcriptase mRNA expression in HeLa cells

PURPOSE

To quantify and correlate human telomerase reverse transcriptase (hTERT) mRNA expression with telomerase activity (TA) after ionizing irradiation of HeLa cells.

MATERIALS AND METHODS

TA and hTERT mRNA expression were evaluated, at 24-h intervals, in HeLa cells cultured for up to 144 h, before and after treatment with increasing doses of 6 MV photon ionizing radiation (5 - 20 Gy), using the telomeric repeat amplification protocol (TRAP) assay and real-time reverse transcriptase polymerase chain reaction (RT-PCR), respectively. Cell viability was determined using the 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. A prototype phantom was constructed for accurate irradiation of HeLa cells.

RESULTS

Treated cells showed a decrease in viability with increasing radiation dose, and a correlation was observed with post-treatment period. TA and hTERT mRNA expression of HeLa cells increased for the first 24 h after irradiation. The maximal increases were approximately two times the un-irradiated cell levels at 24 h post-irradiation, followed by a decrease and a return to the control levels 72 h post-irradiation. The time-course of telomerase activation after 24 h, differed among radiation doses. A dose-dependent G2/M arrest was observed 24 h post-irradiation, along with an increase in polyploidy 48 h post-irradiation and afterwards.

CONCLUSION

A correlation between TA and hTERT mRNA expression and a radiation induced cell cycle dependent modification of hTERT mRNA expression was established for the first 24 h post-irradiation.

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.

Telomere length in mammalian cells exposed to low- and high-LET radiations

Telomeres are specialised nucleoprotein complexes that serve as protective caps of linear eukaryotic chromosomes. The loss of the ends of the chromosomes due to these un-rejoined double strand breaks (DSBs) may not be lethal to the cell, but may instead result in the loss of functional telomeres, chromosome fusions and initiation of breakage/fusion/bridge cycle-induced chromosome instability. The telomeres also participate in the process of DNA repair, as evidenced by 'de novo' synthesis of telomere repeats at DSBs and by the capacity of telomeres to binding the essential components of the DNA repair machinery. Based on the observation that high-LET radiations efficiently induce chromosome aberrations, it was tested whether protons were able to affect telomere structure. Human primary fibroblasts (HFFF2) and mouse embryonic fibroblasts (MEFs) were irradiated with 4 Gy of 3 MeV protons at the radiobiology facility of the INFN-LNL. Experiments with X rays were also carried out. Cells were fixed after either 24 h or 15 d from treatment. A difference in average telomere length, measured by quantitative fluorescence in situ hybridisation (Q-FISH), between X rays and protons treatment was observed. X rays are able to modify telomere length in HFFF2 harvested at a later time. On the other hand, 3 MeV low-energy protons induced, both in HFFF2 and in MEFs, a significant increase in telomere length at short as well as at long harvesting time periods from treatment. These results seem to indicate that lesions characterised by different complexity, as those expected after low-energy protons and those induced by damage similar to that induced by sparsely ionising radiation, are able to modulate telomere elongation at different time periods.

Telomere biology: integrating chromosomal end protection with DNA damage response

Telomeres play the key protective role at chromosomes. Many studies indicate that loss of telomere function causes activation of DNA damage response. Here, we review evidence supporting interdependence between telomere maintenance and DNA damage response and present a model in which these two pathways are combined into a single mechanism for protecting chromosomal integrity. Proteins directly involved in telomere maintenance and DNA damage response include Ku, DNA-PKcs, RAD51D, PARP-2, WRN and RAD50/MRE11/NBS1 complex. Since most of these proteins participate in the repair of DNA double-strand breaks (DSBs), this was perceived by many authors as a paradox, given that telomeres function to conceal natural DNA ends from mechanisms that detect and repair DSBs. However, we argue here that the key function of one particular DSB protein, Ku, is to prevent or control access of telomerase, the enzyme that synthesises telomeric sequences, to both internal DSBs and natural chromosomal ends. This view is supported by observations that Ku has a high affinity for DNA ends; it acts as a negative regulator of telomerase and that telomerase itself can target internal DSBs. Ku then directs other DSB repair/telomere maintenance proteins to either repair DSBs at internal chromosomal sites or prevent uncontrolled elongation of telomeres by telomerase. This model eliminates the above paradox and provides a testable scenario in which the role of DSB repair proteins is to protect chromosomal integrity by balancing repair activities and telomere maintenance. In our model, a close association between telomeres and different DNA damage response factors is not an unexpected event, but rather a logical result of chromosomal integrity maintenance activities.

