Telomerase inhibitors

Innovative anti-proliferative effect of the antiviral favipiravir against MCF-7 breast cancer cells using green nanoemulsion and eco-friendly assessment tools

Telomerase enzyme prevents telomere shortening during division, having human telomerase reverse transcriptase (hTERT) as its catalytic subunit. Favipiravir (FAV), an RNA-dependent RNA polymerases inhibitor, shared structural similarity with hTERT and thus assumed to have cytotoxic effect on cancer cells, in addition to its prophylactic effect to immunocompromised cancer patients. Nanoemulsion (NE) is a potential tumor cells targeting delivery system, thereby enhancing therapeutic efficacy at the intended site, mitigating systemic toxicity, and overcoming multidrug resistance. The objective of this study is to develop a green FAV nanoemulsion (FNE) that is environmentally friendly and safe for patients, while aiming to enhance its cytotoxic effects. The study also highlights the environmental sustainability of the developed RP-HPLC method and assesses its greenness impact. The FNE formulation underwent thermodynamic stability testing and invitro characterization. Greenness was assessed using advanced selected tools like the Analytical Eco-Scale (AES), Analytical Greenness Metric for Sample Preparation (AGREEprep), and green analytical procedure index (GAPI). The cytotoxic potential of FNE was screened against MCF-7 breast cancer and Vero normal cell lines using SRB assay. Stable and ecofriendly FNE was formulated having a particle size (PS) of 25.29 ± 0.57 nm and a zeta potential of -6.79 ± 5.52 mV. The cytotoxic effect of FNE on MCF-7 cells was more potent than FAV with lower IC50 while FNE showed non-toxic effect on VERO normal cell line. Therefore, the FAV nanoemulsion formulation showed targeted cytotoxicity on MCF-7 cells while being non-toxic on normal Vero cells.

Imetelstat, a novel, first-in-class telomerase inhibitor: Mechanism of action, clinical, and translational science

Most cancers and neoplastic progenitor cells have elevated telomerase activity and preservation of telomeres that promote cellular immortality, making telomerase a rational target for the treatment of cancer. Imetelstat is a first-in-class, 13-mer oligonucleotide that binds with high affinity to the template region of the RNA component of human telomerase and acts as a competitive inhibitor of human telomerase enzymatic activity. Pharmacokinetics, pharmacodynamics, exposure-response analyses, efficacy, and safety of imetelstat have been evaluated in vitro, in vivo, and clinically in solid tumor and hematologic malignancies, including lower-risk myelodysplastic syndromes (LR-MDS) and myeloproliferative neoplasms. Imetelstat was approved in the United States in June 2024 for the treatment of adult patients with LR-MDS with transfusion-dependent anemia requiring four or more red blood cell units over 8 weeks who have not responded to or have lost response to or are ineligible for erythropoiesis-stimulating agents, with a recommended dosing regimen of 7.1 mg/kg administered via 2-h intravenous infusion every 4 weeks. In the pivotal trial, significantly more patients treated with imetelstat versus placebo achieved ≥8-week and ≥24-week red blood cell-transfusion independence, and imetelstat was associated with a manageable safety profile characterized primarily by short-lived and manageable neutropenia and thrombocytopenia. This mini-review summarizes the mechanism of action, pharmacokinetic and pharmacodynamic characteristics, clinical development, and clinical efficacy and safety data of imetelstat.

Favipiravir, an antiviral drug, in combination with tamoxifen exerts synergistic effect in tamoxifen-resistant breast cancer cells via hTERT inhibition

Tamoxifen (TAM) is one of the most successful treatments for breast cancer; however, TAM resistance continues to be a significant barrier. TAM resistance has been reported to be associated with increased expression of human telomerase reverse transcriptase (hTERT). This enzyme shares structural similarity with RNA-dependent RNA polymerase (RdRp) enzyme of RNA viruses, suggesting that RdRp inhibitors may also inhibit hTERT. Favipiravir (FAV) is an antiviral drug that inhibits RdRp of RNA viruses. Thus, we propose that FAV may also elicit an antitumor effect by suppressing hTERT. This study aimed to investigate the effect of FAV and TAM on TAM-resistant breast cancer (TAMR-1). The cell viabilities were determined. The levels of CDK1/ hTERT, in addition to regulators of hTERT-targeted signaling pathways were measured. Apoptosis, migration, and cell cycle distribution were also determined. Our data revealed that the combination of TAM and FAV suppressed cell proliferation synergistically (CI < 1) and resulted in a significant change in cell migration and apoptosis. Indeed, this was associated with reduced levels of hTERT and CDK1 and shift in the cell cycle distribution. Our findings suggest that the TAM/FAV combination exhibits synergistic effects against TAMR-1 human breast cancer cells by targeting hTERT.

