Telomere shortening and apoptosis in telomerase-inhibited human tumor cells

Telomere shortening in human diseases

The discovery of telomeres dates back to the early 20th century. In humans, telomeres are heterochromatic structures with tandem DNA repeats of 5'-TTAGGG-3' at the chromosomal ends. Telomere length varies greatly among species and ranges from 10 to 15 kb in humans. With each cell division, telomeres shorten progressively because of the 'end-replication problem'. Short or dysfunctional telomeres are often recognized as DNA DSBs, triggering cell-cycle arrest and result in cellular senescence or apoptotic cell death. Therefore, telomere shortening serves as an important tumor-suppressive mechanism by limiting cellular proliferative capacity by regulating senescence checkpoint activation. Although telomeres serve as a mitotic clock to cells, they also confer capping on chromosomes, with help from telomere-associated proteins. Over the past decades, many studies of telomere biology have demonstrated that telomeres and telomere-associated proteins are implicated in human genetic diseases. In addition, it has become more apparent that accelerated telomere erosion is associated with a myriad of metabolic and inflammatory diseases. Moreover, critically short or unprotected telomeres are likely to form telomeric fusions, leading to genomic instability, the cornerstone for carcinogenesis. In light of these, this minireview summarizes studies on telomeres and telomere-associated proteins in human diseases. Elucidating the roles of telomeres involved in the mechanisms underlying pathogenesis of these diseases may open up new possibilities for novel molecular targets as well as provide important diagnostic and therapeutic implications.

Telomerase reverse transcriptase inhibition stimulates cyclooxygenase 2 expression in cancer cells and synergizes with celecoxib to exert anti-cancer effects

Background:

Telomerase and telomerase reverse transcriptase (hTERT) confer cancer cells sustained proliferation and survival potentials. Targeting telomerase or hTERT is a novel anti-cancer strategy. However, telomerase/hTERT inhibition alone has minimal clinical efficacy. We explored the relationship between hTERT and cyclooxygenase 2 (COX2) and evaluated synergistic anti-cancer effects of targeting both hTERT and COX2.

Methods:

hTERT was depleted in gastric and cervical cancer cells using small interfering RNA (siRNA) and analysed for COX2 expression using quantitative PCR and immunoblotting. Viable cells and apoptotic cells in gastric cancer cells treated with hTERT siRNA or/and the COX2 inhibitor celecoxib were measured using Trypen blue exclusion and flow cytometry. The in vivo anti-cancer effect of hTERT depletion or/and celecoxib was evaluated using mouse xenograft models.

Results:

Knocking down hTERT expression in cancer cells led to robust increases in mRNA and protein levels of COX2. The COX2 promoter activity increased substantially in hTERT-depleted cells. hTERT depletion led to the activation of p38 mitogen-activated protein kinase responsible for the stimulation of COX2 gene transcription. hTERT depletion or celecoxib alone did not affect cancer cell survival, whereas their combination synergistically killed them both in vitro and in vivo.

Conclusion:

hTERT induces COX2 expression and simultaneously targeting hTERT and COX2 synergistically kills cancer cells.

The chromodomain helicase Chd4 is required for Polycomb-mediated inhibition of astroglial differentiation

Polycomb group (PcG) proteins form transcriptional repressor complexes with well-established functions during cell-fate determination. Yet, the mechanisms underlying their regulation remain poorly understood. Here, we extend the role of Polycomb complexes in the temporal control of neural progenitor cell (NPC) commitment by demonstrating that the PcG protein Ezh2 is necessary to prevent the premature onset of gliogenesis. In addition, we identify the chromodomain helicase DNA-binding protein 4 (Chd4) as a critical interaction partner of Ezh2 required specifically for PcG-mediated suppression of the key astrogenic marker gene GFAP. Accordingly, in vivo depletion of Chd4 in the developing neocortex promotes astrogenesis. Collectively, these results demonstrate that PcG proteins operate in a highly dynamic, developmental stage-dependent fashion during neural differentiation and suggest that target gene-specific mechanisms regulate Polycomb function during sequential cell-fate decisions.

