Molecular manifestations and molecular determinants of telomere capping

Telomere Length, Apoptotic, and Inflammatory Genes: Novel Biomarkers of Gastrointestinal Tract Pathology and Meat Quality Traits in Chickens under Chronic Stress (Gallus gallus domesticus)

Simple Summary

The assessment of poultry’s gastrointestinal (GI) tract and meat quality traits are crucial for sustainable poultry production in the tropics. The search for well-conserved and more reliable biomarkers for the GI tract and meat traits has faced many challenges. In this study, we observed the effect of corticosterone (CORT) and age on body weight, buffy coat telomere length, GI tract, and meat quality traits. The critical evaluation of the GI tract and meat traits in this study revealed that telomere length, mitochondria, and acute phase protein genes were altered by chronic stress and were associated with the traits. This study informed us of the potential of telomere length, mitochondria, and acute phase protein genes in the assessment of GI tract pathological conditions and meat quality in the poultry sector for sustainable production.

Abstract

This study was designed to examine the potentials of telomere length, mitochondria, and acute phase protein genes as novel biomarkers of gastrointestinal (GI) tract pathologies and meat quality traits. Chickens were fed a diet containing corticosterone (CORT) for 4 weeks and records on body weight, telomere length, GI tract and muscle histopathological test, meat quality traits, mitochondria, and acute phase protein genes were obtained at weeks 4 and 6 of age. The body weight of CORT-fed chickens was significantly suppressed (p < 0.05). CORT significantly altered the GI tract and meat quality traits. The interaction effect of CORT and age on body weight, duodenum and ileum crypt depth, pH, and meat color was significant (p < 0.05). CORT significantly (p < 0.05) shortened buffy coat telomere length. UCP3 and COX6A1 were diversely and significantly expressed in the muscle, liver, and heart of the CORT-fed chicken. Significant expression of SAAL1 and CRP in the liver and hypothalamus of the CORT-fed chickens was observed at week 4 and 6. Therefore, telomere lengths, mitochondria, and acute phase protein genes could be used as novel biomarkers for GI tract pathologies and meat quality traits.

Telomere Length and Regulatory Genes as Novel Stress Biomarkers and Their Diversities in Broiler Chickens (Gallus gallus domesticus) Subjected to Corticosterone Feeding

Simple Summary

Assessment of poultry welfare is very crucial for sustainable production in the tropics. There is a demand for alternatives to plasma corticosterone levels as they have received much criticism as an unsuitable predictor of animal welfare due to inconsistency. In this study, we noticed no effect of age on plasma corticosterone (CORT) although it was altered by CORT treatment. However, growth performances and organ weight were affected by CORT treatment and age. The broad sense evaluation of telomere length in this study revealed that telomere length in the blood, muscle, liver and heart was shortened by chronic stress induced by corticosterone administration. The expression profile of the telomere regulatory genes was altered by chronic stress. This study informed us of the potential of telomere length and its regulatory genes in the assessment of animal welfare in the poultry sector for sustainable production.

Abstract

This study was designed to characterize telomere length and its regulatory genes and to evaluate their potential as well-being biomarkers. Chickens were fed a diet containing corticosterone (CORT) for 4 weeks and performances, organ weight, plasma CORT levels, telomere lengths and regulatory genes were measured and recorded. Body weights of CORT-fed chickens were significantly suppressed (p < 0.05), and organ weights and circulating CORT plasma levels (p < 0.05) were altered. Interaction effect of CORT and duration was significant (p < 0.05) on heart and liver telomere length. CORT significantly (p < 0.05) shortened the telomere length of the whole blood, muscle, liver and heart. The TRF1, chTERT, TELO2 and HSF1 were significantly (p < 0.05) upregulated in the liver and heart at week 4 although these genes and TERRA were downregulated in the muscles at weeks 2 and 4. Therefore, telomere lengths and their regulators are associated and diverse, so they can be used as novel biomarkers of stress in broiler chickens fed with CORT.

G-quadruplex Stabilization Fuels the ALT Pathway in ALT-positive Osteosarcoma Cells

Most human tumors maintain telomere lengths by telomerase, whereas a portion of them (10–15%) uses a mechanism named alternative lengthening of telomeres (ALT). The telomeric G-quadruplex (G4) ligand RHPS4 is known for its potent antiproliferative effect, as shown in telomerase-positive cancer models. Moreover, RHPS4 is also able to reduce cell proliferation in ALT cells, although the influence of G4 stabilization on the ALT mechanism has so far been poorly investigated. Here we show that sensitivity to RHPS4 is comparable in ALT-positive (U2OS; SAOS-2) and telomerase-positive (HOS) osteosarcoma cell lines, unlinking the telomere maintenance mechanism and RHPS4 responsiveness. To investigate the impact of G4 stabilization on ALT, the cardinal ALT hallmarks were analyzed. A significant induction of telomeric doublets, telomeric clusterized DNA damage, ALT-associated Promyelocytic Leukaemia-bodies (APBs), telomere sister chromatid exchanges (T-SCE) and c-circles was found exclusively in RHPS4-treated ALT cells. We surmise that RHPS4 affects ALT mechanisms through the induction of replicative stress that in turn is converted in DNA damage at telomeres, fueling recombination. In conclusion, our work indicates that RHPS4-induced telomeric DNA damage promotes overactivation of telomeric recombination in ALT cells, opening new questions on the therapeutic employment of G4 ligands in the treatment of ALT positive tumors.

