Activity of a novel G-quadruplex-interactive telomerase inhibitor, telomestatin (SOT-095), against human leukemia cells: involvement of ATM-dependent DNA damage response pathways

Disabling c-Myc in childhood medulloblastoma and atypical teratoid/rhabdoid tumor cells by the potent G-quadruplex interactive agent S2T1-6OTD

We investigated here the effects of S2T1-6OTD, a novel telomestatin derivative that is synthesized to target G-quadruplex-forming DNA sequences, on a representative panel of human medulloblastoma (MB) and atypical teratoid/rhabdoid (AT/RT) childhood brain cancer cell lines. S2T1-6OTD proved to be a potent c-Myc inhibitor through its high-affinity physical interaction with the G-quadruplex structure in the c-Myc promoter. Treatment with S2T1-6OTD reduced the mRNA and protein expressions of c-Myc and hTERT, which is transcriptionally regulated by c-Myc, and decreased the activities of both genes. In remarkable contrast to control cells, short-term (72-hour) treatment with S2T1-6OTD resulted in a dose- and time-dependent antiproliferative effect in all MB and AT/RT brain tumor cell lines tested (IC(50), 0.25-0.39 micromol/L). Under conditions where inhibition of both proliferation and c-Myc activity was observed, S2T1-6OTD treatment decreased the protein expression of the cell cycle activator cyclin-dependent kinase 2 and induced cell cycle arrest. Long-term treatment (5 weeks) with nontoxic concentrations of S2T1-6OTD resulted in a time-dependent (mainly c-Myc-dependent) telomere shortening. This was accompanied by cell growth arrest starting on day 28 followed by cell senescence and induction of apoptosis on day 35 in all of the five cell lines investigated. On in vivo animal testing, S2T1-6OTD may well represent a novel therapeutic strategy for childhood brain tumors.

G-quadruplexes: from guanine gels to chemotherapeutics

G-quartets are square planar arrangements of four guanine bases, which can form extraordinarily stable stacks when present in nucleic acid sequences. Such G-quadruplex structures were long regarded as an in vitro phenomenon, but the widespread presence of suitable sequences in genomes and the identification of proteins that stabilize, modify, or resolve these nucleic acid structures have provided circumstantial evidence for their physiological relevance. The therapeutic potential of small molecules that can stabilize or disrupt G-quadruplex structures has invigorated the field in recent years. Here we review some of the key observations that support biological functions for G-quadruplex DNA as well as the techniques and tools that have enabled researchers to probe these structures and their interactions with proteins and small molecules.

Formation of a unique end-to-end stacked pair of G-quadruplexes in the hTERT core promoter with implications for inhibition of telomerase by G-quadruplex-interactive ligands

The hTERT core promoter contains a G-rich region of 12 consecutive G-tracts, embracing 3 Sp1 binding sites, and has the potential to form multiple G-quadruplexes. From the 12 runs of guanines, 9 putative hTERT G-quadruplex-forming sequences were selected to assay for G-quadruplex formation and stability using circular dichroism and a Taq polymerase stop assay. Results from biophysical and chemical assays demonstrate an approximate inverse correlation between total loop size and structure stability. Investigation of the full-length hTERT G-rich sequence using a Taq polymerase stop assay and dimethyl sulfate footprinting revealed the formation of a unique end-to-end stacked G-quadruplex structure from this sequence. This structure consists of an all parallel G-quadruplex, formed by four consecutive G-tracts, linked to another, atypical G-quadruplex, formed by two pairs of consecutive G-tracts separated by a 26-base loop. This 26-base loop likely forms a stable hairpin structure, which would explain the unexpected stability of this G-quadruplex. Significantly, the formation of this tandem G-quadruplex structure in the full-length sequence masks all three Sp1 binding sites, which is predicted to produce significant inhibition of hTERT promoter activity. Furthermore, our study implies that inhibition of telomerase activity by some G-quadruplex ligands is not only produced by targeting telomeric G-quadruplexes but also by stabilization of the hTERT promoter G-quadruplexes.

Anticancer activity of CX-3543: a direct inhibitor of rRNA biogenesis

Hallmark deregulated signaling in cancer cells drives excessive ribosome biogenesis within the nucleolus, which elicits unbridled cell growth and proliferation. The rate-limiting step of ribosome biogenesis is synthesis of rRNA (building blocks of ribosomes) by RNA Polymerase I (Pol I). Numerous kinase pathways and products of proto-oncogenes can up-regulate Pol I, whereas tumor suppressor proteins can inhibit rRNA synthesis. In tumorigenesis, activating mutations in certain cancer-associated kinases and loss-of-function mutations in tumor suppressors lead to deregulated signaling that stimulates Pol I transcription with resultant increases in ribosome biogenesis, protein synthesis, cell growth, and proliferation. Certain anticancer therapeutics, such as cisplatin and 5-fluorouracil, reportedly exert, at least partially, their activity through disruption of ribosome biogenesis, yet many prime targets for anticancer drugs within the ribosome synthetic machinery of the nucleolus remain largely unexploited. Herein, we describe CX-3543, a small molecule nucleolus-targeting agent that selectively disrupts nucleolin/rDNA G-quadruplex complexes in the nucleolus, thereby inhibiting Pol I transcription and inducing apoptosis in cancer cells. CX-3543 is the first G-quadruplex interactive agent to enter human clinical trials, and it is currently under evaluation against carcinoid/neuroendocrine tumors in a phase II clinical trial.