Induction of apoptosis and inhibition of telomerase activity in human bone marrow and HL-60 p53 null cells treated with anti-cancer drugs

Telomerase plays a key role in the maintenance of chromosomal stability in tumours, and the ability of anti-cancer agents to inhibit telomerase activity is under investigation. In this study, we evaluated the effect of etoposide and taxol, on the telomerase activity and telomere length in human leukaemia p53 null cells and human bone marrow cells, as well as apoptosis and cell cycle modulation. Results showed that after exposure to the drugs, HL-60 cells as well as the human progenitors underwent a block in G2 and subsequently apoptosis, whereas stromal cells from bone marrow did not undergo a block in G2 or enter apoptosis after etoposide exposure. Telomere length increased in stromal cells after treatment with both etoposide and taxol whereas in HL-60 cells only after etoposide treatment with. Bax, bcl-2 and bcl-x change their expression in stromal cells, whereas bcl-x was induced after drug treatment and bcl-2 down regulated in progenitor cells. Our data suggest that telomerase activity and apoptosis are correlated and they seem to be modulated by a common gene, bcl-2.

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.

Reconstitution of active telomerase in primary human foreskin fibroblasts: effects on proliferative characteristics and response to ionizing radiation

PURPOSE

Telomere shortening has been proposed to trigger senescence, and since most primary cells do not express active telomerase, reactivation of telomerase activity was proposed as a safe and non-transforming way of immortalizing cells. However, to study radiation responses, it is as yet unclear whether cells immortalized by telomerase reactivation behave in a similar manner as their parental primary cells.

MATERIALS AND METHODS

Primary human foreskin fibroblasts were transfected with the human catalytic subunit of telomerase, the reverse transcriptase (hTERT), and their growth characteristics and response to DNA damage were characterized.

RESULTS

The sole expression of the human hTERT was sufficient to immortalize the human foreskin fibroblasts. With time in culture, the immortalized cells almost doubled their average telomeric length and the clonal population contained almost no post-mitotic fibroblasts anymore. Up to 300 population doublings, no alterations compared with the parental primary cells were seen in terms of clonogenic radiosensitivity, DNA double-strand break repair, radiation-induced increases in p53 and p21(WAF-1,CIP-1) expression, and the G1/S and G2/M cell cycle checkpoints. Moreover, mitogen-induced mitotic arrest of fibroblasts was still possible in the hTERT-immortalized clones.

CONCLUSIONS

Immortalizing fibroblasts by reconstitution of active telomerase seems a good, reliable manner to generate a large source of cells with a radiation damage response similar to the primary cells.

Expression of telomerase activity and oxidative stress in human hepatocellular carcinoma with cirrhosis

AIM: To study the expression and significance of telomerase activity and oxidative stress in hepatocellular carcinoma (HCC) with cirrhosis.

METHODS: In this study, TRAP-ELISA assay was used to determine telomerase activity in 21 cases of HCC as well as in 23 cases of hepatic cirrhosis. Malondialdehyde (MDA), glutathione S-transferase (GST) and total anti-oxidative capacity (T-AOC) were also examined in the same samples with human MDA, GST and T-AOC kits.

RESULTS: Eighteen of 21 cases of HCC were found to have increased telomerase activity, whereas only three of the 23 non-cancerous cirrhotic samples were found to have weak telomerase activity, and the difference was significant (P < 0.001). No significant difference in telomerase activity was detected according to different tumor size, tumor stage, histological grade, HBsAg, contents of albumin, bilirubin, ALT, AFP, r-GT and platelet. There were significant differences between HCC and cirrhosis in the expression of MDA, GST and T-AOC respectively. Telomerase activity correlated positively with the content of MDA (P < 0.05).