Entropy-Driven Self-Assembly of Single Quantum Dot Sensor for Catalytic Imaging of Telomerase in Living Cells

Telomerase is a highly valuable cancer diagnosis biomarker and a promising cancer therapy target. So far, most telomerase assays are limited by the involvement of tedious procedures, multiple enzymes, and complicated reaction schemes. Sensitive monitoring of low-abundant telomerase in living cells remains a challenge. Herein, we demonstrate an entropy-driven catalytic assembly of quantum dot (QD) sensors for accurate detection and imaging of telomerase activity in living cells. In this sensor, target telomerase specifically catalyzes extension of telomerase primer, and the extended primer subsequently acts as a catalyst to continuously initiate entropy-driven catalytic reaction, generating a large number of fluorophore- and biotin-labeled DNAs that can be self-assembled on the QD surface to induce an efficient Föster resonance energy transfer signal. The proposed sensor requires a single step for both recognition and amplification of the telomerase signal, eliminating the use of either protein enzymes or laborious procedures. Taking advantage of the inherent superiority of single-molecule fluorescence detection and high amplification efficiency of the entropy-driven reaction, this sensor demonstrates single-cell sensitivity for the in vitro assay. Moreover, it is capable of screening the telomerase inhibitor, discriminating different tumor cells from normal ones, and even real-time imaging telomerase in living cells, providing a novel platform for telomerase-associated cancer diagnosis and drug screening.

Inhibition of DKC1 induces telomere-related senescence and apoptosis in lung adenocarcinoma

Background

Lung cancer is one of the most widely spread cancers in the world and half of the non-small cell lung cancers are lung adenocarcinoma (LUAD). Although there were several drugs been approved for LUAD therapy, a large portion of LUAD still cannot be effectively treated due to lack of available therapeutic targets. Here, we investigated the oncogenic roles of DKC1 in LUAD and its potential mechanism and explored the possibility of targeting DKC1 for LUAD therapy.

Methods

The Gene Expression Omnibus (GEO) and The Cancer Genome Atlas Program (TCGA) databases were used to examine the DKC1 transcript levels. Gene expression with clinical information from tissue microarray of LUAD were analyzed for associations between DKC1 expression and LUAD prognosis. In addition, loss- and gain-of-function assays were used for oncogenic function of DKC1 both in vitro and in vivo.

Results

DKC1 is overexpressed in LUAD compared with adjacent normal tissues. High expression of DKC1 predicts the poor overall survival. DKC1 knockdown in LUAD cell lines induced G1 phase arrest and inhibited cell proliferation. Ectopic expression of DKC1 could rescue the growth of LUAD cell lines. In addition, the abundance of DKC1 is positively correlated with telomerase RNA component (TERC) and telomerase reverse transcriptase (TERT) levels in LUAD. DKC1 downregulation resulted in decreased TERC expression, reduced telomerase activity and shorten telomere, and thus eventually led to cell senescence and apoptosis.

Conclusions

Our results show that high DKC1 expression indicates poor prognosis of LUAD and DKC1 downregulation could induce telomere-related cell senescence and apoptosis. This study suggests that DKC1 could serve as a candidate diagnostic biomarker and therapeutic target for LUAD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-021-02827-0.

Pancreatic adenocarcinoma: molecular drivers and the role of targeted therapy

Prognosis from pancreatic ductal adenocarcinoma (PDAC) continues to be poor despite the many efforts channeled to improve its management. Although the mainstay treatment is still traditional chemotherapy, recent advances highlighted a promising potential for targeted therapy in the management of this disease. Those advances emphasize the significance of timely genomic profiling of tumor tissue as well as germline testing of patients to identify potential markers of targeted therapy. While targeted therapy is reserved for a relatively small subset of patients with PDAC, ongoing research is uncovering additional markers, and targeted agents, that will hopefully translate to better outcomes for patients.

Effects of Reverse Transcriptase Inhibitors on Proliferation, Apoptosis, and Migration in Breast Carcinoma Cells

High telomerase activity in human breast cancer is associated with aggressive tumors resulting in decreased survival. Recent studies have shown that telomerase inhibitors may display anticancer properties in some human cancer cell lines. In the present study, we examined the effects of 4 reverse transcriptase inhibitors (RTIs), used for the treatment of HIV; Abacavir (AC), Lamivudine (LV), Stavudine (SV), and Tenofovir (TF) on proliferation, apoptosis, and migration in the normal human mammary epithelial cell line, hTERT-HME1, and the human breast cancer cell line, MCF-7. Cells were treated with AC, LV, SV, or TF alone or in combination with paclitaxel (PAC), a known drug used to treat breast cancer. Conduct of the thiazolyl blue tetrazolium bromide assay demonstrated that AC, SV, and TF had stronger cytotoxic effects on MCF-7 cells than in hTERT-HME1 cells. The combined treatment of RTIs and PAC caused high rates of cell death in MCF-7 and low rates of cell death in HTERT-HME1 by apoptosis. The percentages of apoptotic cells in the treatment of AC and SV in combination with PAC for 48 and 72 hours were higher than PAC. Significantly increased apoptosis and decreased migration levels were found in MCF-7 cells treated with AC and co-treatment of AC+PAC or SV+PAC than HME1 cells. These treatments can also prevent migration capacity more than PAC. Therefore, a combination strategy based on telomerase inhibitors such as AC or SV and anticancer drugs may be more effective in the treatment of certain breast cancers.

