Efficient inhibition of human telomerase activity by antisense oligonucleotides sensitizes cancer cells to radiotherapy

Senescence of Tumor Cells in Anticancer Therapy—Beneficial and Detrimental Effects

Cellular senescence process results in stable cell cycle arrest, which prevents cell proliferation. It can be induced by a variety of stimuli including metabolic stress, DNA damage, telomeres shortening, and oncogenes activation. Senescence is generally considered as a process of tumor suppression, both by preventing cancer cells proliferation and inhibiting cancer progression. It can also be a key effector mechanism for many types of anticancer therapies such as chemotherapy and radiotherapy, both directly and through bioactive molecules released by senescent cells that can stimulate an immune response. Senescence is characterized by a senescence-associated secretory phenotype (SASP) that can have both beneficial and detrimental impact on cancer progression. Despite the negatives, attempts are still being made to use senescence to fight cancer, especially when it comes to senolytics. There is a possibility that a combination of prosenescence therapy—which targets tumor cells and causes their senescence—with senotherapy—which targets senescent cells, can be promising in cancer treatment. This review provides information on cellular senescence, its connection with carcinogenesis and therapeutic possibilities linked to this process.

Oligonucleotide-Functionalized Gold Nanoparticles for Synchronous Telomerase Inhibition, Radiosensitization, and Delivery of Theranostic Radionuclides

Telomerase represents an attractive target in oncology as it is expressed in cancer but not in normal tissues. The oligonucleotide inhibitors of telomerase represent a promising anticancer strategy, although poor cellular uptake can restrict their efficacy. In this study, gold nanoparticles (AuNPs) were used to enhance oligonucleotide uptake. "match" oligonucleotides complementary to the telomerase RNA template subunit (hTR) and "scramble" (control) oligonucleotides were conjugated to diethylenetriamine pentaacetate (DTPA) for 111In-labeling. AuNPs (15.5 nm) were decorated with a monofunctional layer of oligonucleotides (ON-AuNP) or a multifunctional layer of oligonucleotides, PEG(polethylene glycol)800-SH (to reduce AuNP aggregation) and the cell-penetrating peptide Tat (ON-AuNP-Tat). Match-AuNP enhanced the cellular uptake of radiolabeled oligonucleotides while retaining the ability to inhibit telomerase activity. The addition of Tat to AuNPs increased nuclear localization. 111In-Match-AuNP-Tat induced DNA double-strand breaks and caused a dose-dependent reduction in clonogenic survival of telomerase-positive cells but not telomerase-negative cells. hTR inhibition has been reported to sensitize cancer cells to ionizing radiation, and 111In-Match-AuNP-Tat therefore holds promise as a vector for delivery of radionuclides into cancer cells while simultaneously sensitizing them to the effects of the emitted radiation.

Possibility of inducing tumor cell senescence during therapy

The treatment options for cancer include surgery, radiotherapy and chemotherapy. However, the traditional approach of high-dose chemotherapy brings tremendous toxic side effects to patients, as well as potentially causing drug resistance. Drug resistance affects cell proliferation, cell senescence and apoptosis. Cellular senescence refers to the process in which cells change from an active proliferative status to a growth-arrested status. There are multiple factors that regulate this process and cellular senescence is activated by various pathways. Senescent cells present specific characteristics, such as an increased cell volume, flattened cell body morphology, ceased cell division and the expression of β-galactosidase. Tumor senescence can be categorized into replicative senescence and premature senescence. Cellular senescence may inhibit the occurrence and development of tumors, serving as an innovative strategy for the treatment of cancer. The present review mainly focuses on senescent biomarkers, methods for the induction of cellular senescence and its possible application in the treatment of cancer.

Silencing hTERT attenuates cancer stem cell-like characteristics and radioresistance in the radioresistant nasopharyngeal carcinoma cell line CNE-2R

Objective: This study aimed to explore the effect of silencing hTERT on the CSC-like characteristics and radioresistance of CNE-2R cells.

Results: Silencing hTERT suppressed CNE-2R cell proliferation and increased the cell apoptosis rate and radiosensitivity in vitro. Moreover, it could also inhibit the growth of xenografts and increase the apoptosis index and radiosensitivity in vivo. Further study discovered that after silencing hTERT, telomerase activity in CNE-2R cells was markedly suppressed, along with remarkably down-regulated stem cell-related protein levels both in vitro and in vivo.

Conclusion: Silencing hTERT can suppress the CSC-like characteristics of CNE-2R cells to enhance their radiosensitivity, revealing that hTERT may become a potential target for treating radioresistant NPC.