CONCLUSION: Telomerase activation is the early event of carcinogenesis, which is not correlated with clinicopathological factors of HCC. The dysfunction of the anti-oxidative system is closely correlated with the progression from cirrhosis to hepatocellular carcinoma. Oxidative stress may contribute partly to telomerase activation.

Rapid upregulation of telomerase activity in human leukemia HL-60 cells treated with clinical doses of the DNA-damaging drug etoposide

The enzyme telomerase is implicated in cellular resistance to apoptosis, but the mechanism for this resistance remains to be elucidated. The ability of telomerase to synthesize new DNA at telomeres suggests that this enzyme might function in the repair of double-stranded DNA breaks. To distinguish the effects of double-stranded DNA break damage and apoptosis on human telomerase activity, we treated the HL-60 human hematopoietic cancer cell line with clinical doses of the chemotherapeutic drug etoposide (0.5 to 5 microM), which allowed us to distinguish between events associated with DNA damage-induced cell cycle arrest, and events associated with apoptosis. Large (three- to seven-fold) upregulation of telomerase activity occurred soon after etoposide treatment (3 h) in S/G2/M-arresting populations; this upregulation was abolished at onset of apoptotic cell death. No upregulation of telomerase activity was observed in cells treated with a larger dose of etoposide (5 microM) that caused cells to undergo rapid apoptosis without intervening cell cycle arrests. These observations are consistent with a possible role for telomerase upregulation during the DNA damage response.

Ultraviolet irradiation- and dimethyl sulfoxide-induced telomerase activity in ovarian epithelial cell lines

Information about telomerase regulation is incomplete, especially since various studies suggest complexity in telomerase regulation. Given the important association between telomerase and cancer, it is imperative to design and develop a model system in which telomerase activity can be regulated and studied. We employed ultraviolet (UV) radiation or dimethyl sulfoxide (DMSO) to transiently induce telomerase activity in a telomerase-positive cell line and, most importantly, in a telomerase-negative cell line. UV- or DMSO-induced telomerase activity was associated with increased hTRT, but not hTR, mRNA transcription in the telomerase-negative cells. However, no changes in hTRT or hTR mRNA transcription were noted with UV- or DMSO-induced telomerase activity in the telomerase-positive cells. Inhibition of protein synthesis or the phosphotidyl inositol 3-kinase (PI3K) pathway suppressed telomerase induction and/or activity in all cell lines examined, suggesting telomerase activity was dependent on protein synthesis and PI3K-mediated phosphorylation. Furthermore, enhanced telomerase activity was limited to UV and DMSO, since a variety of chemotherapeutic agents failed to induce telomerase activity. Therefore, our data provide a useful culture model system to study telomerase regulation in telomerase-negative and -positive cell lines and from which to obtain information about telomerase as a target for cancer intervention.

Induction of telomerase activity by irradiation in human lymphoblasts

Neuhof, D., Ruess, A., Wenz, F. and Weber, K. J. Induction of Telomerase Activity by Irradiation in Human Lymphoblasts. Radiat. Res. 155, 693-697 (2001). Telomerase activity is a radiation-inducible function, which suggests a role of this enzyme in DNA damage processing. Since the tumor suppressor TP53 plays a central role in the regulation of the cellular response to DNA damage, our study explored the ability of ionizing radiation to change telomerase activity and telomere length in two closely related human lymphoblast cell lines with different TP53 status. TK6 cells (wild-type TP53) and WTK1 cells (mutated TP53) were exposed to different doses of X rays, and telomerase activity was measured by PCR ELISA at different times after irradiation. A dose-dependent increase in telomerase activity was observed. One hour after irradiation with 4 Gy, TK6 and WTK1 cells showed an approximately 2.5-fold increase; for lower doses (0.1 to 1 Gy), telomerase induction was seen only in TK6 cells. Telomerase induction was observed by 0.5 h after irradiation, with a further increase up to 24 h. Irradiated TK6 and WTK1 cells had longer telomeres (+1.3 kb) than unirradiated cells 14 days after exposure. Our data demonstrate a dose-dependent induction of telomerase activity and lengthening of telomeres by ionizing radiation in human lymphoblasts. Induction of telomerase activity by radiation does not generally appear to be controlled by the TP53-dependent DNA damage response pathway. However, for low doses, induction of telomerase requires wild-type TP53.