The secrets of telomerase: Retrospective analysis and future prospects

Telomerase plays a significant role to maintain and regulate the telomere length, cellular immortality and senescence by the addition of guanine-rich repetitive sequences. Chronic inflammation or oxidative stress-induced infection downregulates TERT gene modifying telomerase activity thus contributing to the early steps of gastric carcinogenesis process. Furthermore, telomere-telomerase system performs fundamental role in the pathogenesis and progression of diabetes mellitus as well as in its vascular intricacy. The cessation of cell proliferation in cultured cells by inhibiting the telomerase activity of transformed cells renders the rationale for culling of telomerase as a target therapy for the treatment of metabolic disorders and various types of cancers. In this article, we have briefly described the role of immune system and malignant cells in the expression of telomerase with critical analysis on the gaps and potential for future studies. The key findings regarding the secrets of the telomerase summarized in this article will help in future treatment modalities for the prevention of various types of cancers and metabolic disorders notably diabetes mellitus.

Therapeutic strategies for targeting telomerase in cancer

Telomere and telomerase play important roles in abnormal cell proliferation, metastasis, stem cell maintenance, and immortalization in various cancers. Therefore, designing of drugs targeting telomerase and telomere is of great significance. Over the past two decades, considerable knowledge regarding telomere and telomerase has been accumulated, which provides theoretical support for the design of therapeutic strategies such as telomere elongation. Therefore, the development of telomere-based therapies such as nucleoside analogs, non-nucleoside small molecules, antisense technology, ribozymes, and dominant negative human telomerase reverse transcriptase are being prioritized for eradicating a majority of tumors. While the benefits of telomere-based therapies are obvious, there is a need to address the limitations of various therapeutic strategies to improve the possibility of clinical applications. In this study, current knowledge of telomere and telomerase is discussed, and therapeutic strategies based on recent research are reviewed.

A simple “mix-and-detection” method for the sensitive detection of telomerase from cancer cells under absolutely isothermal conditions

We develop a simple “mix-and-detection” method for the sensitive detection of telomerase from cancer cells under absolutely isothermal conditions.

Inhibitors of telomerase and poly(ADP-ribose) polymerases synergize to limit the lifespan of pancreatic cancer cells

Imetelstat (GRN163L) is a potent and selective inhibitor of telomerase. We have previously reported that GRN163L could shorten telomeres and limit the lifespan of CD18/HPAF and CAPAN1 pancreatic cancer cells. Here, we examined the effects of GRN163L on two other pancreatic cancer cell lines: AsPC1 and L3.6pl. In both lines, chronic exposure to GRN163L led to an initial shortening of telomeres followed by a stabilization of extremely short telomeres. In AsPC1 cells, telomere attrition eventually led to the induction of crisis and the loss of the treated population. In L3.6pl cells, crisis was transient and followed by the emergence of GRN163L-resistant cells, which could grow at increasing concentrations of GRN163L. The Shelterin complex is a telomere-associated complex that limits the access of telomerase to telomeres. The telomerase inhibitory function of this complex can be enhanced by drugs that block the poly(ADP-ribosyl)ation of its TRF1 and/or TRF2 subunits. Combined treatment of the GRN163L-resistant L3.6pl cells with GRN163L and 3-aminobenzamide (3AB), a general inhibitor of poly(ADP-ribose) polymerases, led to additional telomere shortening and limited the lifespan of the resistant cells. Results from this work suggest that inhibitors of telomerase and poly(ADP-ribose) polymerases can cooperate to limit the lifespan of pancreatic cancer cells.

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.

Role of Telomeres and Telomerase in Aging and Cancer

Telomeres progressively shorten throughout life. A hallmark of advanced malignancies is the ability for continuous cell divisions that almost universally correlates with the stabilization of telomere length by the reactivation of telomerase. The repression of telomerase and shorter telomeres in humans may have evolved, in part, as an anticancer protection mechanism. Although there is still much we do not understand about the regulation of telomerase, it remains a very attractive and novel target for cancer therapeutics. This review focuses on the current state of advances in the telomerase area, identifies outstanding questions, and addresses areas and methods that need refinement.

SIGNIFICANCE

Despite many recent advances, telomerase remains a challenging target for cancer therapy. There are few telomerase-directed therapies, and many of the assays used to measure telomeres and telomerase have serious limitations. This review provides an overview of the current state of the field and how recent advances could affect future research and treatment approaches. Cancer Discov; 6(6); 584-93. ©2016 AACR.