Methods: An RNAi lentiviral vector specific to the hTERT gene was constructed to infect CNE-2R cells, the hTERT silencing effect was verified through qPCR and Western blot assays, and telomerase activity was detected by PCR-ELISA. Moreover, radiosensitivity in vitro was detected through colony formation assays, CCK-8 assays and flow cytometry. Tumor growth and radioresistance were also evaluated using xenograft models, while the apoptosis index in xenografts was measured through TUNEL assay. Levels of stem cell-related proteins were determined in vitro and in vivo.

Radiolabeled Oligonucleotides Targeting the RNA Subunit of Telomerase Inhibit Telomerase and Induce DNA Damage in Telomerase-Positive Cancer Cells

Telomerase is expressed in the majority (>85%) of tumors, but has restricted expression in normal tissues. Long-term telomerase inhibition in malignant cells results in progressive telomere shortening and reduction in cell proliferation. Here we report the synthesis and characterization of radiolabeled oligonucleotides that target the RNA subunit of telomerase, hTR, simultaneously inhibiting enzymatic activity and delivering radiation intracellularly. Oligonucleotides complementary (Match) and noncomplementary (Scramble or Mismatch) to hTR were conjugated to diethylenetriaminepentaacetic dianhydride (DTPA), allowing radiolabeling with the Auger electron-emitting radionuclide indium-111 (111In). Match oligonucleotides inhibited telomerase activity with high potency, which was not observed with Scramble or Mismatch oligonucleotides. DTPA-conjugation and 111In-labeling did not change telomerase inhibition. In telomerase-positive cancer cells, unlabeled Match oligonucleotides had no effect on survival, however, 111In-labeled Match oligonucleotides significantly reduced clonogenic survival and upregulated the DNA damage marker γH2AX. Minimal radiotoxicity and DNA damage was observed in telomerase-negative cells exposed to 111In-Match oligonucleotides. Match oligonucleotides localized in close proximity to nuclear Cajal bodies in telomerase-positive cells. In comparison with Match oligonucleotides, 111In-Scramble or 111In-Mismatch oligonucleotides demonstrated reduced retention and negligible impact on cell survival. This study indicates the therapeutic activity of radiolabeled oligonucleotides that specifically target hTR through potent telomerase inhibition and DNA damage induction in telomerase-expressing cancer cells and paves the way for the development of novel oligonucleotide radiotherapeutics targeting telomerase-positive cancers. SIGNIFICANCE: These findings present a novel radiolabeled oligonucleotide for targeting telomerase-positive cancer cells that exhibits dual activity by simultaneously inhibiting telomerase and promoting radiation-induced genomic DNA damage.

Cellular Senescence: Aging, Cancer, and Injury

Cellular senescence is a permanent state of cell cycle arrest that occurs in proliferating cells subjected to different stresses. Senescence is, therefore, a cellular defense mechanism that prevents the cells to acquire an unnecessary damage. The senescent state is accompanied by a failure to re-enter the cell cycle in response to mitogenic stimuli, an enhanced secretory phenotype and resistance to cell death. Senescence takes place in several tissues during different physiological and pathological processes such as tissue remodeling, injury, cancer, and aging. Although senescence is one of the causative processes of aging and it is responsible of aging-related disorders, senescent cells can also play a positive role. In embryogenesis and tissue remodeling, senescent cells are required for the proper development of the embryo and tissue repair. In cancer, senescence works as a potent barrier to prevent tumorigenesis. Therefore, the identification and characterization of key features of senescence, the induction of senescence in cancer cells, or the elimination of senescent cells by pharmacological interventions in aging tissues is gaining consideration in several fields of research. Here, we describe the known key features of senescence, the cell-autonomous, and noncell-autonomous regulators of senescence, and we attempt to discuss the functional role of this fundamental process in different contexts in light of the development of novel therapeutic targets.

Effect of therapies-mediated modulation of telomere and/or telomerase on cancer cells radiosensitivity

Cancer is one of the leading causes of death in the world. Many strategies of cancer treatment such as radiotherapy which plays a key role in cancer treatment are developed and used nowadays. However, the side effects post-cancer radiotherapy and cancer radioresistance are two major causes of the limitation of cancer radiotherapy effectiveness in the cancer patients. Moreover, reduction of the limitation of cancer radiotherapy effectiveness by reducing the side effects post-cancer radiotherapy and cancer radioresistance is the aim of several radiotherapy-oncologic teams. Otherwise, Telomere and telomerase are two cells components which play an important role in cancer initiation, cancer progression and cancer therapy resistance such as radiotherapy resistance. For resolving the problems of the limitation of cancer radiotherapy effectiveness especially the cancer radio-resistance problems, the radio-gene-therapy strategy which is the use of gene-therapy via modulation of gene expression combined with radiotherapy was developed and used as a new strategy to treat the patients with cancer. In this review, we summarized the information concerning the implication of telomere and telomerase modulation in cancer radiosensitivity.