Discovery of potent telomerase activators: Unfolding new therapeutic and anti-aging perspectives

Telomere length, a marker of cellular aging, decreases with age and it has been associated with aging‑related diseases. Environmental factors, including diet and lifestyle factors, affect the rate of telomere shortening which can be reversed by telomerase. Telomerase activation by natural molecules has been suggested to be an anti‑aging modulator that can play a role in the treatment of aging‑related diseases. This study aimed to investigate the effect of natural compounds on telomerase activity in human peripheral blood mononuclear cells (PBMCs). The tested compounds included Centella asiatica extract formulation (08AGTLF), Astragalus extract formulation (Nutrient 4), TA‑65 (containing Astragalus membranaceus extract), oleanolic acid (OA), maslinic acid (MA), and 3 multi‑nutrient formulas (Nutrients 1, 2 and 3) at various concentrations. The mean absorbance values of telomerase activity measured following treatment with some of the above‑mentioned formulations were statistically significantly higher compared to those of the untreated cells. In particular, in order of importance with respect to telomerase activation from highest to lowest, 08AGTLF, OA, Nutrient 4, TA‑65, MA, Nutrient 3 and Nutrient 2, triggered statistically significant increase in telomerase activity compared to the untreated cells. 08AGTLF reached the highest levels of telomerase activity reported to date, at least to our knowledge, increasing telomerase activity by 8.8 folds compared to untreated cells, while Nutrient 4 and OA were also potent activators (4.3‑fold and 5.9‑fold increase, respectively). On the whole, this study indicates that the synergistic effect of nutrients and natural compounds can activate telomerase and produce more potent formulations. Human clinical studies using these formulations are required to evaluate their mode of action. This would reveal the health benefits of telomerase activation through natural molecules and would shed new light onto the treatment of aging‑related diseases.

Association of nutraceutical supplements with longer telomere length

Telomeres are nucleotide tandem repeats located at the tip of eukaryotic chromosomes that maintain genomic integrity. The gradual shortening of telomeres leads to cellular senescence and apoptosis, a key mechanism of aging and age‑related chronic diseases. Epigenetic factors, such as nutrition, exercise and tobacco can affect the rate at which telomeres shorten and can modify the risk of developing chronic diseases. In this study, we evaluated the effects of a combination of nutraceutical supplements (NS) on telomere length (TL) in healthy volunteers with no medical history of any disease. Participants (n=47) were selected from healthy outpatients visiting a private clinic and were divided into the experimental group (n=16), that received the NS and the control group (n=31). We estimated the length of single telomeres in metaphase spread leukocytes, isolated from peripheral blood, using quantitative‑fluorescent in situ hybridization (Q‑FISH) analysis. The length of the whole telomere genome was significantly increased (P<0.05) for the mean, 1st quartile and median measurements in the experimental group. Similar findings were observed for short TL (20th percentile) (P<0.05) for the median and 3rd quartile measurements in the NS group, compared to the control group. The beneficial effects of the supplements on the length of short telomeres remained significant (P<0.05) following adjustment for age and sex. Telomeres were moderately longer in female patients compared to the male patients. On the whole, the findings of this study suggest that the administration of NS may be linked to sustaining the TL.

Association between leukocyte telomere length and hostility in US army service members

Hostility is a common form of emotionally charged anger which can lead to maladaptive and unhealthy behaviors. Significant association between shortened telomeres and greater levels of hostility has been observed in civilian populations, but has not yet been comprehensively studied in military populations. Our study investigates the relationship between hostility, post-traumatic stress disorder (PTSD), and leukocyte telomere length (LTL) in a sample of United States Army Special Operations personnel (n = 474) who deployed to Iraq and/or Afghanistan as part of combat operations. Hostility was measured with five items from the Brief Symptom Inventory (BSI). PTSD was determined using the PTSD Checklist (PCL) total score. The LTL was assessed using quantitative polymerase chain reaction methods and regression analyses were conducted to determine the association of hostility and telomere length. PTSD subjects reported higher hostility scores compared with those without PTSD. Among the participants with PTSD, those with medium or high level of hostility had shorter LTL than those with low level hostility (P < 0.01). Stepwise regression indicated that hostility level and age, but not gender and PTSD, were negatively correlated with LTL. Univariate regression showed that total hostility score was negatively associated with LTL (CI= -0.06 to -0.002, Beta= -0.095, p < 0.039) as well as a significant correlation between LTL and hostility impulses (HI) (CI= -0.108 to -0.009, Beta= -0.106, p < 0.021) and hostility controlling (HC) (CI= -0.071 to -0.002, Beta= -0.095, p < 0.004). Multiple regression analyses revealed that, while HC has no significant association with LTL, HI was still negatively correlated with LTL (p = 0.021). Our data indicates that LTL is associated with HI levels. Prevention and treatment efforts designed to reduce hostility may help mitigate risk for LTL shortening, a process of cellular aging, and thus slow accelerated aged-related health outcomes.

Rnr1's role in telomere elongation cannot be replaced by Rnr3: a role beyond dNTPs?

Telomeres, the nucleoprotein complexes at the end of eukaryotic chromosomes, protect them from degradation and ensure the replicative capacity of cells. In most human tumors and in budding yeast, telomere length is maintained by the activity of telomerase, an enzyme that adds dNTPs according to an internal RNA template. The dNTPs are generated with the help of the ribonucleotide reductase (RNR) complex. We have recently generated strains lacking the large subunit of RNR, Rnr1, which were kept viable by the expression of RNR complexes containing the Rnr1 homolog, Rnr3. Interestingly, we found that these Rnr1-deficient strains have short telomeres that are stably maintained, but cannot become efficiently elongated by telomerase. Thus, a basic maintenance of short telomeres is possible under conditions, where Rnr1 activity is absent, but a sustained elongation of short telomeres fully depends on Rnr1 activity. We show that Rnr3 cannot compensate for this telomeric function of Rnr1 even when overall cellular dNTP values are restored. This suggests that Rnr1 plays a role in telomere elongation beyond increasing cellular dNTP levels. Furthermore, our data indicate that telomerase may act in two different modes, one that is capable of coping with the "end-replication problem" and is functional even in the absence of Rnr1 and another required for the sustained elongation of short telomeres, which fully depends on the presence of Rnr1. Supply of dNTPs for telomere elongation is provided by the Mec1^ATR checkpoint, both during regular DNA replication and upon replication fork stalling. We discuss the implications of these results on telomere maintenance in yeast and cancer cells.