Depletion of telomerase RNA inhibits growth of gastrointestinal tumors transplanted in mice

AIM: To explore effects of telomerase RNA-targeting phosphorothioate antisense oligodeoxynucleotides (PS-ASODN) on growth of human gastrointestinal stromal tumors transplanted in mice.

METHODS: A SCID mouse model for transplantation of human gastrointestinal stromal tumors (GISTs) was established using tumor cells from a patient who was diagnosed with GIST and consequently had been treated with imatinib. GIST cells cultured for 10 passages were used for inoculation into mice. Transfection of PS-ASODN was carried out with Lipotap Liposomal Transfection Reagent. GISTs that subsequently developed in SCID mice were subjected to intra-tumoral injection once daily from day 7 to day 28 post-inoculation, and mice were divided into the following four groups according to treatment: PS-ASODN group (5.00 μmoL/L of oligonucleotide, each mouse received 0.2 mL once daily); imatinib group (0.1 mg/g body weight); liposome negative control group (0.01 mL/g); and saline group (0.01 mL/g). On day 28, the mice were sacrificed, and tumor attributes including weight and longest and shortest diameters were measured. Tumor growth was compared between treatment groups, and telomerase activity was measured by enzyme-linked immunosorbent assay. Apoptosis was examined by flow cytometry. Real-time polymerase chain reaction was used to detect expression of the mRNA encoding the apoptosis inhibition B-cell leukemia/lymphoma 2 (bcl-2) gene.

RESULTS: In the PS-ASODN group, tumor growth was inhibited by 59.437%, which was markedly higher than in the imatinib group (11.071%) and liposome negative control group (2.759%) [tumor inhibition = (mean tumor weight of control group - mean tumor weight of treatment group)/(mean tumor weight of control group) × 100%]. Telomerase activity was significantly lower (P < 0.01) in the PS-ASODN group (0.689 ± 0.158) compared with the imatinib group (1.838 ± 0.241), liposome negative control group (2.013 ± 0.273), and saline group (2.004 ± 0.163). Flow cytometry revealed that the apoptosis rate of tumor cells treated with PS-ASODN was 20.751% ± 0.789%, which was higher (P < 0.01) than that of the imatinib group (1.163% ± 0.469%), liposome negative control group (1.212% ± 0.310%), and saline group (1.172% ± 0.403%). Expression of bcl-2 mRNA in the transplanted GISTs was markedly downregulated (P < 0.01) in the PS-ASODN group (7.245 ± 0.739) compared with the imatinib group (14.153 ± 1.618) and liposome negative control group (16.396 ± 1.351).

CONCLUSION: PS-ASODN can repress GIST growth, mediated perhaps by inhibition of telomerase activity and downregulation of bcl-2 expression.

Inhibition of telomerase activity by oleanane triterpenoid CDDO-Me in pancreatic cancer cells is ROS-dependent

Methyl-2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oate (CDDO-Me) is a synthetic derivative of oleanolic acid, a triterpene, with apoptosis-inducing activity in a wide range of cancer cells. Induction of apoptosis by CDDO-Me is associated with the generation of reactive oxygen species (ROS) and inhibition of telomerase activity. In the present study, we investigated the role of ROS in inhibition of telomerase by CDDO-me. Treatment of MiaPaCa-2 and Panc-1 pancreatic cancer cell lines with CDDO-Me induced the production of hydrogen peroxide and superoxide anions and inhibited the telomerase activity. Pretreatment of cells with N-acetylcycsteine, a general purpose antioxidant or overexpression of glutathione peroxidase (GPx) or superoxide dismutase-1 (SOD-1) blocked the telomerase inhibitory activity of CDDO-Me. Furthermore, blocking ROS generation also prevented the inhibition of hTERT gene expression, hTERT protein production and expression of a number of hTERT-regulatory proteins by CDDO-Me (e.g., c-Myc, Sp1, NF-κB and p-Akt). Data also showed that Akt plays an important role in the activation of telomerase activity. Together, these data suggest that inhibition of telomerase activity by CDDO-Me is mediated through a ROS-dependent mechanism; however, more work is needed to fully understand the role of ROS in down-regulation of hTERT gene and hTERT-regulatory proteins by CDDO-Me.