New dimeric carbazole-benzimidazole mixed ligands for the stabilization of human telomeric G-quadruplex DNA and as telomerase inhibitors. A remarkable influence of the spacer

The development of G-quadruplex (G4) DNA binding small molecules has become an important strategy for selectively targeting cancer cells. Herein, we report the design and evolution of a new kind of carbazole-based benzimidazole dimers for their efficient telomerase inhibition activity. Spectroscopic titrations reveal the ligands high affinity toward the G4 DNA with significantly higher selectivity over duplex-DNA. The electrophoretic mobility shift assay shows that the ligands efficiently promote the formation of G4 DNA even at a lower concentration of the stabilizing K(+) ions. The TRAP-LIG assay demonstrates the ligand's potential telomerase inhibition activity and also establishes that the activity proceeds via G4 DNA stabilization. An efficient nuclear internalization of the ligands in several common cancer cells (HeLa, HT1080, and A549) also enabled differentiation between normal HFF cells in co-cultures of cancer and normal ones. The ligands induce significant apoptotic response and antiproliferative activity toward cancer cells selectively when compared to the normal cells.

Novel Efficient Cell-Penetrating, Peptide-Mediated Strategy for Enhancing Telomerase Inhibitor Oligonucleotides

At present, there are several therapeutic approaches for targeting telomerase in tumors. One in particular, currently undergoing clinical trials, is based on synthetic lipid-modified oligonucleotide antagonists aimed at inhibiting the ribonucleoprotein subunit of human telomerase. However, while enabling efficient uptake, the lipid modifications reduce the potency of the therapeutic oligonucleotides compared to nonmodified oligonucleotides. Moreover, lipid modification may increase oligonucleotide accumulation in the liver causing undesirable hepatotoxicity. Noncovalent complexation strategies for cell-penetrating peptide (CPP)-mediated delivery present an option to circumvent the need for potency-reducing modifications, while allowing for a highly efficient uptake, and could significantly improve the efficiency of telomerase-targeting cancer therapeutics. Delivery of a nonlipidated locked nucleic acid/2'-O-methyl mixmer significantly inhibits the telomerase activity in treated HeLa cells. The inhibitory effect was further improved through addition of a CPP. Furthermore, calculated IC50-values for the oligonucleotide delivered by CPPs into HeLa cells are more than 20 times lower than telomerase inhibitor Imetelstat, currently undergoing clinical trials. These results emphasize the potential of CPP-mediated delivery of future pharmaceuticals and provide means by which to enhance an already promising therapeutic strategy for cancer treatment.

Calorimetric and spectroscopic investigations of the binding of metallated porphyrins to G-quadruplex DNA

BACKGROUND

The human telomere contains tandem repeat of (TTAGG) capable of forming a higher order DNA structure known as G-quadruplex. Porphyrin molecules such as TMPyP4 bind and stabilize G-quadruplex structure.

METHODS

Isothermal titration calorimetry (ITC), circular dichroism (CD), and mass spectroscopy (ESI/MS), were used to investigate the interactions between TMPyP4 and the Co(III), Ni(II), Cu(II), and Zn(II) complexes of TMPyP4 (e.g. Co(III)-TMPyP4) and a model human telomere G-quadruplex (hTel22) at or near physiologic ionic strength ([Na(+)] or [K(+)]≈0.15M).

RESULTS

The apo-TMPyP4, Ni(II)-TMPyP4, and Cu(II)-TMPyP4 all formed complexes having a saturation stoichiometry of 4:1, moles of ligand per mole of DNA. Binding of apo-TMPyP4, Ni(II)-TMPyP4, and Cu(II)-TMPyP4 is described by a "four-independent-sites model". The two highest-affinity sites exhibit a K in the range of 10(8) to 10(10)M(-1) with the two lower-affinity sites exhibiting a K in the range of 10(4) to 10(5)M(-1). Binding of Co(III)-TMPyP4, and Zn(II)-TMPyP4, is best described by a "two-independent-sites model" in which only the end-stacking binding mode is observed with a K in the range of 10(4) to 10(5)M(-1).

CONCLUSIONS

In the case of apo-TMPyP4, Ni(II)-TMPyP4, and Cu(II)-TMPyP4, the thermodynamic signatures for the two binding modes are consistent with an "end stacking" mechanism for the higher affinity binding mode and an "intercalation" mechanism for the lower affinity binding mode. In the case of Co(III)-TMPyP4 and Zn(II)-TMPyP4, both the lower affinity for the "end-stacking" mode and the loss of the intercalative mode for forming the 2:1 complexes with hTel22 are attributed to the preferred metal coordination geometry and the presence of axial ligands.

GENERAL SIGNIFICANCE

The preferred coordination geometry around the metal center strongly influences the energetics of the interactions between the metallated-TMPyP4 and the model human telomeric G-quadruplex.