Implications of telomeres and telomerase in endometrial pathology

BACKGROUND

Eukaryotic chromosomal ends are linear and are protected by nucleoprotein complexes known as telomeres. The complex structural anatomy and the diverse functions of telomeres as well as the unique reverse transcriptase enzyme, telomerase that maintains telomeres are under intensive scientific scrutiny. Both are involved in many human diseases including cancer, but also in ageing and chronic disease such as diabetes. Their intricate involvement in many cellular processes and pathways is being dynamically deciphered in many organs including the endometrium. This review summarizes our current knowledge on the topic of telomeres and telomerase and their potential role in providing plausible explanations for endometrial aberrations related to common gynaecological pathologies.

OBJECTIVE AND RATIONALE

This review outlines the recent major findings in telomere and telomerase functions in the context of endometrial biology. It highlights the contemporary discoveries in hormonal regulation, normal endometrial regeneration, stem cells and common gynaecological diseases such as endometriosis, infertility, recurrent reproductive failure and endometrial cancer (EC).

SEARCH METHODS

The authors carried out systematic PubMed (Medline) and Ovid searches using the key words: telomerase, telomeres, telomere length, human telomerase reverse transcriptase, telomeric RNA component, with endometrium, hormonal regulation, endometrial stem/progenitor cells, endometrial regeneration, endometriosis, recurrent miscarriage, infertility, endometrial hyperplasia, EC and uterine cancer. Publications used in this review date from 1995 until 31st June 2016.

OUTCOMES

The human endometrium is a unique somatic organ, which displays dynamic telomerase activity (TA) related to the menstrual cycle. Telomerase is implicated in almost all endometrial pathologies and appears to be crucial to endometrial stem cells. In particular, it is vital for normal endometrial regeneration, providing a distinct route to formulate possible curative, non-hormonal therapies to treat chronic endometrial conditions. Furthermore, our current understanding of telomere maintenance in EC is incomplete. Data derived from other malignancies on the role of telomerase in carcinogenesis cannot be extrapolated to EC because unlike in other cancers, TA is already present in proliferating healthy endometrial cells.

WIDER IMPLICATIONS

Since telomerase is pivotal to endometrial regeneration, further studies elucidating the role of telomeres, telomerase, their associated proteins and their regulation in normal endometrial regeneration as well as their role in endometrial pathologies are essential. This approach may allow future development of novel treatment strategies that are not only non-hormonal but also potentially curative.

Transient ALT activation protects human primary cells from chromosome instability induced by low chronic oxidative stress

Cells are often subjected to the effect of reactive oxygen species (ROS) as a result of both intracellular metabolism and exposure to exogenous factors. ROS-dependent oxidative stress can induce 8-oxodG within the GGG triplet found in the G-rich human telomeric sequence (TTAGGG), making telomeres highly susceptible to ROS-induced oxidative damage. Telomeres are nucleoprotein complexes that protect the ends of linear chromosomes and their dysfunction is believed to affect a wide range of cellular and/or organismal processes. Acute oxidative stress was shown to affect telomere integrity, but how prolonged low level oxidative stress, which may be more physiologically relevant, affects telomeres is still poorly investigated. Here, we explored this issue by chronically exposing human primary fibroblasts to a low dose of hydrogen peroxide. We observed fluctuating changes in telomere length and fluctuations in the rates of chromosome instability phenotypes, such that when telomeres shortened, chromosome instability increased and when telomeres lengthened, chromosome instability decreased. We found that telomere length fluctuation is associated with transient activation of an alternative lengthening of telomere (ALT) pathway, but found no evidence of cell death, impaired proliferation, or cell cycle arrest, suggesting that ALT activation may prevent oxidative damage from reaching levels that threaten cell survival.

Hypothesis: Paralog Formation from Progenitor Proteins and Paralog Mutagenesis Spur the Rapid Evolution of Telomere Binding Proteins

Through elegant studies in fungal cells and complex organisms, we propose a unifying paradigm for the rapid evolution of telomere binding proteins (TBPs) that associate with either (or both) telomeric DNA and telomeric proteins. TBPs protect and regulate telomere structure and function. Four critical factors are involved. First, TBPs that commonly bind to telomeric DNA include the c-Myb binding proteins, OB-fold single-stranded binding proteins, and G-G base paired Hoogsteen structure (G4) binding proteins. Each contributes independently or, in some cases, cooperatively, to provide a minimum level of telomere function. As a result of these minimal requirements and the great abundance of homologs of these motifs in the proteome, DNA telomere-binding activity may be generated more easily than expected. Second, telomere dysfunction gives rise to genome instability, through the elevation of recombination rates, genome ploidy, and the frequency of gene mutations. The formation of paralogs that diverge from their progenitor proteins ultimately can form a high frequency of altered TBPs with altered functions. Third, TBPs that assemble into complexes (e.g., mammalian shelterin) derive benefits from the novel emergent functions. Fourth, a limiting factor in the evolution of TBP complexes is the formation of mutually compatible interaction surfaces amongst the TBPs. These factors may have different degrees of importance in the evolution of different phyla, illustrated by the apparently simpler telomeres in complex plants. Selective pressures that can utilize the mechanisms of paralog formation and mutagenesis to drive TBP evolution along routes dependent on the requisite physiologic changes.