Prediction of telomerase inhibitory activity for acridinic derivatives based on chemical structure

Telomerase is a reverse transcriptase enzyme that activates in more than 85% of cancer cells and it is associated with the acquisition of a malignant phenotype. Some experimental strategies have been suggested in order to avoid the enzyme effect on unstopped telomere elongation. One of them, the stabilization of the G-quartet structure, has been widely studied. Nevertheless, no QSAR studies to predict this activity have been developed. In the present study a classification model was carried out to identify, through molecular descriptors with structural fragments and groups information, those acridinic derivatives with better inhibitory concentration on telomerase enzyme. A linear discriminant model was developed to classify a data set of 90 acridinic derivatives (48 more potent derivatives with IC(50) < 1 microM and 42 less potent with IC(50) > or = 1 microM). The final model fit the data with sensitivity of 87.50% and specificity of 82.85%, for a final accuracy of 85.33%. The predictive ability of the model was assessed by a prediction set (15 compounds of 90% and 82.29% of prediction accuracy); a tenfold full cross-validation procedure (removing 15 compounds in each cycle, 84.80% of good prediction) and the prediction of inhibitory concentration on telomerase enzyme for external data of 10 novel acridines (90% of good prediction). The results of this study suggest that the established model has a strong predictive ability and can be prospectively used in the molecular design and action mechanism analysis of this kind of compounds with anticancer activity.

G-quadruplex nucleic acids as therapeutic targets

Nucleic acid sequences containing several short runs of guanine nucleotides can form complex higher order structures, termed quadruplexes. Their occurrence has been most extensively characterised at the telomeric ends of eukaryotic chromosomes, whose DNA comprises such sequences, and where the extreme 3' ends are single-stranded. This enables relatively facile formation of quadruplex arrangements under the influence of a quadruplex-selective small molecule to compete effectively with telomeric protein-DNA interactions. Occurrences of quadruplexes within the human and other genomes have been mapped by bioinformatics surveys, which have revealed over-representations in promoter regions, especially of genes involved in replication, such as oncogenes, as well as in 5'UTR regions. The highly distinctive nature of quadruplex topologies suggests that they can act as novel therapeutic targets, for example in the selective inhibition of transcription of a given oncogene, using designed small molecules to stabilise a particular quadruplex. This offers the prospect of an alternative to, for example, direct kinase targeting with small molecules, without the attendant issues of active-site resistance. We survey here the basis of these approaches, together with current progress, and discuss the mechanistic issues posed by quadruplex targeting.

Synthesis of a potent G-quadruplex-binding macrocyclic heptaoxazole

A novel G-quadruplex binder, L1H1-7OTD (shown in color by atom type), was developed. This macrocyclic heptaoxazole potently and selectively stabilizes telomeric DNA in an intramolecular antiparallel G-quadruplex conformation. L1H1-7OTD shows selective cytotoxicity toward HeLa cells, a telomerase-positive cell line.

Antitumor activity of TMPyP4 interacting G-quadruplex in retinoblastoma cell lines

To investigate the molecular mechanism of the antitumor activity of the cationic porphyrin 5, 10, 15, 20-tetra-(N-methyl-4-pyridyl)porphyrin (TMPyP4) in retinoblastoma cell lines, Y79 and WERI-Rb1 cells were treated with TMPyP4 for 0-72 h, after which growth inhibition, modulation of the cell cycle and the induction of apoptosis were examined. In addition, the effect of TMPyP4 on the susceptibility to irradiation was evaluated in Y79 and WERI-Rb1 cells. In vitro telomeric repeat amplification protocol assay showed TMPyP4 (10-100 microM) directly blocked telomerase elongation, suggesting that TMPyP4 can form stable guanine (G)-quadruplexes in extending telomere repeats in substrate oligonucleotides. The antiproliferative activities of TMPyP4 assessed with the MTS assay and expressed in terms of IC(50): Y79 cells, 60 microM; WERI-Rb1 cells, 45 microM. Treatment with TMPyP4 at doses of 10, 20, 50 or 100 microM for 48 or 72 h significantly inhibited the growth of Y79 and WERI-Rb1 cells. Apoptosis, as assessed with CaspACE FITC-VAD-FMK, was induced by TMPyP4 in a dose-dependent manner. Induction of apoptosis by TMPyP4 was associated with increased expression of phosphorylated DNA damage response factor H2AX (Ser139), phosphorylated p53 (Ser46) protein and activation of mitogen-activated protein kinases in Y79 and WERI-Rb1 cells. Moreover, TMPyP4 significantly enhanced the susceptibility to irradiation in both cell lines. This study provides insight into the molecular mechanism of the antitumor effects of TMPyP4. G-quadruplex structure may be a potential therapeutic target in retinoblastoma.

Telomerase regulation in hematological cancers: a matter of stemness?

Human telomerase is a nuclear ribonucleoprotein enzyme complex that catalyzes the synthesis and extension of telomeric DNA. This enzyme is highly expressed and active in most malignant tumors while it is usually not or transiently detectable in normal somatic cells, suggesting that it plays an important role in cellular immortalization and tumorigenesis. As most leukemic cells are generally telomerase-positive and have often shortened telomeres, our understanding of how telomerase is deregulated in these diseases could help to define novel therapies targeting the telomere/telomerase complex. Nonetheless, considering that normal hematopoietic stem cells and some of their progeny do express a functional telomerase, it is tempting to consider such an activity in leukemias as a sustained stemness feature and important to understand how telomere length and telomerase activity are regulated in the various forms of leukemias.

Targeting the telosome: therapeutic implications

Since telomere integrity is required to guarantee the unlimited replicative potential of cancer cells, telomerase, the enzyme responsible for telomere length maintenance in most human tumors, and lately also telomeres themselves have become extremely attractive targets for new anticancer interventions. At the current status of knowledge, it is still not possible to define the best therapeutic target between telomerase and telomeres. It is noteworthy that interfering with telomeres, through direct targeting of telomeric DNA or proteins involved in the telosome complex, could negatively affect the proliferative potential not only of tumors expressing telomerase activity but also of those that maintain their telomeres through alternative lengthening or still unknown mechanisms. This review presents the different therapeutic approaches proposed thus far and developed in preclinical tumor models and discusses the perspectives for their use in the clinical setting.