In-silico modeling studies of G-quadruplex with soy isoflavones having anticancerous activity

Telomere forms t-loop and G-quadruplex as the protective structure and the formation of these structures hinder the telomerase enzyme action. The binding affinities of ligand which stabilize the G-quadruplex represent good correlation with telomerase inhibition depicted in the anti-cancerous action. Most of the potent G-quadruplex stabilizing compounds suffer from the poor drug like properties. Herein, natural dietary compounds isoflavones were taken for the theoretical study to examine their stabilizing effect on G-quadruplex structure. The experimental G-quadruplex complexes were reproduced to obtain and validate the theoretical parameters. The obtained theoretical binding energies are in significant correlation with the experimental data. Analysis of binding shows isoflavones to be groove binders, and differential nature of quadruplex grooves might be beneficial in the selectivity aspects. Among all, derrubone was found to have better selectivity as well as affinity for the G-quadruplex comparable to well known ligand TMPyP4. The GBSA rescoring result enlightens the various interaction terms involved in the binding process. Cumulative stabilizing effects coming from VDW, ES, and GB energy terms attest to optimal binding of derrubone molecule which can be considered as a lead for the higher phases of drug designing. These findings are of great value in terms of unexplored groove binding modes and the studied natural compounds might be helpful to direct the focus of synthetic chemists in designing of new generation of antitumor agents.

Pharmacodynamics of telomerase inhibition and telomere shortening by noncytotoxic suramin

We reported that suramin is an effective chemosensitizer at noncytotoxic concentrations (<50 μM); this effect was observed in multiple types of human xenograft tumors in vitro and in vivo. Clinical evaluation of noncytotoxic suramin is ongoing. Because (a) suramin inhibits reverse transcriptase, (b) telomerase is a reverse transcriptase, and (c) inhibition of telomerase enhances tumor chemosensitivity, we studied the pharmacodynamics of noncytotoxic suramin on telomerase activity and telomere length in cultured cells and tumors grown in animals. In three human cancer cells that depend on telomerase for telomere maintenance (pharynx FaDu, prostate PC3, breast MCF7), suramin inhibited telomerase activity in cell extracts and intact cells at concentrations that exhibited no cytotoxicity (IC50 of telomerase was between 1 and 3 μM vs. >60 μM for cytotoxicity), and continuous treatment at 10-25 μM for 6 weeks resulted in gradual telomere shortening (maximum of 30%) and cell senescence (measured by β-galactosidase activity and elevation of mRNA levels of two senescence markers p16 and p21). In contrast, noncytotoxic suramin did not shorten the telomere in telomerase-independent human osteosarcoma Saos-2 cells. In mice bearing FaDu tumors, treatment with noncytotoxic suramin for 6 weeks resulted in telomere erosion in >95% of the tumor cells with an average telomere shortening of >40%. These results indicate noncytotoxic suramin inhibits telomerase, shortens telomere and induces cell senescence, and suggest telomerase inhibition as a potential mechanism of its chemosensitization.

Molecular Modeling and Evaluation of Novel Dibenzopyrrole Derivatives as Telomerase Inhibitors and Potential Drug for Cancer Therapy

During previous years, many studies on synthesis, as well as on anti-tumor, anti-inflammatory and anti-bacterial activities of the pyrazole derivatives have been described. Certain pyrazole derivatives exhibit important pharmacological activities and have proved to be useful template in drug research. Considering importance of pyrazole template, in current work the series of novel inhibitors were designed by replacing central ring of acridine with pyrazole ring. These heterocyclic compounds were proposed as a new potential base for telomerase inhibitors. Obtained dibenzopyrrole structure was used as a novel scaffold structure and extension of inhibitors was done by different functional groups. Docking of newly designed compounds in the telomerase active site (telomerase catalytic subunit TERT) was carried out. All dibenzopyrrole derivatives were evaluated by three docking programs: CDOCKER, Ligandfit docking (Scoring Functions) and AutoDock. Compound C_9g, C_9k and C_9l performed best in comparison to all designed inhibitors during the docking in all methods and in interaction analysis. Introduction of pyrazole and extension of dibenzopyrrole in compounds confirm that such compound may act as potential telomerase inhibitors.

Alternative splicing regulation of telomerase: a new paradigm?

Alternative splicing affects approximately 95% of eukaryotic genes, greatly expanding the coding capacity of complex genomes. Although our understanding of alternative splicing has increased rapidly, current knowledge of splicing regulation has largely been derived from studies of highly expressed mRNAs. Telomerase is a key example of a protein that is alternatively spliced, but it is expressed at very low levels and although it is known that misregulation of telomerase splicing is a hallmark of nearly all cancers, the details of this process are unclear. Here we review work showing that hTERT expression is in part regulated by atypical alternative splicing, perhaps due to its exceptionally low expression level. We propose that these differential regulatory mechanisms may be widely applicable to other genes and may provide new opportunities for the development of cancer therapeutics.