Oxidative stress induces persistent telomeric DNA damage responsible for nuclear morphology change in mammalian cells

One main function of telomeres is to maintain chromosome and genome stability. The rate of telomere shortening can be accelerated significantly by chemical and physical environmental agents. Reactive oxygen species are a source of oxidative stress and can produce modified bases (mainly 8-oxoG) and single strand breaks anywhere in the genome. The high incidence of guanine residues in telomeric DNA sequences makes the telomere a preferred target for oxidative damage. Our aim in this work is to evaluate whether chromosome instability induced by oxidative stress is related specifically to telomeric damage. We treated human primary fibroblasts (MRC-5) in vitro with hydrogen peroxide (100 and 200 µM) for 1 hr and collected data at several time points. To evaluate the persistence of oxidative stress-induced DNA damage up to 24 hrs after treatment, we analysed telomeric and genomic oxidative damage by qPCR and a modified comet assay, respectively. The results demonstrate that the genomic damage is completely repaired, while the telomeric oxidative damage persists. The analysis of telomere length reveals a significant telomere shortening 48 hrs after treatment, leading us to hypothesise that residual telomere damage could be responsible for the telomere shortening observed. Considering the influence of telomere length modulation on genomic stability, we quantified abnormal nuclear morphologies (Nucleoplasmic Bridges, Nuclear Buds and Micronuclei) and observed an increase of chromosome instability in the same time frame as telomere shortening. At subsequent times (72 and 96 hrs), we observed a restoration of telomere length and a reduction of chromosome instability, leaving us to conjecture a correlation between telomere shortening/dysfunction and chromosome instability. We can conclude that oxidative base damage leads to abnormal nuclear morphologies and that telomere dysfunction is an important contributor to this effect.

Construction of a tumor-specific bioluminescent eukaryotic expression vector and analysis of its expression in vitro and in vivo

The aims of this study were to construct a tumor-specific bioluminescent eukaryotic vector driven by the hTERT gene promoter and to establish a stable HeLa cell line expressing a modified firefly luciferase gene. PhTERTp-luc and pGL4.17 (luc2/Neo) were digested with SacI and HindIII, respectively, and the recombinant vector phTERTp-luc-neo was generated by ligating the desired fragments. The expression of phTERTp-luc-neo was tested in a non-transformed cell line (MRC-5), and in telomerase-positive (HeLa, MCF-7 and 293T) and -negative (U2OS and SaOS) transformed cell lines using a luciferase assay. Results showed that the recombinant vector had higher luciferase activity in telomerase-positive transformed cell lines. PhTERTp-luc-neo was transfected into a HeLa cell line, selected by G418 and bioluminescence imaging, and a cell clone HeLa-luc that constitutively expressed both neomycin and luciferase was obtained. We also conducted experiments in animals to observe luciferase activity in vivo using stable cell lines that were subcutaneously implanted into BALB/c nude mice and tumor growth was monitored by bioluminescence imaging. The HeLa-luc cell line retained its oncogenicity and tumors were detected on the fifth day following implantation by bioluminescence imaging. This study has formed a basis for the study of the expression and regulation of hTERT and early tumor detection. It also provides a convenient, sensitive and reliable platform for cervical cancer research.

Robust activation of the human but not mouse telomerase gene during the induction of pluripotency

Pluripotent stem cells (PSCs) express telomerase and have unlimited proliferative potential. To study telomerase activation during reprogramming, 3 classes of embryonic stem cell (ESC)-like clones were isolated from mouse fibroblasts containing a transgenic hTERT reporter. Class I expressed few pluripotency markers, whereas class II contained many, but not Oct4, Nanog, and Sox2. Neither class of cells differentiated efficiently. Class III cells, the fully reprogrammed induced PSCs (iPSCs), expressed all pluripotency markers, formed teratomas indistinguishable from those of mESCs, and underwent efficient osteogenic differentiation in vitro. Interestingly, whereas the endogenous mTERT gene expression was only moderately increased during reprogramming, the hTERT promoter was strongly activated in class II cells and was further elevated in class III cells. Treatment of class II cells with chemical inhibitors of MEKs and glycogen synthase kinase 3 resulted in their further reprogramming into class III cells, accompanied by a strong activation of hTERT promoter. In reprogrammed human cells, the endogenous telomerase level, although variable among different clones, was dramatically elevated. Only in cells with the highest telomerase were telomeres restored to the lengths in hESCs. Our data, for the first time, demonstrated that the hTERT promoter was strongly activated in discrete steps, revealing a critical difference in human and mouse cell reprogramming. Because telomere elongation is crucial for self-renewal of hPSCs and replicative aging of their differentiated progeny, these findings have important implications in the generation and applications of iPSCs.

Chromatin and epigenetic regulation of the telomerase reverse transcriptase gene

Telomerase expression and telomere maintenance are critical for long-term cell proliferation and survival, and they play important roles in development, aging, and cancer. Cumulating evidence has indicated that regulation of the rate-limiting subunit of human telomerase reverse transcriptase gene (hTERT) is a complex process in normal cells and many cancer cells. In addition to a number of transcriptional activators and repressors, the chromatin environment and epigenetic status of the endogenous hTERT locus are also pivotal for its regulation in normal human somatic cells and in tumorigenesis.

Cellular senescence: unravelling complexity

Cellular senescence might be a tumour suppressing mechanism as well as a contributor to age-related loss of tissue function. It has been characterised classically as the result of the loss of DNA sequences called telomeres at the end of chromosomes. However, recent studies have revealed that senescence is in fact an intricate process, involving the sequential activation of multiple cellular processes, which have proven necessary for the establishment and maintenance of the phenotype. Here, we review some of these processes, namely, the role of mitochondrial function and reactive oxygen species, senescence-associated secreted proteins and chromatin remodelling. Finally, we illustrate the use of systems biology to address the mechanistic, functional and biochemical complexity of senescence.

G-quadruplex structure: a target for anticancer therapy and a probe for detection of potassium

G-Quadruplexes are four-stranded DNA structures that play important regulatory roles in the maintenance of telomere length by inhibiting telomerase activity. Telomeres are specialized functional DNA-protein structures consisting of a variable number of tandem G-rich repeats together with a group of specific proteins. Telomere losses during cell replication are compensated by telomerase, which adds telomeric repeats onto the chromosome ends in the presence of its substrate--the 3'-overhang. Recently, quadruplexes have been considered as a potential therapeutic target for human cancer because they can inhibit telomerase activity, and some quadruplex-interacting drugs can induce senescence and apoptosis of cancer cells. In addition, due to the potassium preference to the other cations, especially sodium ions, quadruplexes have been suggested for developing potassium detection probes with higher sensitivity and selectivity. This review will illustrate these two aspects to provide further understanding of G-quadruplex structures.