Genetic regulation of embryo death and senescence

The survival of the preimplantation mammalian embryo depends not only on providing the proper conditions for normal development but also on acquiring the mechanisms by which embryos cope with adversity. The ability of the early conceptus to resist stress as development proceeds may be regulated by diverse factors such as the attainment of a cell death program and protective mechanisms involving stress-induced genes and/or cell cycle modulators. This paper reviews the recent research on the genetic regulation of early embryo cell death and senescence focussing on the bovine species where possible. The different modes of cell death will be explained, clarifying the confusing cell death terminology, by advocating the recommendations set forth by the Cell Death Nomenclature Committee to extend to the embryology research field. Specific pro-death and anti-death genes will be discussed with reference to their expression patterns during early mammalian embryogenesis.

Human CDK10 gene isoforms

The CDK10/PISSLRE gene has been shown to encode two different CDK-like putative kinases. The function(s) of the gene products are unknown, although a role at the G2/M transition has been suggested. We characterised two novel cDNAs. CDK10 mRNA quantity was not found to be correlated with cell proliferation status in HeLa or WI38 cell cultures or in human tissues. Relative levels of the four CDK10 isoforms were studied by RT-PCR, of which three were principally expressed. The two initially cloned isoforms predominated in human tissues, except in brain and muscle. Relative isoform levels did not vary during the cell cycle in culture, except when cells entered into the cell cycle. Finally, the predominant isoforms were shown to have different translation initiation sites and to have different subcellular distribution, due to an alternatively spliced nuclear localisation signal.

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.

Upregulation of telomerase activity by X-irradiation in mouse leukaemia cells is independent of Tert, Terc, Tnks and Myc transcription

X-irradiation of two mouse myeloid leukaemia cell lines was found to lead to increased telomerase activities. Maximal increases in activity at 24 h post-irradiation were approximately three times control unirradiated cell levels. These maxima were reached at between 3-5 Gy depending upon cell line. Peak activity was reached at 8h, remained elevated to 24 h and returned to control levels by 48 h. In contrast, X-irradiation did not activate telomerase in a telomerase-negative human fibroblast line, while in cultured normal mouse bone marrow cells irradiation appeared to reduce activities. No simple relationship between radiation-induced increases in telomerase activity in the myeloid leukaemia lines and the proportions of cells in the S or M phases of the cell cycle was apparent. Radiation-induced increases in activity were significantly reduced by inhibitors of transcription (actinomycin D, alpha-amanatin) and protein synthesis (cycloheximide). These data are consistent with two possibilities: (i) X-irradiation leads to increased transcription and/or translation of a component of telomerase, thus increasing activities; or (ii) X-irradiation induces the transcription of a positive regulator of telomerase activity. Northern blot analysis did not indicate that transcription of mTert, the catalytic subunit of telomerase, or mTerc, the RNA component, was elevated after irradiation. Similarly, no significant changes in the expression of Myc or Tnks, the tankyrase gene, two suspected telomerase regulators, were detected. These data are therefore consistent with the induction by X-irradiation of a positive regulator of telomerase activity other than Tnks or Myc or the core protein and RNA components of the enzyme.

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.

Telomere shortening triggers a p53-dependent cell cycle arrest via accumulation of G-rich single stranded DNA fragments

It has been repeatedly suspected that telomere shortening might be one possible trigger of the p53-dependent cell cycle arrest, although the mechanism of this arrest remained unclear. Telomeres in human cells under mild oxidative stress accumulate single-strand damage faster than interstitial repetitive sequences. In MRC-5 fibroblasts and U87 glioblastoma cells, which both express wild-type p53, oxidative stress-mediated production of single-strand damage in telomeres is concomitant to the accumulation of p53 and p21 and to cell cycle arrest. This response can be modeled by treatment of cells with short single stranded telomeric G-rich DNA fragments. The arrest is transient in U87 cells. Recovery from it is accompanied by up-regulation of telomerase activity and elongation of telomeres. Overexpression of mutated p53 is sufficient to reverse the phenotype of inhibition as well as the delayed activation of telomerase. These data suggest that the production of G-rich single stranded fragments during the course of telomere shortening is sufficient to trigger a p53 dependent cell cycle arrest.