Phosphorothioate‑modified antisense oligonucleotides against human telomerase reverse transcriptase sensitize cancer cells to radiotherapy

Emergence of resistance, unavoidable systemic toxicity and unsatisfactory efficacy arethe main obstacles for traditional cancer therapy. Combination with phosphorothioate modified antisense oligonucleotides (PS‑ASODN) against human telomerase reverse transcriptase (hTERT) may enhance the therapeutic effect of irradiation. However, the effect of PS‑ASODN against hTERT on the anti‑tumor effects of irradiation in liver cancer remain unclear. In the current study, Walker 256 cells were transfected with hTERT PS‑ASODN. Cell proliferation and cell viability were measured using the MTT assay and cell senescence was examined by SA‑β‑gal staining. Telomerase activity was determined by telomeric repeat amplification protocol‑polymerase chain reaction‑ELISA. Cell apoptosis was assayed by flow cytometry and DNA damage was determined by the comet assay.The PS‑ASODN was demonstrated to have an inhibitory effect on cell proliferation and accelerated effect on cell senescence by inhibiting telomerase activity. PS‑ASODN promoted the irradiation‑induced inhibition of cell viability and telomerase activity, and irradiation‑induced DNA damage and cell apoptosis via the activation of apoptosis‑associated proteins. Taken together, these results indicated that combined treatment of PS‑ASODN with irradiation significantly enhanced tumor inhibition. Therefore, PS‑ASODN provides an experimental foundation for gene therapy and is proposed for application in clinical treatment of liver cancer combined with radiotherapy.

Targeting human telomerase RNA component using antisense oligonucleotide induces rapid cell death and increases ATO-induced apoptosis in APL cells

The impressive advances carried out in designing pharmacological strategies with the aim of telomerase inhibition in cancers emerged a consensus that telomerase-targeted therapies could be exciting prospect in repertoire of future cancer strategies. The results of the present study indicated that targeting telomerase using an oligonucleotide-based molecule against human telomerase RNA template (hTR ASODN) reduced the survival rate of NB4 cells and induced a caspase-3-dependent apoptosis. Our finding was even noticeable in the synergistic experiments, where we found an enhanced reduction in the viability of the cells after short-term treatment with ATO in combination with the inhibitor. The resulting data delineated that short-term treatment of the cells with hTR ASODN either as single agent or in combination with ATO resulted in apoptotic cell death through activation of DNA damage response via up-regulation of p73 and ATM coupled with down-regulation of c-Myc. Moreover, we found that induction of p21 and subsequent disturbance of the death promoter to death repressor genes may contribute to the enhanced growth suppressive effect of the drugs combination. Overall, our findings support the idea that telomerase activity may have pivotal role in attenuating ATO effectiveness and combination of ATO with telomerase inhibitor seems to be a novel promising strategy, which may increase APL cure rates.

Targeting telomerase and telomeres to enhance ionizing radiation effects in in vitro and in vivo cancer models

One of the hallmarks of cancer consists in the ability of tumor cells to divide indefinitely, and to maintain stable telomere lengths throughout the activation of specific telomere maintenance mechanisms (TMM). Therefore in the last fifteen years, researchers proposed to target telomerase or telomeric structure in order to block limitless replicative potential of cancer cells providing a fascinating strategy for a broad-spectrum cancer therapy. In the present review, we report in vitro and in vivo evidence regarding the use of chemical agents targeting both telomerase or telomere structure and showing promising antitumor effects when used in combination with ionizing radiation (IR). RNA interference, antisense oligonucleotides (e.g., GRN163L), non-nucleoside inhibitors (e.g., BIBR1532) and nucleoside analogs (e.g., AZT) represent some of the most potent strategies to inhibit telomerase activity used in combination with IR. Furthermore, radiosensitizing effects were demonstrated also for agents acting directly on the telomeric structure such as G4-ligands (e.g., RHPS4 and Telomestatin) or telomeric-oligos (T-oligos). To date, some of these compounds are under clinical evaluation (e.g., GRN163L and KML001). Advantages of Telomere/Telomerase Targeting Compounds (T/TTCs) coupled with radiotherapy may be relevant in the treatment of radioresistant tumors and in the development of new optimized treatment plans with reduced dose adsorbed by patients and consequent attenuation of short- end long-term side effects. Pros and cons of possible future applications in cancer therapy based on the combination of T/TCCs and radiation treatment are discussed.