A genome wide analysis of the response to uncapped telomeres in budding yeast reveals a novel role for the NAD+ biosynthetic gene BNA2 in chromosome end protection

BACKGROUND

Telomeres prevent the ends of eukaryotic chromosomes from being recognized as damaged DNA and protect against cancer and ageing. When telomere structure is perturbed, a co-ordinated series of events promote arrest of the cell cycle so that cells carrying damaged telomeres do not divide. In order to better understand the eukaryotic response to telomere damage, budding yeast strains harboring a temperature sensitive allele of an essential telomere capping gene (cdc13-1) were subjected to a transcriptomic study.

RESULTS

The genome-wide response to uncapped telomeres in yeast cdc13-1 strains, which have telomere capping defects at temperatures above approximately 27 degrees C, was determined. Telomere uncapping in cdc13-1 strains is associated with the differential expression of over 600 transcripts. Transcripts affecting responses to DNA damage and diverse environmental stresses were statistically over-represented. BNA2, required for the biosynthesis of NAD+, is highly and significantly up-regulated upon telomere uncapping in cdc13-1 strains. We find that deletion of BNA2 and NPT1, which is also involved in NAD+ synthesis, suppresses the temperature sensitivity of cdc13-1 strains, indicating that NAD+ metabolism may be linked to telomere end protection.

CONCLUSIONS

Our data support the hypothesis that the response to telomere uncapping is related to, but distinct from, the response to non-telomeric double-strand breaks. The induction of environmental stress responses may be a conserved feature of the eukaryotic response to telomere damage. BNA2, which is involved in NAD+ synthesis, plays previously unidentified roles in the cellular response to telomere uncapping.

Association of hsp90 to the hTERT promoter is necessary for hTERT expression in human oral cancer cells

Enhanced expression of human telomerase reverse transcriptase (hTERT) occurs frequently during cellular immortalization. The current study was undertaken to determine the mechanism regulating the hTERT promoter activity during cellular immortalization of human oral keratinocytes. Normal human oral keratinocytes (NHOKs) were immortalized with Bmi-1 and the E6 oncoprotein of human papillomavirus type 16 to establish the telomerase-positive HOK-Bmi-1/E6 cell line. Using DNA-protein-binding assay, we found that heat shock protein 90 (hsp90) physically interacts with the hTERT promoter in vitro. The hsp90 interaction with the promoter was detected more strongly in the telomerase-positive HOK-Bmi-1/E6 cells compared with that in senescing NHOK. Chromatin immunoprecipitation confirmed the in vivo interaction between hsp90 and the hTERT promoter in SCC4 cells, a telomerase-positive oral cancer cell line, but not in the NHOK. To determine the physiological significance of this interaction, SCC4 cells were exposed to geldanamycin (GA), a competitive inhibitor of hsp90. GA exposure led to decrease in telomerase activity, hTERT promoter activity and hTERT messenger RNA expression in SCC4 cells, even in the absence of de novo protein synthesis. Also, it abolished the in vivo interaction of the hTERT promoter region with hsp90 but not with Sp1 or c-Myc. These results indicate that physical interaction between hsp90 and the hTERT promoter occurs in telomerase-positive cells but not in normal human cells and is necessary for the enhanced hTERT expression and telomerase activity in cancer cells.

A systems-level approach to mapping the telomere length maintenance gene circuitry

The ends of eukaryotic chromosomes are protected by telomeres, nucleoprotein structures that are essential for chromosomal stability and integrity. Understanding how telomere length is controlled has significant medical implications, especially in the fields of aging and cancer. Two recent systematic genome-wide surveys measuring the telomere length of deleted mutants in the yeast Saccharomyces cerevisiae have identified hundreds of telomere length maintenance (TLM) genes, which span a large array of functional categories and different localizations within the cell. This study presents a novel general method that integrates large-scale screening mutant data with protein–protein interaction information to rigorously chart the cellular subnetwork underlying the function investigated. Applying this method to the yeast telomere length control data, we identify pathways that connect the TLM proteins to the telomere-processing machinery, and predict new TLM genes and their effect on telomere length. We experimentally validate some of these predictions, demonstrating that our method is remarkably accurate. Our results both uncover the complex cellular network underlying TLM and validate a new method for inferring such networks.

DNA damage in telomeres and mitochondria during cellular senescence: is there a connection?

Cellular senescence is the ultimate and irreversible loss of replicative capacity occurring in primary somatic cell culture. It is triggered as a stereotypic response to unrepaired nuclear DNA damage or to uncapped telomeres. In addition to a direct role of nuclear DNA double-strand breaks as inducer of a DNA damage response, two more subtle types of DNA damage induced by physiological levels of reactive oxygen species (ROS) can have a significant impact on cellular senescence: Firstly, it has been established that telomere shortening, which is the major contributor to telomere uncapping, is stress dependent and largely caused by a telomere-specific DNA single-strand break repair inefficiency. Secondly, mitochondrial DNA (mtDNA) damage is closely interrelated with mitochondrial ROS production, and this might also play a causal role for cellular senescence. Improvement of mitochondrial function results in less telomeric damage and slower telomere shortening, while telomere-dependent growth arrest is associated with increased mitochondrial dysfunction. Moreover, telomerase, the enzyme complex that is known to re-elongate shortened telomeres, also appears to have functions independent of telomeres that protect against oxidative stress. Together, these data suggest a self-amplifying cycle between mitochondrial and telomeric DNA damage during cellular senescence.

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.