Abnormal growth factor/cytokine network in gastric cancer

Gastric cancer cells express a broad spectrum of the growth factor/cytokine receptor systems that organize the complex interaction between cancer cells and stromal cells in tumor microenvironment, which confers cell growth, apoptosis, morphogenesis, angiogenesis, progression and metastasis. However, these abnormal growth factor/cytokine networks differ in the two histological types of gastric cancer. Importantly, activation of nuclear factor-kB pathway by Helicobacter pylori infection may act as a key player for induction of growth factor/cytokine networks in gastritis and pathogenesis of gastric cancer. Better understanding of these events will no doubt provide new approaches for biomarkers of diagnosis and effective therapeutic targeting of gastric cancer.

Strategies targeting telomerase inhibition

Telomerase plays a pivotal role in cellular immortality and tumorigenesis. Its activity is normally not detectable in most somatic cells while it is reactivated in the vast majority of cancer cells. Therefore, inhibition of telomerase has been viewed as a promising anticancer approach due to its specificity for cancer cells. Studies so far have shown that telomerase inhibition can inhibit the proliferation of cancer cells or cause apoptosis while it has no effect on most normal cells. Strategies currently being applied to induce telomerase inhibition target virtually all of the major components of the ribonucleoprotein holoenzyme and related cell signal pathways that regulate its activity. These strategies include inhibition of telomerase through targeting at the telomerase reverse transcriptase (TERT) catalytic subunit, the telomerase RNA (TR) component, and associated proteins. Other strategies have been developed to target the proteins associated with telomerase at the telomeric ends of chromosomes such as tankyrase. The specific mechanisms that mediate those inhibition effects include small molecules, antisense RNA, and ribozymes. Although the beneficial evidence of telomerase inhibition is obvious, limitations of strategies remain to be resolved to increase the feasibility of clinical application. This analysis will summarize recent developments of strategies in telomerase inhibition.

Telomere maintenance in laser capture microdissection-purified Barrett's adenocarcinoma cells and effect of telomerase inhibition in vivo

PURPOSE

The aims of this study were to investigate telomere function in normal and Barrett's esophageal adenocarcinoma (BEAC) cells purified by laser capture microdissection and to evaluate the effect of telomerase inhibition in cancer cells in vitro and in vivo.

EXPERIMENTAL DESIGN

Epithelial cells were purified from surgically resected esophagi. Telomerase activity was measured by modified telomeric repeat amplification protocol and telomere length was determined by real-time PCR assay. To evaluate the effect of telomerase inhibition, adenocarcinoma cell lines were continuously treated with a specific telomerase inhibitor (GRN163L) and live cell number was determined weekly. Apoptosis was evaluated by Annexin labeling and senescence by beta-galactosidase staining. For in vivo studies, severe combined immunodeficient mice were s.c. inoculated with adenocarcinoma cells and following appearance of palpable tumors, injected i.p. with saline or GRN163L.

RESULTS

Telomerase activity was significantly elevated whereas telomeres were shorter in BEAC cells relative to normal esophageal epithelial cells. The treatment of adenocarcinoma cells with telomerase inhibitor, GRN163L, led to loss of telomerase activity, reduction in telomere length, and growth arrest through induction of both the senescence and apoptosis. GRN163L-induced cell death could also be expedited by addition of the chemotherapeutic agents doxorubicin and ritonavir. Finally, the treatment with GRN163L led to a significant reduction in tumor volume in a subcutaneous tumor model.

CONCLUSIONS

We show that telomerase activity is significantly elevated whereas telomeres are shorter in BEAC and suppression of telomerase inhibits proliferation of adenocarcinoma cells both in vitro and in vivo.

G-quadruplexes: targets in anticancer drug design

G-quadruplexes are special secondary structures adopted in some guanine-rich DNA sequences. As guanine-rich sequences are present in important regions of the eukaryotic genome, such as telomeres and the regulatory regions of many genes, such structures may play important roles in the regulation of biological events in the body. G-quadruplexes have become valid targets for new anticancer drugs in the past few decades. Many leading compounds that target these structures have been reported, and a few of them have entered preclinical or clinical trials. Nonetheless, the selectivity of this kind of antitumor compound has yet to be improved in order to suppress the side effects caused by nonselective binding. As drug design targets, the topology and structural characteristics of quadruplexes, their possible biological roles, and the modes and sites of small-ligand binding to these structures should be understood clearly. Herein we provide a summary of published research that has set out to address the above problem to provide useful information on the design of small ligands that target G-quadruplexes. This review also covers research methodologies that have been developed to study the binding of ligands to G-quadruplexes.