An investigation of G-quadruplex structural polymorphism in the human telomere using a combined approach of hydrodynamic bead modeling and molecular dynamics simulation

Guanine-rich oligonucleotides can adopt noncanonical tertiary structures known as G-quadruplexes, which can exist in different forms depending on experimental conditions. High-resolution structural methods, such as X-ray crystallography and NMR spectroscopy, have been of limited usefulness in resolving the inherent structural polymorphism associated with G-quadruplex formation. The lack of, or the ambiguous nature of, currently available high-resolution structural data, in turn, has severely hindered investigations into the nature of these structures and their interactions with small-molecule inhibitors. We have used molecular dynamics in conjunction with hydrodynamic bead modeling to study the structures of the human telomeric G-quadruplex-forming sequences at the atomic level. We demonstrated that molecular dynamics can reproduce experimental hydrodynamic measurements and thus can be a powerful tool in the structural study of existing G-quadruplex sequences or in the prediction of new G-quadruplex structures.

Telomerase inhibitor Imetelstat (GRN163L) limits the lifespan of human pancreatic cancer cells

Telomerase is required for the unlimited lifespan of cancer cells. The vast majority of pancreatic adenocarcinomas overexpress telomerase activity and blocking telomerase could limit their lifespan. GRN163L (Imetelstat) is a lipid-conjugated N3'→P5' thio-phosphoramidate oligonucleotide that blocks the template region of telomerase. The aim of this study was to define the effects of long-term GRN163L exposure on the maintenance of telomeres and lifespan of pancreatic cancer cells. Telomere size, telomerase activity, and telomerase inhibition response to GRN163L were measured in a panel of 10 pancreatic cancer cell lines. The cell lines exhibited large differences in levels of telomerase activity (46-fold variation), but most lines had very short telomeres (2-3 kb in size). GRN163L inhibited telomerase in all 10 pancreatic cancer cell lines, with IC50 ranging from 50 nM to 200 nM. Continuous GRN163L exposure of CAPAN1 (IC50 = 75 nM) and CD18 cells (IC50 = 204 nM) resulted in an initial rapid shortening of the telomeres followed by the maintenance of extremely short but stable telomeres. Continuous exposure to the drug eventually led to crisis and to a complete loss of viability after 47 (CAPAN1) and 69 (CD18) doublings. Crisis In these cells was accompanied by activation of a DNA damage response (γ-H2AX) and evidence of both senescence (SA-β-galactosidase activity) and apoptosis (sub-G1 DNA content, PARP cleavage). Removal of the drug after long-term GRN163L exposure led to a reactivation of telomerase and re-elongation of telomeres in the third week of cultivation without GRN163L. These findings show that the lifespan of pancreatic cancer cells can be limited by continuous telomerase inhibition. These results should facilitate the design of future clinical trials of GRN163L in patients with pancreatic cancer.

Advances in the molecular design of potential anticancer agents via targeting of human telomeric DNA

Telomerase is an attractive drug target to develop new generation drugs against cancer.

Luminescent silver nanoclusters anchored by oligonucleotides detect human telomerase ribonucleic acid template

Luminescent silver nanoclusters were anchored by designed oligonucleotides. After hybridizing with human telomerase RNA template, the luminescence of the cluster decreased linearly with respect to the concentration of the complementary strand (25-250 nM). The cluster is therefore a potential candidate for human telomerase detection.

A novel electrochemical biosensor for sensitive detection of telomerase activity based on structure-switching DNA

Telomerase has been considered to be an important tumor biomarker for early cancer diagnostics and a valuable target for therapy treatment. A novel electrochemical biosensor based on structure-switching DNA probe with ferrocene (Fc) as the electroactive reporter to detect telomerase activity was developed. The developed approach displayed desirable dynamic range from 10(2) to 6 × 10(4) Hela cells mL(-1) with a detection limit of 100 Hela cells mL(-1). This biosensor afforded good reproducibility, stability and simple operations. It provided a useful platform for practical use in quantitative telomerase activity assay for clinical applications. Telomerase inhibitor performance was also investigated and the results indicated the approach was suitable for telomerase inhibitor screening research.

Targeting TGF-β signaling in cancer

INTRODUCTION

The transforming growth factor-β (TGF-β) signaling pathway has a pivotal role in tumor suppression and yet, paradoxically, in tumor promotion. Functional context dependent insights into the TGF-β pathway are crucial in developing TGF-β-based therapeutics for cancer.

AREAS COVERED

This review discusses the molecular mechanism of the TGF-β pathway and describes the different ways of tumor suppression by TGF-β. It is then explained how tumors can evade these effects and how TGF-β contributes to further growing and spreading of some of the tumors. In the last part of the review, the data on targeting TGF-β pathway for cancer treatment is assessed. This review focuses on anti-TGF-β based treatment and other options targeting activated pathways in tumors where the TGF-β tumor suppressor pathway is lost. Pre-clinical as well up to date results of the most recent clinical trials are given.

EXPERT OPINION

Targeting the TGF-β pathway can be a promising direction in cancer treatment. However, several challenges still exist, the most important are differentiating between the carcinogenic effects of TGF-β and its other physiological roles, and delineating the tumor suppressive versus the tumor promoting roles of TGF-β in each specific tumor. Future studies are needed in order to find safer and more effective TGF-β-based drugs.