Mitochondrial dysfunction accounts for the stochastic heterogeneity in telomere-dependent senescence

Aging is an inherently stochastic process, and its hallmark is heterogeneity between organisms, cell types, and clonal populations, even in identical environments. The replicative lifespan of primary human cells is telomere dependent; however, its heterogeneity is not understood. We show that mitochondrial superoxide production increases with replicative age in human fibroblasts despite an adaptive UCP-2–dependent mitochondrial uncoupling. This mitochondrial dysfunction is accompanied by compromised [Ca²⁺]_i homeostasis and other indicators of a retrograde response in senescent cells. Replicative senescence of human fibroblasts is delayed by mild mitochondrial uncoupling. Uncoupling reduces mitochondrial superoxide generation, slows down telomere shortening, and delays formation of telomeric γ-H2A.X foci. This indicates mitochondrial production of reactive oxygen species (ROS) as one of the causes of replicative senescence. By sorting early senescent (SES) cells from young proliferating fibroblast cultures, we show that SES cells have higher ROS levels, dysfunctional mitochondria, shorter telomeres, and telomeric γ-H2A.X foci. We propose that mitochondrial ROS is a major determinant of telomere-dependent senescence at the single-cell level that is responsible for cell-to-cell variation in replicative lifespan.

After a limited number of cell divisions, somatic cells lose the capacity for proliferation, called cellular replicative senescence. Senescence, which is triggered by the loss of DNA sequences at the ends of chromosomes (telomeres), is often seen as an example of a regular “biological clock.” However, cell senescence is heterogeneous, with large differences in lifespan between individual cell lineages. This heterogeneity is clearly related to stress, specifically oxidative stress. It was not known, however, whether stress-induced “premature” senescence involves telomeres or is caused by telomere-independent DNA damage responses. Mitochondria are the most important source of reactive oxygen species (ROS) in cells under physiological conditions. We found that mitochondrial function deteriorated while cells approached senescence, leading to increased ROS production. Delaying mitochondrial dysfunction led to postponed replicative senescence and slowing of telomere shortening. Prematurely senescing cells sorted out of young cultures displayed mitochondrial dysfunction, increased oxidative stress, and short telomeres. We propose that replicative telomere-dependent senescence is not “clocked,” but rather is a stochastic process triggered largely by random mitochondrial dysfunction.

Mitochondrial uncoupling is used to study the relationship between telomere length, the production of reactive oxygen species, and replicative senescence.

A new steroid derivative stabilizes g-quadruplexes and induces telomere uncapping in human tumor cells

Human telomeric DNA consists of tandem repeats of the sequence d(TTAGGG) with a 3' single-stranded extension (the G-overhang). The stabilization of G-quadruplexes in the human telomeric sequence by small-molecule ligands inhibits the activity of telomerase and results in telomere uncapping, leading to senescence or apoptosis of tumor cells. Therefore, the search for new and selective G-quadruplex ligands is of considerable interest because a selective ligand might provide a telomere-targeted therapeutic approach to treatment of cancer. We have screened a bank of derivatives from natural and synthetic origin using a temperature fluorescence assay and have identified two related compounds that induce G-quadruplex stabilization: malouetine and steroid FG. These steroid derivatives have nonplanar and nonaromatic structures, different from currently known G-quadruplex ligands. Malouetine is a natural product isolated from the leaves of Malouetia bequaaertiana E. Woodson and is known for its curarizing and DNA-binding properties. Steroid FG, a funtumine derivative substituted with a guanylhydrazone moiety, interacted selectively with the telomeric G-quadruplex in vitro. This derivative induced senescence and telomere shortening of HT1080 tumor cells at submicromolar concentrations, corresponding to the phenotypic inactivation of telomerase activity. In addition, steroid FG induced a rapid degradation of the telomeric G-overhang and the formation of anaphase bridges, characteristics of telomere uncapping. Finally, the expression of protection of telomere 1 (POT1) induced resistance to the growth effect of steroid FG. These results indicate that these steroid ligands represent a new class of telomere-targeted agents with potential as antitumor drugs.

Telomerase RNA inhibition using antisense oligonucleotide against human telomerase RNA linked to a 2',5'-oligoadenylate

Telomerase, a ribonucleoprotein enzyme, is detected in the vast majority of cancers, including malignant gliomas, but not in most normal somatic cells. To inhibit telomerase function effectively, we have adopted the 2',5'-oligoadenylate (2-5A) antisense system. 2-5A is a mediator of one pathway of interferon actions by activating RNase L, resulting in single-stranded RNA cleavage. By linking 2-5A to an antisense oligonucleotide, RNase L degrades the targeted RNA specifically and effectively. Therefore, we have synthesized the antisense oligonucleotide against human telomerase RNA component (hTR) linked to 2-5A (2-5A-anti-hTR) and have demonstrated its antitumor effect on telomerase-positive cancer cells in vitro and in vivo.

Formation of non-random extrachromosomal elements during development, differentiation and oncogenesis

Extrachromosomal elements (EEs) were first discovered as minute chromatin bodies [Cox et al. Minute chromatin bodies in malignant tumors of childhood. Lancet 1965;62:55-8], and subsequently characterized as small circular DNA molecules physically separated from chromosomes. They include episomes, minichromosomes, small polydispersed DNAs or double minutes. This review focuses on eukaryotic EEs generated by genome rearrangements under physiological or pathological conditions. Some of those rearrangements occur randomly, but others are strictly non-random, highly regulated, and involve specific chromosomal locations (V(D)J-recombination, telomere maintenance mechanisms, c-myc deregulation). The multiple mechanisms of EEs formation are strongly interconnected and frequently linked to gene amplification. Identification of genes located on EEs will undoubtedly allow a better understanding of genome dynamics and oncogenic pathways.