Activity of a novel Aurora kinase inhibitor against the T315I mutant form of BCR-ABL: in vitro and in vivo studies

Despite promising results from clinical studies of ABL kinase inhibitors, a challenging problem that remains is the T315I mutation against which neither nilotinib nor dasatinib show significant activity. In the present study, we investigated the activity of a novel Aurora kinase inhibitor, VE-465, against leukemia cells expressing wild-type BCR-ABL or the T315I mutant form of BCR-ABL. We observed a dose-dependent reduction in the level of BCR-ABL autophosphorylation in VE-465-treated cells. Exposure to the combination of VE-465 and imatinib exerted an enhanced apoptotic effect in K562 cells. Combined treatment with VE-465 and imatinib caused more attenuation of the levels of phospho-AKT and c-Myc in K562 cells. Further, the isobologram indicated the synergistic effect of simultaneous exposure to VE-465 and imatinib in K562 cells. To assess the in vivo efficacy of VE-465, athymic nude mice were injected intravenously with BaF3 cells expressing wild-type BCR-ABL or the T315I mutant form. The vehicle-treated mice died of a condition resembling acute leukemia by 28 days; however, nearly all mice treated with VE-465 (75 mg/kg, twice daily; intraperitoneally for 14 days) survived for more than 56 days. Histopathological analysis of vehicle-treated mice revealed infiltration of the spleen. In contrast, histopathological analysis of organs from VE-465-treated mice demonstrated normal tissue architecture. Taken together, the present study shows that VE-465 exhibits a desirable therapeutic index that can reduce the in vivo growth of T315I mutant form and wild-type BCR-ABL-expressing cells in an efficacious manner.

Telomerase inhibitor GRN163L inhibits myeloma cell growth in vitro and in vivo

Human telomerase, the reverse transcriptase which extends the life span of a cell by adding telomeric repeats to chromosome ends, is expressed in most cancer cells but not in the majority of normal somatic cells. Inhibition of telomerase therefore holds great promise as anticancer therapy. We have synthesized a novel telomerase inhibitor GRN163L, a lipid-attached phosphoramidate oligonucleotide complementary to template region of the RNA subunit of telomerase. Here, we report that GRN163L is efficiently taken up by human myeloma cells without any need of transfection and is resistant to nucleolytic degradation. The exposure of myeloma cells to GRN163L led to an effective inhibition of telomerase activity, reduction of telomere length and apoptotic cell death after a lag period of 2-3 weeks. Mismatch control oligonucleotides had no effect on growth of myeloma cells. The in vivo efficacy of GRN163L was confirmed in two murine models of human multiple myeloma. In three independent experiments, significant reduction in tumor cell growth and better survival than control mice was observed. Furthermore, GRN163L-induced myeloma cell death could be significantly enhanced by Hsp90 inhibitor 17AAG. These data provide the preclinical rationale for clinical evaluation of GRN163L in myeloma and in combination with 17AAG.

Targeting human telomeric G-quadruplex DNA with oxazole-containing macrocyclic compounds

Oxazole-containing macrocycles, which include the natural product telomestatin, represent a promising class of anticancer agents that target G-quadruplex DNA. Two synthetic hexaoxazole-containing macrocyclic compounds (HXDV and HXLV-AC) have been characterized with regard to their cytotoxic activities versus human cancer cells, as well as the mode, thermodynamics, and specificity with which they bind to the intramolecular (3+1) G-quadruplex structural motif formed in the presence of K+ ions by human telomeric DNA. Both compounds exhibit cytotoxic activities versus human lymphoblast (RPMI 8402) and oral carcinoma (KB3-1) cells, with associated IC50 values ranging from 0.4 to 0.9microM. The compounds bind solely to the quadruplex nucleic acid form, but not to the duplex or triplex form. Binding to the quadruplex is associated with a stoichiometry of two ligand molecules per DNA molecule, with one ligand molecule binding to each end of the host quadruplex via a nonintercalative "terminal capping" mode of interaction. For both compounds, quadruplex binding is primarily entropy driven, while also being associated with a negative change in heat capacity. These thermodynamic properties reflect contributions from favorable ligand-induced alterations in the loop configurational entropies of the quadruplex, but not from changes in net hydration. The stoichiometry and mode of binding revealed by our studies have profound implications with regard to the number of ligand molecules that can potentially bind the 3-overhang region of human telomeric DNA.

Antitumor activity of G-quadruplex-interactive agent TMPyP4 in K562 leukemic cells

The cationic porphyrin TMPyP4 can bind to and stabilize DNA guanine-quadruplexes. We investigated the molecular mechanism of the antitumor activity of TMPyP4 in K562 cells and human telomere reverse transcriptase subunit (hTERT)-transfected K562 cells in which telomerase activity, followed by telomere elongation, was enhanced. Treatment with 100 microM TMPyP4 significantly inhibited the growth of both types of cell, with decreases of cells in the G(1) phase and increases of those in the S and G(2)/M phases after 48 h, preceding cell death after 72 h. cDNA microarray analysis revealed upregulation of 33 genes and downregulation of 54 genes in K562 cells treated with 100 microM TMPyP4 for 48 h. Moreover, TMPyP4 decreased c-Myc protein expression, increased the expression of p21(CIP1) and p57(KIP2) proteins, and activated p38 mitogen-activated protein kinase, c-Jun N-terminal kinase, and extracellular signal-regulated kinase. These findings may provide a rationale for the development of guanine-quadruplex-interactive agents as novel antileukemic therapies.

Human telomere, oncogenic promoter and 5'-UTR G-quadruplexes: diverse higher order DNA and RNA targets for cancer therapeutics

Guanine-rich DNA sequences can form G-quadruplexes stabilized by stacked G–G–G–G tetrads in monovalent cation-containing solution. The length and number of individual G-tracts and the length and sequence context of linker residues define the diverse topologies adopted by G-quadruplexes. The review highlights recent solution NMR-based G-quadruplex structures formed by the four-repeat human telomere in K⁺ solution and the guanine-rich strands of c-myc, c-kit and variant bcl-2 oncogenic promoters, as well as a bimolecular G-quadruplex that targets HIV-1 integrase. Such structure determinations have helped to identify unanticipated scaffolds such as interlocked G-quadruplexes, as well as novel topologies represented by double-chain-reversal and V-shaped loops, triads, mixed tetrads, adenine-mediated pentads and hexads and snap-back G-tetrad alignments. The review also highlights the recent identification of guanine-rich sequences positioned adjacent to translation start sites in 5′-untranslated regions (5′-UTRs) of RNA oncogenic sequences. The activity of the enzyme telomerase, which maintains telomere length, can be negatively regulated through G-quadruplex formation at telomeric ends. The review evaluates progress related to ongoing efforts to identify small molecule drugs that bind and stabilize distinct G-quadruplex scaffolds associated with telomeric and oncogenic sequences, and outlines progress towards identifying recognition principles based on several X-ray-based structures of ligand–G-quadruplex complexes.