Reciprocal regulation by TLR4 and TGF-β in tumor-initiating stem-like cells

Tumor-initiating stem-like cells (TICs) are resistant to chemotherapy and associated with hepatocellular carcinoma (HCC) caused by HCV and/or alcohol-related chronic liver injury. Using HCV Tg mouse models and patients with HCC, we isolated CD133(+) TICs and identified the pluripotency marker NANOG as a direct target of TLR4, which drives the tumor-initiating activity of TICs. These TLR4/NANOG-dependent TICs were defective in the TGF-β tumor suppressor pathway. Functional oncogene screening of a TIC cDNA library identified Yap1 and Igf2bp3 as NANOG-dependent genes that inactivate TGF-β signaling. Mechanistically, we determined that YAP1 mediates cytoplasmic retention of phosphorylated SMAD3 and suppresses SMAD3 phosphorylation/activation by the IGF2BP3/AKT/mTOR pathway. Silencing of both YAP1 and IGF2BP3 restored TGF-β signaling, inhibited pluripotency genes and tumorigenesis, and abrogated chemoresistance of TICs. Mice with defective TGF-β signaling (Spnb2(+/-) mice) exhibited enhanced liver TLR4 expression and developed HCC in a TLR4-dependent manner. Taken together, these results suggest that the activated TLR4/NANOG oncogenic pathway is linked to suppression of cytostatic TGF-β signaling and could potentially serve as a therapeutic target for HCV-related HCC.

Inducible immortality in hTERT-human mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) are attractive candidates for tissue engineering and cell-based therapy because of their multipotentiality and availability in adult donors. However, in vitro expansion and differentiation of these cells is limited by replicative senescence. The proliferative capacity of hMSCs can be enhanced by ectopic expression of telomerase, allowing for long-term culture. However, hMSCs with constitutive telomerase expression demonstrate unregulated growth and even tumor formation. To address this problem, we used an inducible Tet-On gene expression system to create hMSCs in which ectopic telomerase expression can be induced selectively by the addition of doxycycline (i-hTERT hMSCs). i-hTERT hMSCs have inducible hTERT expression and telomerase activity, and are able to proliferate significantly longer than wild type hMSCs when hTERT expression is induced. They stop proliferating when hTERT expression is turned off and can be rescued when expression is re-induced. They retain multipotentiality in vitro even at an advanced age. We also used a selective inhibitor of telomere elongation to show that the mechanism driving immortalization of hMSCs by hTERT is dependent upon maintenance of telomere length. Thanks to their extended lifespan, preserved multipotentiality and controlled growth, i-hTERT hMSCs may prove to be a useful tool for the development and testing of novel stem cell therapies.

N-(1-Pyrenyl) maleimide inhibits telomerase activity in a cell free system and induces apoptosis in Jurkat cells

Telomerase activity is repressed in normal human somatic cells, but is activated in most cancers, suggesting that telomerase may be an important target for cancer therapy. Agents that interact selectively with telomerase are anticipated to exert specific action on cancer cells. In this study, we evaluated maleimide derivatives for their potency and selectivity of telomerase inhibition. Among the several N-substituted derivatives of maleimide tested, N-(1-Pyrenyl) maleimide was shown to exert the greatest inhibition of telomerase in a cell free system, with an IC50 value of 0.25 μM. Importantly, we demonstrated that N-(1-pyrenyl) maleimide induces apoptosis in Jurkat T cells and displays the greatest differential cytotoxicity against hematopoietic cancer cells. These results suggest that N-(1-pyrenyl) maleimide is an attractive maleimide to be tested and developed as anti-cancer drug.

Functionalized halloysite nanotube-based carrier for intracellular delivery of antisense oligonucleotides

Halloysites are cheap, abundantly available, and natural with high mechanical strength and biocompatibility. In this paper, a novel halloysite nanotube [HNT]-based gene delivery system was explored for loading and intracellular delivery of antisense oligodeoxynucleotides [ASODNs], in which functionalized HNTs [f-HNTs] were used as carriers and ASODNs as a therapeutic gene for targeting survivin. HNTs were firstly surface-modified with γ-aminopropyltriethoxysilane in order to facilitate further biofunctionalization. The f-HNTs and the assembled f-HNT-ASODN complexes were characterized by transmission electron microscopy [TEM], dynamic light scattering, UV-visible spectroscopy, and fluorescence spectrophotometry. The intracellular uptake and delivery efficiency of the complexes were effectively investigated by TEM, confocal microscopy, and flow cytometry. In vitro cytotoxicity studies of the complexes using MTT assay exhibited a significant enhancement in the cytotoxic capability. The results exhibited that f-HNT complexes could efficiently improve intracellular delivery and enhance antitumor activity of ASODNs by the nanotube carrier and could be used as novel promising vectors for gene therapy applications, which is attributed to their advantages over structures and features including a unique tubular structure, large aspect ratio, natural availability, rich functionality, good biocompatibility, and high mechanical strength.