The telomere/telomerase binding factor PinX1 is a new target to improve the radiotherapy effect of oesophageal squamous cell carcinomas

Chemoradiotherapy (CRT) is a standard treatment for oesophageal squamous cell carcinoma (ESCC) in its advanced stages. The telomerase/telomere interacting protein PinX1 contributes to telomere maintenance, tumourigenicity, and influences the DNA damage agent-induced apoptotic response in telomerase-positive cancer cells. However, the clinical and biological significance of PinX1 in human ESCCs remains unclear. We examined the expression dynamics of PinX1 by immunohistochemistry in a learning cohort (n = 98) and a validation cohort (n = 59) of ESCC patients treated with definite chemoradiotherapy (CRT). A series of in vivo and in vitro assays were performed to elucidate the effect of PinX1 on ESCC cells' CRT response and underlying mechanisms. Knockdown of PinX1 did not affect ESCC cells' chemosensitivities to 5-fluorouracil and cisplatin, but substantially increased ESCC cells' therapeutic efficacy of radiation both in vitro and in vivo. Ectopic overexpression of PinX1 dramatically enhanced ESCC cells' resistance to radiotherapy. Furthermore, we demonstrated that PinX1 resistance to radiotherapy (RT) was attributed to PinX1 maintaining telomere stability, reducing ESCC cell death by RT-induced mitosis catastrophe (MC). High expression of Pinx1 correlated positively with ESCC's resistance to CRT, and was a strong and independent predictor for short disease-specific survival (DSS) of ESCC patients. Our data suggest that PinX1 could serve as a novel predictor for a CRT response to ESCC patients, and the pathway of PinX1-mediated telomere stability might represent a new target to improve the RT effect of ESCC.

Induction of senescence in cancer cells by the G-quadruplex stabilizer, BMVC4, is independent of its telomerase inhibitory activity

BACKGROUND AND PURPOSE

Telomerase is the enzyme responsible for extending G-strand telomeric DNA and represents a promising target for treatment of neoplasia. Inhibition of telomerase can be achieved by stabilization of G-quadruplex DNA structures. Here, we characterize the cellular effects of a novel G-quadruplex stabilizing compound, 3,6-bis(4-methyl-2-vinylpyrazinium iodine) carbazole (BMVC4).

EXPERIMENTAL APPROACH

The cellular effects of BMVC4 were characterized in both telomerase-positive and alternative lengthening of telomeres (ALT) cancer cells. The molecular mechanism of how BMVC4 induced senescence is also addressed.

KEY RESULTS

BMVC4-treated cancer cells showed typical senescence phenotypes. BMVC4 induced senescence in both ALT and telomerase-overexpressing cells, suggesting that telomere shortening through telomerase inhibition might not be the cause for senescence. A large fraction of DNA damage foci was not localized to telomeres in BMVC4-treated cells and BMVC4 suppressed c-myc expression through stabilizing the G-quadruplex structure located at its promoter. These results indicated that the cellular targets of BMVC4 were not limited to telomeres. Further analyses showed that BMVC4 induced DNA breaks and activation of ataxia telangiectasia-mutated mediated DNA damage response pathway.

CONCLUSIONS AND IMPLICATIONS

BMVC4, a G-quadruplex stabilizer, induced senescence by activation of pathways of response to DNA damage that was independent of its telomerase inhibitory activity. Thus, BMVC4 has the potential to be developed as a chemotherapeutic agent against both telomerase positive and ALT cancer cells.

Telomerase reverse transcriptase (TERT) is a therapeutic target of oleanane triterpenoid CDDO-Me in prostate cancer

Methyl-2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oate (CDDO-Me) is an synthetic oleanane triterpenoid with strong antiprolifertive and proapoptotic activities in cancer cells. However, the effect of CDDO-Me on human telomerase reverse transcriptase (hTERT) and its telomerase activity in prostate cancer cells has not been studied. We investigated the role of hTERT in mediating the anticancer activity of CDDO-Me in prostate cancer cells in vitro and in vivo. The inhibition of cell proliferation and induction of apoptosis by CDDO-Me in LNCaP and PC-3 prostate cancer cell lines was associated with the inhibition of hTERT gene expression, hTERT telomerase activity and a number of proteins that regulate hTERT transcriptionally and post-translationally. Furthermore, ablation of hTERT protein increased the sensitivity of cancer cells to CDDO-Me, whereas its overexpression rendered them resistant to CDDO-Me. In addition, inhibition of progression of preneoplastic lesions (i.e., low and high-grade prostate intraepithelial neoplasms, PINs) to adenocarcinoma of the prostate by CDDO-Me in TRAMP mice was associated with significant decrease in TERT and its regulatory proteins in the prostate gland. These data provide evidence that telomerase is a potential target of CDDO-Me for the prevention and treatment of prostate cancer.