Short telomeres and high telomerase activity in T-cell prolymphocytic leukemia

To test the role of telomere biology in T-cell prolymphocytic leukemia (T-PLL), a rare aggressive disease characterized by the expansion of a T-cell clone derived from immuno-competent post-thymic T-lymphocytes, we analyzed telomere length and telomerase activity in subsets of peripheral blood leukocytes from 11 newly diagnosed or relapsed patients with sporadic T-PLL. Telomere length values of the leukemic T cells (mean+/-s.d.: 1.53+/-0.65 kb) were all below the 1st percentile of telomere length values observed in T cells from healthy age-matched controls whereas telomere length of normal T- and B cells fell between the 1st and 99th percentile of the normal distribution. Leukemic T cells exhibited high levels of telomerase and were sensitive to the telomerase inhibitor BIBR1532 at doses that showed no effect on normal, unstimulated T cells. Targeting the short telomeres and telomerase activity in T-PLL seems an attractive strategy for the future treatment of this devastating disease.

Mechanism of acridine-based telomerase inhibition and telomere shortening

The trisubstituted acridine compound BRACO-19 has been developed as a ligand for stabilising G-quadruplex structures. It is shown here that BRACO-19 produces short- and long-term growth arrest in cancer cell lines, and is significantly less potent in a normal cell line. BRACO-19 reduces telomerase activity and long-term telomere length attrition is observed. It is also shown that BRACO-19 binds to telomeric single-stranded overhang DNA, consistent with quadruplex formation, and the single-stranded protein hPOT1 has been shown to be displaced from the overhang in vitro and in cellular experiments. It is concluded that the cellular activity of BRACO-19 can be ascribed both to the uncapping of 3' telomere ends and to telomere shortening that may preferentially affect cells with short telomeres.

Targeting telomeres and telomerase

Telomeres and telomerase represent, at least in theory, an extremely attractive target for cancer therapy. The objective of this review is to present the latest view on the mechanism(s) of action of telomerase inhibitors, with an emphasis on a specific class of telomere ligands called G-quadruplex ligands, and to discuss their potential use in oncology.

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.

Telomere protection mechanisms change during neurogenesis and neuronal maturation: newly generated neurons are hypersensitive to telomere and DNA damage

Telomeres are DNA-protein complexes at the ends of eukaryotic chromosomes that play an important role in maintaining the integrity of the genome. In proliferative stem cells and cancer cells, telomere length is maintained by telomerase, and telomere structure and functions are regulated by telomere-associated proteins. We find that telomerase levels are high in embryonic cortical neural progenitor cells (NPCs) and low in newly generated neurons (NGNs) and mature neurons (MNs). In contrast, telomere repeat-binding factor 2 (TRF2) expression is undetectable in early brain development in vivo and in cultured NPCs and is expressed at progressively higher levels as NPCs cease proliferation and differentiate into postmitotic neurons. The telomere-disrupting agent telomestatin induces a DNA damage response and apoptosis in NGNs (which have low levels of TRF2 and telomerase), whereas NPCs (which have high levels of telomerase) and MNs (which have high levels of TRF2) are resistant to telomere damage. Overexpression of TRF2 in NGNs protects them against death induced by telomestatin and other DNA-damaging agents. Knockdown of TRF2 expression in MNs and knock-out of telomerase reverse transcriptase in NPCs increased their sensitivity to telomere- and DNA-damaging agents but did not affect the vulnerability of NGNs. These findings suggest that TRF2 and telomerase function as distinct telomere protection mechanisms during the processes of neurogenesis and neuronal maturation and that hypersensitivity of NGNs to telomere damage results from relative deficiencies of both telomerase and TRF2.

Defining the mode, energetics and specificity with which a macrocyclic hexaoxazole binds to human telomeric G-quadruplex DNA

Oxazole-containing macrocycles represent a promising class of anticancer agents that target G-quadruplex DNA. We report the results of spectroscopic studies aimed at defining the mode, energetics and specificity with which a hexaoxazole-containing macrocycle (HXDV) binds to the intramolecular quadruplex formed by the human telomeric DNA model oligonucleotide d(T2AG3)4 in the presence of potassium ions. HXDV binds solely to the quadruplex nucleic acid form, but not to the duplex or triplex form. HXDV binds d(T2AG3)4 with a stoichiometry of two drug molecules per quadruplex, with these binding reactions being coupled to the destacking of adenine residues from the terminal G-tetrads. HXDV binding to d(T2AG3)4 does not alter the length of the quadruplex. These collective observations are indicative of a nonintercalative 'terminal capping' mode of interaction in which one HXDV molecule binds to each end of the quadruplex. The binding of HXDV to d(T2AG3)4 is entropy driven, with this entropic driving force reflecting contributions from favorable drug-induced alterations in the configurational entropy of the host quadruplex as well as in net hydration. The 'terminal capping' mode of binding revealed by our studies may prove to be a general feature of the interactions between oxazole-containing macrocyclic ligands (including telomestatin) and intramolecular DNA quadruplexes.

Telomeres rather than telomerase a key target for anti-cancer therapy?