Targeting telomerase with radiolabeled inhibitors

The expression of telomerase in approximately 85% of cancers and its absence in the majority of normal cells makes it an attractive target for cancer therapy. However the lag period between initiation of telomerase inhibition and growth arrest makes direct inhibition alone an insufficient method of treatment. However, telomerase inhibition has been shown to enhance cancer cell radiosensitivity. To investigate the strategy of simultaneously inhibiting telomerase while delivering targeted radionuclide therapy to cancer cells, 123I-radiolabeled inhibitors of telomerase were synthesized and their effects on cancer cell survival studied. An 123I-labeled analogue of the telomerase inhibitor MST-312 inhibited telomerase with an IC50 of 1.58 μM (MST-312 IC50: 0.23 μM). Clonogenic assays showed a dose dependant effect of 123I-MST-312 on cell survival in a telomerase positive cell line, MDA-MB-435.

Downregulation of Ubiquitin-conjugating Enzyme UBE2D3 Promotes Telomere Maintenance and Radioresistance of Eca-109 Human Esophageal Carcinoma Cells

Ubiquitin-conjugating enzyme UBE2D3 is an important member of the ubiquitin-proteasome pathways. Our previous study showed that the expression of UBE2D3 was negatively related to human telomerase reverse transcriptase (hTERT) and radioresistance in human breast cancer cells. However, in esophageal carcinoma, the exact effects and mechanisms of UBE2D3 in radioresistance remain unclear. This study shows that UBE2D3 knockdown was associated with significant increases in radioresistance to X-rays, telomerase activity, telomere length, and telomere shelterins. UBE2D3 knockdown-mediated radioresistance was related to a decrease in the spontaneous and ionizing radiation-induced apoptosis, resulting from a decrease in the Bax/Bcl-2 ratio. Furthermore, UBE2D3 downregulation was associated with increased G1-S phase transition and prolonged IR-induced G2/M arrest through over expression of cyclin D1, decrease of CDC25A expression and promotion of the ATM/ATR-Chk1-CDC25C pathway. Moreover, UBE2D3 downregulation reduced spontaneous DNA double-strand breaks and accelerated the repair of DNA damage induced by IR. The current data thus demonstrate that UBE2D3 downregulation enhances radioresistance by increased telomere homeostasis and prolonged IR-induced G2/M arrest, but decreases the IR-induced apoptosis and the number of DNA damage foci. These results suggest that UBE2D3 might be a potential molecular target to improve radiotherapy effects in esophageal carcinoma.

Radiosensitization to X-ray radiation by telomerase inhibitor MST-312 in human hepatoma HepG2 cells

AIMS

Previous studies in malignant cells have shown that irradiation-induced upregulation of telomerase activity, not only protected damaged telomeres, but also contributed to DNA damage repair by chromosomal healing and increased resistance to irradiation. The purpose of the present study was to investigate the radiosensitizing effect of telomerase inhibitor MST-312 and the corresponding mechanism in the human hepatoma cell line HepG2.

MAIN METHODS

Cell proliferation, telomerase activity, cell cycle distribution, DNA damage and repair, expression of p53, mitochondrial membrane potential, and cell apoptosis were measured with the MTT assay, real-time fluorescent quantitative PCR, flow cytometry, immunofluorescence, western blots, JC-1 staining, and Hoechst 33258 staining, respectively.

KEY FINDINGS

MST-312 effectively inhibited telomerase activity and showed relative weak toxicity to HepG2 cells at 4 μM. Compared with irradiation alone, 4 μM MST-312 pretreatment, followed by X-ray treatment, significantly reduced clonogenic potential. Aggravated DNA damage and increased sub-G1 cell fractions were observed. Further investigation found that homologous recombination (HR) repair protein Rad51 foci nuclear formation was blocked, and expression of p53 was elevated. These led to the collapse of mitochondrial membrane potential, and enhanced the apoptotic rate.

SIGNIFICANCE

These data demonstrated that disturbances of telomerase function could enhance the radiosensitivity of HepG2 cells to X-ray irradiation by impairing HR repair processes. In addition, telomerase inhibitor MST-312 may be useful as an adjuvant treatment in combination with irradiation.