Telomestatin-induced telomere uncapping is modulated by POT1 through G-overhang extension in HT1080 human tumor cells

Telomestatin is a potent G-quadruplex ligand that interacts with the 3' telomeric overhang, leading to its degradation, and induces a delayed senescence and apoptosis of cancer cells. POT1 and TRF2 were recently identified as specific telomere-binding proteins involved in telomere capping and t-loop maintenance and whose interaction with telomeres is modulated by telomestatin. We show here that the treatment of HT1080 human tumor cells by telomestatin induces a rapid decrease of the telomeric G-overhang and of the double-stranded telomeric repeats. Telomestatin treatment also provokes a strong decrease of POT1 and TRF2 from their telomere sites, suggesting that the ligand triggers the uncapping of the telomere ends. The effect of the ligand is associated with an increase of the gamma-H2AX foci, one part of them colocalizing at telomeres, thus indicating the occurrence of a DNA damage response at the telomere, but also the presence of additional DNA targets for telomestatin. Interestingly, the expression of GFP-POT1 in HT1080 cells increases both telomere and G-overhang length. As compared with HT1080 cells, HT1080GFP-POT1 cells presented a resistance to telomestatin treatment characterized by a protection to the telomestatin-induced growth inhibition and the G-overhang shortening. This protection is related to the initial G-overhang length rather than to its degradation rate and is overcome by increased telomestatin concentration. Altogether these results suggest that telomestatin induced a telomere dysfunction in which G-overhang length and POT1 level are important factors but also suggest the presence of additional DNA sites of action for the ligand.

The Pattern of Chromosome-Specific Variations in Telomere Length in Humans Shows Signs of Heritability and Is Maintained through Life

This paper characterizes the distribution of telomere length on individual chromosome arms in humans. By fluorescent in situ hybridization (FISH), followed by computer-assisted analysis of digital images, it is shown that the distribution of telomere length on individual chromosome arms is not random, but that humans have a common telomere profile. This profile exists in lymphocytes, amniocytes and fibroblasts, and seems to be conserved during life. A closer look at the overall pattern of the profile shows that the length of the telomeres in general follows the total chromosome length. In addition to the common profile, it is found that each person has specific characteristics, which are also conserved throughout life. Studying both twins and families we have obtained indications that these individual characteristics are at least partly inherited. Altogether, our results suggest that the length of individual telomeres might occasionally play a role in the heritability of life span.

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.

The telomerase inhibitor telomestatin induces telomere shortening and cell death in Arabidopsis

The cellular response to telomere dysfunction in plants was investigated with the use of telomestatin, an inhibitor of human telomerase activity. Telomestatin bound to plant telomeric repeat sequence, and inhibited telomerase activity in suspension-cultured cells of Arabidopsis thaliana and Oryza sativa (rice) in a dose-dependent manner. The inhibitor did not affect transcript level of the TERT gene, which encodes the catalytic subunit of telomerase, in the plant cells. Inhibition of telomerase activity by telomestatin resulted in rapid shortening of telomeres and the induction of cell death by an apoptosis-like mechanism in Arabidopsis cells. These results suggest that telomerase contributes to the survival of proliferating plant cells by maintaining telomere length, and that telomere erosion triggers cell death.

Suppression of gross chromosomal rearrangements by the multiple functions of the Mre11-Rad50-Xrs2 complex in Saccharomyces cerevisiae

The Mre11-Rad50-Xrs2 complex in Saccharomyces cerevisiae has roles in the intra-S checkpoint, homologous recombination, non-homologous end joining, meiotic recombination, telomere maintenance and the suppression of gross chromosomal rearrangements (GCRs). The discovery of mutations in the genes encoding the human homologues of two MRX subunits that underlie the chromosome fragility syndromes, Ataxia telangiectasia-like disorder and Nijmegen breakage syndrome suggest that the MRX complex also functions in suppression of GCRs in human cells. Previously, we demonstrated that the deletion mutations in each of the MRX genes increased the rate of GCRs up to 1000-fold compared to wild-type rates. However, it has not been clear which molecular function of the MRX complex is important for suppression of GCRs. Here, we present evidence that at least three different activities of the MRX complex are important for suppression of GCRs. These include the nuclease activity of Mre11, an activity related to MRX complex formation and another activity that has a close link with the telomere maintenance function of the MRX complex. An activity related to MRX complex formation is especially important for the suppression of translocation type of GCRs. However, the non-homologous end joining function of MRX complex does not appear to participate in the suppression of GCRs.

Increased genome instability and telomere length in the elg1-deficient Saccharomyces cerevisiae mutant are regulated by S-phase checkpoints

Gross chromosomal rearrangements (GCRs) are frequently observed in cancer cells. Abnormalities in different DNA metabolism including DNA replication, cell cycle checkpoints, chromatin remodeling, telomere maintenance, and DNA recombination and repair cause GCRs in Saccharomyces cerevisiae. Recently, we used genome-wide screening to identify several genes the deletion of which increases GCRs in S. cerevisiae. Elg1, which was discovered during this screening, functions in DNA replication by participating in an alternative replication factor complex. Here we further characterize the GCR suppression mechanisms observed in the elg1Delta mutant strain in conjunction with the telomere maintenance role of Elg1. The elg1Delta mutation enhanced spontaneous DNA damage and resulted in GCR formation. However, DNA damage due to inactivation of Elg1 activates the intra-S checkpoints, which suppress further GCR formation. The intra-S checkpoints activated by the elg1Delta mutation also suppress GCR formation in strains defective in the DNA replication checkpoint. Lastly, the elg1Delta mutation increases telomere size independently of other previously known telomere maintenance proteins such as the telomerase inhibitor Pif1 or the telomere size regulator Rif1. The increase in telomere length caused by the elg1Delta mutation was suppressed by a defect in the DNA replication checkpoint, which suggests that DNA replication surveillance by Dpb11-Mec1/Tel1-Dun1 also has an important role in telomere length regulation.