It was in the 1930s that telomeres (from the Greek telos = end and meros = part) were first recognized as essential structures at the ends of the chromosomes and were shown to be important for chromosomal stability (Muller HJ: The remaking of chromosomes. The Collecting Net-Woods Hole 1938: 13: 181-198, McClintock B, The stability of broken ends of chromosomes in Zea mays. Genetics 1041: 26: 234-282). However, it was only in 1978 that the first telomeric sequence was identified -- in the protocoa Tetrahymena, a single cell organism that at a certain stage of development has many identical minichromosomes with twice as many telomeres (Blackburn EH and Gall JG. A tandemly repeated sequence at the termini of the extrachromosomal ribosomal RNA genes in Tetrahymena. J. Mol. Biol. 1978: 120: 33-53.). Today we know that telomeres form specialized, three-dimensional DNA-protein structures and fulfil important capping functions. Besides, telomeric DNA is essential as ''access DNA'' for those cells that are not able to counteract loss of DNA during replication because they do not express telomerase, the enzyme responsible for telomere length maintenance. Since telomerase is mostly found in tumor cells and inhibition correlates with telomere shortening and finally growth inhibition, telomerase and lately also the telomeres themselves have become attractive targets for anti-cancer therapy. This review aims to critically throw light on different therapeutical approaches and comes to the conclusion that telomeres may be the better targets for cancer therapeutics.

Physiological relevance of telomeric G-quadruplex formation: a potential drug target

The concept of a G-quartet, a unique structural arrangement intrinsic to guanine-rich DNA, was first introduced by Gellert and colleagues over 40 years ago. For decades, it has been uncertain whether the G-quartet and the structure that it gives rise to, the G-quadruplex, are purely in vitro phenomena. Nevertheless, the presence of signature G-rich motifs in the eukaryotic genome, and the plethora of proteins that bind to, modify or resolve this nucleic acid structure in vitro have provided circumstantial evidence for its physiological relevance. More recently, direct visualisation of G-quadruplex DNA at native telomeres was achieved, bolstering the evidence for its existence in the cell. Furthermore, G-quadruplex folded telomeric DNA has been found to perturb telomere function and to impede the action of telomerase, an enzyme overexpressed in >85% of human cancers, hence opening up a novel avenue for cancer therapy in the form of G-quadruplex stabilising agents.

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.

Telomerase inhibition with a novel G-quadruplex-interactive agent, telomestatin: in vitro and in vivo studies in acute leukemia

The telomerase complex is responsible for telomere maintenance and represents a promising neoplasia therapeutic target. Recently, we have demonstrated that treatment with a G-quadruplex-interactive agent, telomestatin reproducibly inhibited telomerase activity in the BCR-ABL-positive leukemic cell lines. In the present study, we investigated the mechanisms of apoptosis induced by telomerase inhibition in acute leukemia. We have found the activation of caspase-3 and poly-(ADP-ribose) polymerase in telomestatin-treated U937 cells (PD20) and dominant-negative DN-hTERT-expressing U937 cells (PD25). Activation of p38 mitogen-activated protein (MAP) kinase and MKK3/6 was also found in telomestatin-treated U937 cells (PD20) and dominant-negative DN-hTERT-expressing U937 cells (PD25); however, activation of JNK and ASK1 was not detected in these cells. To examine the effect of p38 MAP kinase inhibition on growth properties and apoptosis in telomerase-inhibited cells, we cultured DN-hTERT-expressing U937 cells with or without SB203580. Dominant-negative-hTERT-expressing U937 cells stopped proliferation on PD25; however, a significant increase in growth rate was observed in the presence of SB203580. Treatment of SB203580 also reduced the induction of apoptosis in DN-hTERT-expressing U937 cells (PD25). These results suggest that p38 MAP kinase has a critical role for the induction of apoptosis in telomerase-inhibited leukemia cells. Further, we evaluated the effect of telomestatin on the growth of U937 cells in xenograft mouse model. Systemic intraperitoneal administration of telomestatin in U937 xenografts decreased tumor telomerase levels and reduced tumor volumes. Tumor tissue from telomestatin-treated animals exhibited marked apoptosis. None of the mice treated with telomestatin displayed any signs of toxicity. Taken together, these results lay the foundations for a program of drug development to achieve the dual aims of efficacy and selectivity in vivo.

8-chloroadenosine induced HL-60 cell growth inhibition, differentiation, and G(0)/G(1) arrest involves attenuated cyclin D1 and telomerase and up-regulated p21(WAF1/CIP1)

8-Chloroadenosine, an active dephosphorylated metabolite of the antineoplastic agent 8-chloroadenosine 3',5'-monophosphate (8-Cl-cAMP), induces growth inhibition in multiple carcinomas. Here we report that 8-chloroadenosine inhibits growth in human promyelocytic leukemia HL-60 cells by a G(0)/G(1) phase arrest and terminates cell differentiation along the granulocytic lineage. The mechanism of 8-chloroadenosine-induced G(0)/G(1) arrest is independent of apoptosis. The expressions of cyclin D1 and c-myc in HL-60 are suppressed by 8-chloroadenosine, whereas the cyclin-dependent kinases inhibitor p21(WAF1/CIP1) is up-regulated. 8-Chloroadenosine has less effect on the expressions of cyclin-dependent kinase (cdk)2 and cdk4, G(1) phase cyclin-dependent kinases, and only moderately induces the expression of transforming growth factor beta1 (TGFbeta1) and the mitotic inhibitor p27(KIP1). Telomerase activity is reduced in extracts of 8-chloroadenosine treated HL-60 cells, but 8-chloroadenosine does not directly inhibit the catalytic activity of telomerase in vitro. Therefore, anti-proliferation of HL-60 cells by 8-chloroadenosine involves coordination of cyclin D1 suppression, reduction of telomerase activity, and up-regulation of p21(WAF1/CIP1) that arrest cell-cycle progression at G(0)/G(1) phase and terminate cell differentiation.