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

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

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

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

Telomerase as a useful target in cancer fighting-the breast cancer case

Telomerase was initially considered as a relevant factor distinguishing cancer from normal cells. During detailed studies, it appeared that its expression and activity is not only limited to cancer cells however, but in this particular cells, the telomerase is much more abundant. Thus, it has become a very promising target for an anticancer therapy. It was revealed in many studies that regulation of telomerase is a multifactorial process in mammalian cells, involving regulation of expression of telomerase subunits coding genes, post-translational protein–protein interactions, and protein phosphorylation. Numerous proto-oncogenes and tumor suppressor genes are engaged in this mechanism, and the complexity of telomerase control is studied in the context of tumor development as well as aging. Additionally, since numerous studies reveal a correlation between short telomeres and increased genome instability or cell mortality, the telomerase control appears to be one of the crucial factors to study in order to improve the cancer diagnostics and therapy or prevention. Interestingly, almost 100 % of adenocarcinoma, including breast cancer cells, expresses telomerase which makes it a good target for telomerase-related therapy. Additionally, telomerase is also supposed to be associated with drug resistance. Thus, targeting the enzyme might result in attenuation of this phenomenon. Moreover, since stem cells existence was reported, it must be considered whether targeting telomerase can bring some serious side effects and result in stem cells viability or their regenerative potential decrease. Thus, we review some molecular mechanisms engaged in therapy based on targeting telomerase in breast cancer cells.

Biochemical markers predicting response to radiation- and radiochemo-therapy in cancer patients

In last years there is increasing interest in radiogenomics and the characterization of DNA array molecular profiles that can predict tumor and no tumor tissues radioresponse. Ongoing studies carried out worldwide in the banking of tumor and no tumor samples give evidence that perspective markers for response prediction in individual patient to intended radiation therapy can be some apoptotic indexes, spectrum a number of specific proteins, and DNA-based microarray molecular profiling analysis as well determination of single nucleotide polymorphisms in genome of the patients. So far there are only a few robust reports of molecular markers predicting tumor and no tumor tissues response to radiation. The results of new studies, which in future should be validated in larger definitive trials, are likely to see in nearest years. It is needed to determine technologies of methods and to define more precisely areas of its applications.

Human telomerase expression regulation

Since telomerase has been recognized as a relevant factor distinguishing cancer cells from normal cells, it has become a very promising target for anti-cancer therapy. A correlation between short telomere length and increased mortality was revealed in many studies. The telomerase expression/activity appears to be one of the most crucial factors to study to improve cancer therapy and prevention. However, this multisubunit enzymatic complex can be regulated at various levels. Thus, several strategies have been proposed to control telomerase in cancer cells such as anti-sense technology against TR and TERT, ribozymes against TERT, anti-estrogens, progesterone, vitamin D, retinoic acid, quadruplex stabilizers, telomere and telomerase targeting agents, modulation of interaction with other proteins involved in the regulation of telomerase and telomeres, etc. However, the transcription control of key telomerase subunits seems to play the crucial role in whole complexes activity and cancer cells immortality. Thus, the research of telomerase regulation can bring significant insight into the knowledge concerning stem cells metabolism but also ageing. This review summarizes the current state of knowledge of numerous telomerase regulation mechanisms at the transcription level in human that might become attractive anti-cancer therapy targets.

Association of telomerase activity with radio- and chemosensitivity of neuroblastomas

BACKGROUND

Telomerase activity compensates shortening of telomeres during cell division and enables cancer cells to escape senescent processes. It is also supposed, that telomerase is associated with radio- and chemoresistance. In the here described study we systematically investigated the influence of telomerase activity (TA) and telomere length on the outcome of radio- and chemotherapy in neuroblastoma.

METHODS

We studied the effects on dominant negative (DN) mutant, wild type (WT) of the telomerase catalytic unit (hTERT) using neuroblastoma cell lines. The cells were irradiated with 60Co and treated with doxorubicin, etoposide, cisplatin and ifosfamide, respectively. Viability was determined by MTS/MTT-test and the GI50 was calculated. Telomere length was measured by southernblot analysis and TA by Trap-Assay.

RESULTS

Compared to the hTERT expressing cells the dominant negative cells showed increased radiosensitivity with decreased telomere length. Independent of telomere length, telomerase negative cells are significantly more sensitive to irradiation. The effect of TA knock-down or overexpression on chemosensitivity were dependent on TA, the anticancer drug, and the chemosensitivity of the maternal cell line.

CONCLUSIONS

Our results supported the concept of telomerase inhibition as an antiproliferative treatment approach in neuroblastomas. Telomerase inhibition increases the outcome of radiotherapy while in combination with chemotherapy the outcome depends on drug- and cell line and can be additive/synergistic or antagonistic. High telomerase activity is one distinct cancer stem cell feature and the here described cellular constructs in combination with stem cell markers like CD133, Aldehyddehydrogenase-1 (ALDH-1) or Side population (SP) may help to investigate the impact of telomerase activity on cancer stem cell survival under therapy.

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.