Overexpression of Bcl-2 is associated with apoptotic resistance to the G-quadruplex ligand 12459 but is not sufficient to confer resistance to long-term senescence

The triazine derivative 12459 is a potent G-quadruplex interacting agent that inhibits telomerase activity. This agent induces time- and dose-dependent telomere shortening, senescence-like growth arrest and apoptosis in the human A549 tumour cell line. We show here that 12459 induces a delayed apoptosis that activates the mitochondrial pathway. A549 cell lines selected for resistance to 12459 and previously characterized for an altered hTERT expression also showed Bcl-2 overexpression. Transfection of Bcl-2 into A549 cells induced a resistance to the short-term apoptotic effect triggered by 12459, suggesting that Bcl-2 is an important determinant for the activity of 12459. In sharp contrast, the Bcl-2 overexpression was not sufficient to confer resistance to the senescence-like growth arrest induced by prolonged treatment with 12459. We also show that 12459 provokes a rapid degradation of the telomeric G-overhang in conditions that paralleled the apoptosis induction. In contrast, the G-overhang degradation was not observed when apoptosis was induced by camptothecin. Bcl-2 overexpression did not modify the G-overhang degradation, suggesting that this event is an early process uncoupled from the final apoptotic pathway.

Telomeres and telomerase: their mechanisms of action and the effects of altering their functions

The molecular features of telomeres and telomerase are conserved among most eukaryotes. How telomerase and telomeres function and how they interact to promote the chromosome-stabilizing properties of telomeres are discussed here.

POT1 and TRF2 cooperate to maintain telomeric integrity

Mammalian telomeric DNA contains duplex TTAGGG repeats and single-stranded overhangs. POT1 (protection of telomeres 1) is a telomere-specific single-stranded DNA-binding protein, highly conserved in eukaryotes. The biological function of human POT1 is not well understood. In the present study, we demonstrate that POT1 plays a key role in telomeric end protection. The reduction of POT1 by RNA interference led to the loss of telomeric single-stranded overhangs and induced apoptosis, chromosomal instability, and senescence in cells. POT1 and TRF2 interacted with each other to form a complex with telomeric DNA. A dominant negative TRF2, TRF2(DeltaBDeltaM), bound to POT1 and prevented it from binding to telomeres. POT1 overexpression protected against TRF2(DeltaBDeltaM)-induced loss of telomeric single-stranded overhangs, chromosomal instability, and senescence. These results demonstrate that POT1 and TRF2 share in part in the same pathway for telomere capping and suggest that POT1 binds to the telomeric single-stranded DNA in the D-loop and cooperates with TRF2 in t-loop maintenance.

The cellular level of telomere dysfunction determines induction of senescence or apoptosis in vivo

Telomere dysfunction induces two types of cellular response: cellular senescence and apoptosis. We analysed the extent to which the cellular level of telomere dysfunction and p53 gene status affect these cellular responses in mouse liver using the experimental system of TRF2 inhibition by a dominant-negative version of the protein (TRF2delta B delta M). We show that the level of telomere dysfunction correlates with the level of TRF2delta B delta M protein expression resulting in chromosomal fusions, aberrant mitotic figures and aneuploidy of liver cells. These alterations provoked p53-independent apoptosis, but a strictly p53-dependent senescence response in distinct populations of mouse liver cells depending on the cellular level of TRF2delta B delta M expression. Apoptosis was associated with higher expression of TRF2delta B delta M, whereas cellular senescence was associated with low levels of TRF2delta B delta M) expression. Our data provide experimental evidence that induction of senescence or apoptosis in vivo depends on the cellular level of telomere dysfunction and differentially on p53 gene function.

Telomeres and telomerase

Telomeres are the protective DNA-protein complexes found at the ends of eukaryotic chromosomes. Telomeric DNA consists of tandem repeats of a simple, often G-rich, sequence specified by the action of telomerase, and complete replication of telomeric DNA requires telomerase. Telomerase is a specialized cellular ribonucleoprotein reverse transcriptase. By copying a short template sequence within its intrinsic RNA moiety, telomerase synthesizes the telomeric DNA strand running 5' to 3' towards the distal end of the chromosome, thus extending it. Fusion of a telomere, either with another telomere or with a broken DNA end, generally constitutes a catastrophic event for genomic stability. Telomerase acts to prevent such fusions. The molecular consequences of telomere failure, and the molecular contributors to telomere function, with an emphasis on telomerase, are discussed here.

Natural and pharmacological regulation of telomerase

The extremities of eukaryotic chromosomes are called telomeres. They have a structure unlike the bulk of the chromosome, which allows the cell DNA repair machinery to distinguish them from 'broken' DNA ends. But these specialised structures present a problem when it comes to replicating the DNA. Indeed, telomeric DNA progressively erodes with each round of cell division in cells that do not express telomerase, a specialised reverse transcriptase necessary to fully duplicate the telomeric DNA. Telomerase is expressed in tumour cells but not in most somatic cells and thus telomeres and telomerase may be proposed as attractive targets for the discovery of new anticancer agents.

New ways not to make ends meet: telomerase, DNA damage proteins and heterochromatin

Telomeres are stabilized, and telomeric DNA is replenished, by the action of the ribonucleoprotein reverse transcriptase telomerase. Telomere capping functions include the ability of telomeres to protect chromosome ends from cellular DNA-damage responses such as cell cycle arrest or apoptosis. This property of telomeres is especially important for cancer cells, which continue proliferating despite chromosome aberrations. Telomere capping is influenced by multiple, mutually reinforcing factors including telomere length, although telomere length is only one of several determinants of telomere functionality. For example, many cancer cells express high levels of telomerase yet maintain relatively short telomeres. We consider three aspects of telomere capping that have emerged relatively recently: (1) a new role for telomerase in telomere capping independent of its function in telomere elongation. Support for this novel function comes from experiments showing an increase in replicative potential with the reactivation of telomerase, without net telomere lengthening; (2) the role at telomeres of DNA damage proteins. We propose a model in which two factors specifically target telomeres for the action of telomerase, as opposed to recombination or non-homologous end-joining: binding by telomeric proteins that limits DNA damage responses at telomeres, and the affinity of the telomerase RNP for telomeric proteins and DNA; and (3) we discuss a potential protective role of amplified subtelomeric DNAs, which may aid capping of telomeres maintained by non-telomerase based mechanisms through the formation of heterochromatin.