The importance of the telomere and telomerase system in hematological malignancies

Telomeres are specialized chromosomal end structures composed of repeat TTAGGG sequences in humans. They shorten with each cell division and thus serve as the "mitotic clock" of the cell. One of their main functions is the maintenance of chromosomal integrity and their excessive shortening is associated with DNA instability. Telomerase, a unique reverse transcriptase, is inactive in most somatic human cells and is up-regulated in most cancer cells. Recently, the biology of the telomere/telomerase system has attracted much attention because of its possible role in carcinogenesis and aging. In this article we review the biology of this system and its relevance to normal and malignant hematopoietic cells. The biological, diagnostic and prognostic value of telomere/telomerase biology is discussed, as well as its potential future applications in cancer therapeutics.

Senescence and telomere shortening induced by novel potent G-quadruplex interactive agents, quindoline derivatives, in human cancer cell lines

Agents stabilizing G-quadruplexes have the potential to interfere with telomere replication by blocking the elongation step catalysed by telomerase or telomerase-independent mechanism and could therefore act as antitumor agents. In this study, we found that quindoline derivatives interacted preferentially with intramolecular G-quadruplex structures and were novel potent telomerase inhibitors. Treatment with quindoline derivatives reproducibly inhibited telomerase activity in human leukemia K562 cells and colon cancer SW620 cells. N'-(10H-Indolo [3,2-b] quinolin-11-yl)-N, N-dimethyl-propane-1,3-diamine (SYUIQ-5), (one of quindoline derivatives), when added to K562 and SW620 cell culture at nonacute cytotoxic concentrations, increased time of population doublings of K562 and SW620 cells, induced a marked cessation in cell growth and cellular senescence phenotype after 35 and 18 days, respectively. Growth cessation was accompanied by a shortening of telomere length, and induction of p16, p21 and p27 protein expression. However, another compound SYUIQ-7 with greater IC(50) for telomerase had no obvious cellular effect in nonacute cytotoxic concentrations. These results indicate that quindoline derivatives as novel potent G-quadruplex interactive agents induce senescence and telomere shortening in cancer cells and therefore are promising agents for cancer treatment.

G-Quadruplex stabilization by telomestatin induces TRF2 protein dissociation from telomeres and anaphase bridge formation accompanied by loss of the 3′ telomeric overhang in cancer cells

Inhibition of telomerase activity by telomerase inhibitors induces a gradual loss of telomeres, and this in turn causes cancer cells to enter to a crisis stage. Here, we report the telomerase inhibitor telomestatin, which is known to stabilize G-quadruplex structures at 3' single-stranded telomeric overhangs (G-tails), rapidly dissociates TRF2 from telomeres in cancer cells within a week, when given at a concentration that does not cause normal cells to die. The G-tails were dramatically reduced upon short-term treatment with the drug in cancer cell lines, but not in normal fibroblasts and epithelial cells. In addition, telomestatin also induced anaphase bridge formation in cancer cell lines. These effects of telomestatin were similar to those of dominant negative TRF2, which also causes a prompt loss of the telomeric G-tails and induces an anaphase bridge. These results indicate that telomestatin exerts its anticancer effect not only through inhibiting telomere elongation, but also by rapidly disrupting the capping function at the very ends of telomeres. Unlike conventional telomerase inhibitors that require long-term treatments, the G-quadruplex stabilizer telomestatin induced prompt cell death, and it was selectively effective in cancer cells. This study also identifies the TRF2 protein as a therapeutic target for treating many types of cancer which have the TRF2 protein at caps of the telomere DNA of each chromosome.

Telomerase inhibition, telomere shortening, cell growth suppression and induction of apoptosis by telomestatin in childhood neuroblastoma cells

Neuroblastoma is a tumour derived from primitive cells of the sympathetic nervous system and is the most common extracranial solid tumour in childhood. Unfavourable tumours are characterised not only by structural changes, including 1p deletion and amplification of the MYCN proto-oncogene, but also by high telomerase activity. Telomeric G-rich single-stranded DNA can adopt in vitro an intramolecular quadruplex structure, which has been shown to inhibit telomerase activity. In this study, we examined telomestatin, a G-quadruplex interactive agent, for its ability to inhibit telomere maintenance of neuroblastoma cells. Telomere length was determined by the terminal restriction fragment method, telomerase activity was measured by a quantitative telomeric repeat amplification protocol, and the expression of human telomerase by quantitative real-time polymerase chain reaction (RT-PCR). Short-term treatment with telomestatin resulted in dose-dependent cytotoxicity and induction of apoptosis. Long-term treatment with telomestatin at non-cytotoxic, but still telomerase activity-inhibiting, concentrations resulted in telomere shortening, growth arrest and induction of apoptosis. These results suggest that the effect of telomestatin is dose-dependent and at least 2-fold. Prolonged low-dose treatment with telomestatin limits the cellular lifespan of NB cells through disruption of telomere maintenance.

Ascididemin and meridine stabilise G-quadruplexes and inhibit telomerase in vitro

Ascididemin and Meridine are two marine compounds with pyridoacridine skeletons known to exhibit interesting antitumour activities. These molecules have been reported to behave like DNA intercalators. In this study, dialysis competition assay and mass spectrometry experiments were used to determine the affinity of ascididemin and meridine for DNA structures among duplexes, triplexes, quadruplexes and single-strands. Our data confirm that ascididemin and meridine interact with DNA but also recognize triplex and quadruplex structures. These molecules exhibit a significant preference for quadruplexes over duplexes or single-strands. Meridine is a stronger quadruplex ligand and therefore a stronger telomerase inhibitor than ascididemin (IC50=11 and >80 muM, respectively in a standard TRAP assay).