Accelerated telomere shortening and telomerase activation in Fanconi's anaemia

Fanconi's anaemia (FA) is an autosomal recessive disorder characterized by progressive bone marrow failure that often evolves towards acute leukaemia. FA also belongs to a group of chromosome instability diseases. Because telomeres are directly involved in chromosomal stability and in cell proliferation capacity, we examined telomere metabolism in peripheral blood mononuclear cells (PBMC). Telomere length was significantly shorter in 54 FA patient samples, compared to 51 controls (P<0.0001). In addition, mean telomere terminal restriction fragment lengths (TRF) in nine heterozygous patient samples did not differ from those of controls. In 14 samples from FA patients with severe aplastic anaemia (SFA), telomere length was significantly shorter than in 22 samples of age-matched FA patients with moderate haematological abnormalities (NSFA) (P<0.001). However, no correlation was found between TRF length and the presence of bone marrow clonal abnormalities in 16 additional, separately analysed, patient samples. Sequential measurement of TRF in six FA patients showed an accelerated rate of telomere shortening. Accordingly, telomere shortening rate was inversely correlated with clinical status. Telomerase, the enzyme that counteracts telomere shortening, was 4.8-fold more active in 25 FA patients than in 15 age-matched healthy controls. A model for the FA disease process is proposed.

Involvement of telomeric sequences in chromosomal aberrations

The genomes of higher eukaryotes are not homogeneous in terms of structure or function. Many examples of chromosomal regions particularly prone to involvement in aberrations have been reported. The molecular structures of some of these regions have now been determined, most notably the folate-sensitive fragile sites and FRA16B-a distamycin A-sensitive fragile site. In addition, a number of cytological studies suggest that telomeric sequences can in some circumstances be involved in chromosomal aberrations more frequently than expected. Here, the roles of telomeric DNA sequences, both terminal and interstitial, and telomerase in chromosomal aberration formation are reviewed.

Induction of telomerase activity by in vivo X-irradiation of mouse splenocytes and its possible role in chromosome healing

Telomeres serve as protective caps for the chromosome ends. They are one of the functional elements required for the stable transmission of eukaryotic chromosomes. Telomerase, a ribonucleoprotein, stabilises the telomere length by adding telomere repeats on to chromosome ends. Telomeres and telomerase can play a role in the formation of chromosome aberrations and especially in healing of the chromosome or chromatid breaks produced by radiation-induced DNA damage. Telomerase-independent processes also appear to be capable of capping broken chromosome ends. We have studied the expression of telomerase, telomere status and chromosome rearrangements in mouse splenocytes following different doses (0.5, 1.0, 2.0 or 3.0 Gy) of X-irradiation in vivo up to 224 days post-exposure. A dose-dependent increase in telomerase activity up to 2 Gy X-ray exposure was observed immediately after irradiation. The increased enzyme activity was detected even up to day 224 post-irradiation, the last time point studied, especially at higher doses (2 Gy and 3 Gy). A significant difference in average telomere length, measured by quantitative fluorescence in situ hybridisation (Q-FISH) on metaphase chromosomes, noticed immediately after irradiation indicates terminal deletion or altered telomere chromatin. However, telomere length was not statistically significant from the control at the later time points studied. Presence of telomere repeats at the chromosomal breakage sites revealed by FISH with peptide nucleic acid (PNA) telomeric probe indicates a possible role of telomerase-dependent or independent processes in chromosome healing and telomere capture in mammalian cells. We found that approximately 25 to 50% of the newly formed telomeres at the breakage sites are in the range of 200 bp to 1 kb, which might suggest that these repeats could have been added by telomerase which showed a corresponding increase following irradiation.