Targeting telomerase-expressing cancer cells

The role of telomeres and telomerase as a target for cancer therapeutics is an area of continuing interest. This review is intended to provide an update on the field, pointing to areas in which our knowledge remains deficient and exploring the details of the most promising areas being advanced into clinical trials. Topics that will be covered include the role of dysfunctional telomeres in cellular aging and how replicative senescence provides an initial barrier to the emergence of immortalized cells, a hallmark of cancer. As an important translational theme, this review will consider possibilities for selectively targeting telomeres and telomerase to enhance cancer therapy. The role of telomerase as an immunotherapy, as a gene therapy approach using telomerase promoter driven oncolytic viruses and as a small oligonucleotide targeted therapy (Imetelstat) will be discussed.

Is telomerase a viable target in cancer?

The ideal cancer treatment would specifically target cancer cells yet have minimal or no adverse effects on normal somatic cells. Telomerase, the ribonucleoprotein reverse transcriptase that maintains the ends of human chromosome, is an attractive cancer therapeutic target for exactly this reason [1]. Telomerase is expressed in more than 85% of cancer cells, making it a nearly universal cancer marker, while the majority of normal somatic cells are telomerase negative. Telomerase activity confers limitless replicative potential to cancer cells, a hallmark of cancer which must be attained for the continued growth that characterizes almost all advanced neoplasms [2]. In this review we will summarize the role of telomeres and telomerase in cancer cells, and how properties of telomerase are being exploited to create targeted cancer therapies including telomerase inhibitors, telomerase-targeted immunotherapies and telomerase-driven virotherapies. A frank and balanced assessment of the current state of telomerase inhibitors with caveats and potential limitations will be included.

Targeting human telomerase for cancer therapeutics

The enzyme telomerase is involved in the replication of telomeres, specialized structures that cap and protect the ends of chromosomes. Its activity is required for maintenance of telomeres and for unlimited lifespan, a hallmark of cancer cells. Telomerase is overexpressed in the vast majority of human cancer cells and therefore represents an attractive target for therapy. Several approaches have been developed to inhibit this enzyme through the targeting of its RNA or catalytic components as well as its DNA substrate, the single-stranded 3'-telomeric overhang. Telomerase inhibitors are chemically diverse and include modified oligonucleotides as well as small diffusable molecules, both natural and synthetic. This review presents an update of recent investigations pertaining to these agents and discusses their biological properties in the context of the initial paradigm that the exposure of cancer cells to these agents should lead to progressive telomere shortening followed by a delayed growth arrest response.

Intracellular efficacy of tumor-targeting group I intron-based trans-splicing ribozyme

BACKGROUND

Group I intron-based trans-splicing ribozyme, which can specifically reprogram human telomerase reverse transcriptase (hTERT) RNA, could be a useful tool for tumor-targeted gene therapy. In the present study, the therapeutic feasibility of this ribozyme was investigated by analyzing trans-splicing efficacy in vivo as well as in cells.

METHODS

We assessed transgene activation, degree of ribozyme expression, targeted hTERT mRNA level, or the level of trans-splicing products in hTERT(+) cells or in human tumor nodules xenografted in animals after ribozyme administration.

RESULTS

The activity and efficacy of the trans-splicing ribozyme in cells was dependent on the amount of endogenous hTERT mRNA and/or the accumulation of ribozyme RNA in cells. Intracellular activity of the ribozyme reached a plateau when no more targetable substrate mRNA was available or the ribozyme RNA level was fully saturated. In addition, the efficacy of ribozyme in xenografted tumor tissues was dependent on the dose of the delivered ribozyme-encoding adenoviral vector, indicating the potential of the ribozyme expression level as a determining factor for the in vivo efficacy of the trans-splicing ribozyme. On the basis of these results, we enhanced the intracellular ribozyme activity by increasing the ribozyme expression level transcriptionally and/or post-transcriptionally.

CONCLUSIONS

We analyzed ribozyme efficacy and determined the most influential factors of its trans-splicing reaction in mammalian cell lines as well as in vivo. The present study could provide insights into the optimization of the trans-splicing ribozyme-based RNA replacement approach to cancer treatment.

Development of potential anticancer agents that target the telomere sequence

The immortality of cancer cells is due to the relatively high concentration of telomerase enzyme that maintains the telomere sequence during cell division. Human telomeric DNA consists of repeats of the sequence d(5'-TTAGGG-3'). Deoxyribonucleic guanidine (DNG) is a DNA analog in which positively charged guanidine [-NH-C(NH2+)-NH-] replaces the negatively charged phosphodiester of DNA. The synthesized DNG hexamer AgAgTgCgCpC and dodecamer AgAgTgCgCgCAgAgTgCgCpC are complementary to the non-coding telomere sequence d(5'-TTAGGG-3'). We have found that binding of the complementary DNG hexamer to the telomere is favored over that of DNA telomere by 10(2.5)-fold and binding the dodecamer with 2-mismatched DNA is favored by 10(5)-fold. We have shown that DNG binding to RNA is favored over binding to DNA. A complementary complex of DNG with RNA at the active site of telomerase enzyme would be very stable.