Preferential binding of a G-quadruplex ligand to human chromosome ends

The G-overhangs of telomeres are thought to adopt particular conformations, such as T-loops or G-quadruplexes. It has been suggested that G-quadruplex structures could be stabilized by specific ligands in a new approach to cancer treatment consisting in inhibition of telomerase, an enzyme involved in telomere maintenance and cell immortality. Although the formation of G-quadruplexes was demonstrated in vitro many years ago, it has not been definitively demonstrated in living human cells. We therefore investigated the chromosomal binding of a tritiated G-quadruplex ligand, 3H-360A (2,6-N,N'-methyl-quinolinio-3-yl)-pyridine dicarboxamide [methyl-3H]. We verified the in vitro selectivity of 3H-360A for G-quadruplex structures by equilibrium dialysis. We then showed by binding experiments with human genomic DNA that 3H-360A has a very potent selectivity toward G-quadruplex structures of the telomeric 3'-overhang. Finally, we performed autoradiography of metaphase spreads from cells cultured with 3H-360A. We found that 3H-360A was preferentially bound to chromosome terminal regions of both human normal (peripheral blood lymphocytes) and tumor cells (T98G and CEM1301). In conclusion, our results provide evidence that a specific G-quadruplex ligand interacts with the terminal ends of human chromosomes. They support the hypothesis that G-quadruplex ligands induce and/or stabilize G-quadruplex structures at telomeres of human cells.

IL-2 increases human telomerase reverse transcriptase activity transcriptionally and posttranslationally through phosphatidylinositol 3'-kinase/Akt, heat shock protein 90, and mammalian target of rapamycin in transformed NK cells

Human telomerase activity is induced by Ag receptor ligation in T and B cells. However, it is unknown whether telomerase activity is increased in association with activation and proliferation of NK cells. We found that telomerase activity in a human NK cell line (NK-92), which requires IL-2 for proliferation, was increased within 24 h after stimulation with IL-2. Levels of human telomerase reverse transcriptase (hTERT) mRNA and protein correlated with telomerase activity. ERK1/2 and Akt kinase (Akt) were activated by IL-2 stimulation. LY294002, an inhibitor of PI3K, abolished expression of hTERT mRNA and protein expression and abolished hTERT activity, whereas PD98059, which inhibits MEK1/2 and thus ERK1/2, had no effect. In addition, radicicol, an inhibitor of heat shock protein 90 (Hsp90), and rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), blocked IL-2-induced hTERT activity and nuclear translocation of hTERT but not hTERT mRNA expression. hTERT was coimmunoprecipitated with Akt, Hsp90, mTOR, and p70 S6 kinase (S6K), suggesting that these molecules form a physical complex. Immunoprecipitates of Akt, Hsp90, mTOR, and S6K from IL-2-stimulated NK-92 cells contained telomerase activity. Furthermore, the findings that Hsp90 and mTOR immunoprecipitates from primary samples contained telomerase activity are consistent with the results from NK-92 cells. These results indicate that IL-2 stimulation induces hTERT activation and that the mechanism of IL-2-induced hTERT activation involves transcriptional or posttranslational regulation through the pathway including PI3K/Akt, Hsp90, mTOR, and S6K in NK cells.

Telomerase: a potential therapeutic target for cancer

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

Recent developments in cancer chemotherapy oriented towards new targets

Malignant diseases are one of the major causes of death in the western world. Patients are treated by surgery, radiation and chemotherapy. Chemotherapeutic treatment is used to decrease the tumour burden and to eliminate malignant cells. However, in most cases, resistance against chemotherapy develops. Therefore, there is a permanent need for new additional treatment strategies and chemotherapeutic combination regimens. In the present review article, the authors try to highlight the most promising approaches and summarise a selection of potential targets and compounds which might become alternative treatment options against malignant diseases. Due to the high number of scientific articles and the rapid developments in the area of cancer research, the authors can only mention a few selected targets and treatment options; however, the review focuses on new and notably important targets and compounds.

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.

d(G3T4G4) forms unusual dimeric G-quadruplex structure with the same general fold in the presence of K+, Na+ or NH4+ ions

We have recently communicated that DNA oligonucleotide d(G(3)T(4)G(4)) forms a dimeric G-quadruplex in the presence of K(+) ions [J. Am. Chem. Soc.2003, 125, 7866-7871]. The high-resolution NMR structure of d(G(3)T(4)G(4))(2) G-quadruplex exhibits G-quadruplex core consisting of three stacked G-quartets. The two overhanging G3 and G11 residues are located at the opposite sides of the end G-quartets and are not involved in G-quartet formation. d(G(3)T(4)G(4))(2) G-quadruplex represents the first bimolecular G-quadruplex where end G-quartets are spanned by diagonal (T4-T7) as well as edge-type loops (T15-T18). Three of the G-rich strands are parallel while one is anti-parallel. The G12-G22 strand demonstrates a sharp reversal in strand direction between residues G19 and G20 that is accommodated with the leap over the middle G-quartet. The reversal in strand direction is achieved without any extra intervening residues. Here we furthermore examined the influence of different monovalent cations on the folding of d(G(3)T(4)G(4)). The resolved imino and aromatic proton resonances as well as (sequential) NOE connectivity patterns showed only minor differences in key intra- and interquartet NOE intensities in the presence of K(+), Na(+) and NH(4)(+) ions, which were consistent with subtle structural differences while retaining the same folding topology of d(G(3)T(4)G(4))(2) G-quadruplex.

Telomeres, telomerase, and apoptosis

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