Effect of targeted silencing of hTERT mRNA by lentivirus-mediated siRNA on A549 lung cancer cells in vitro

In our present study, we took advantage of the characteristics of RNA interference technology, which can efficiently, stably, and specifically silence target genes, and designed a small interfering RNA (siRNA) that could specifically target hTERT mRNA. We used a lentiviral vector (LV) to deliver the hTERT siRNA into telomerase-positive A549 lung cancer cells and investigated the effect of hTERT siRNA on the hTERT mRNA levels, hTERT protein levels, cell proliferation, and apoptosis in the lung cancer cells. The results from quantitative PCR, Western blotting, and the MTT assay showed that the expression levels of both hTERT mRNA and protein in the cells were significantly decreased and that the cell proliferation rate started to significantly slow down at 48 h after transfection with hTERT-LV. Our study demonstrated that siRNA sequences specifically targeting hTERT mRNA, which were packaged into lentivirus particles and then used to transfect the lung cancer cell line A549, can specifically silence the mRNA of the target gene, hTERT, and then reduce the hTERT protein expression level, which, in turn, reduces cell proliferation, inhibits cell growth, and induces apoptosis.

Telomerase reverse transcriptase promotes epithelial-mesenchymal transition and stem cell-like traits in cancer cells

Telomerase activation through induction of telomerase reverse transcriptase (hTERT) contributes to malignant transformation by stabilizing telomeres. Clinical studies demonstrate that higher hTERT expression is associated with cancer progression and poor outcomes, but the underlying mechanism is unclear. Because epithelial-mesenchymal transition (EMT) and cancer stem cells (CSCs) are key factors in cancer metastasis and relapse, and hTERT has been shown to exhibit multiple biological activities independently of its telomere-lengthening function, we address a potential role of hTERT in EMT and CSCs using gastric cancer (GC) as a model. hTERT overexpression promotes, whereas its inhibition suppresses, EMT and stemness of GC cells, respectively. Transforming growth factor (TGF)-β1 and β-catenin-mediated EMT was abolished by small interfering RNA depletion of hTERT expression. hTERT interacts with β-catenin, enhances its nuclear localization and transcriptional activity, and occupies the β-catenin target vimentin promoter. All these hTERT effects were independent of its telomere-lengthening function or telomerase activity. hTERT and EMT marker expression correlates positively in GC samples. Mouse experiments demonstrate the in vivo stimulation of hTERT on cancer cell colonization. Collectively, hTERT stimulates EMT and induces stemness of cancer cells, thereby promoting cancer metastasis and recurrence. Thus, targeting hTERT may prevent cancer progression by inhibiting EMT and CSCs.

Antitumor effect of new multiple antigen peptide based on HLA-A0201-restricted CTL epitopes of human telomerase reverse transcriptase (hTERT)

The development of peptide vaccines aimed at enhancing immune responses against tumor cells is becoming a promising area of research. Human telomerase reverse transcriptase (hTERT) is an ideal universal target for novel immunotherapies against cancers. The aim of this work was to verify whether the multiple antigen peptides (MAP) based on HLA-A0201-restricted CTL epitopes of hTERT could trigger a better and more sustained CTL response and kill multiple types of hTERT-positive tumor cells in vitro and ex vivo. Dendritic cells (DC) pulsed with MAP based on HLA-A0201-restricted CTL epitopes of hTERT (hTERT-540, hTERT-865 and hTERT-572Y) were used to evaluate immune responses against various tumors and were compared to the immune responses resulting from the use of corresponding linear epitopes and a recombinant adenovirus-hTERT vector. A 4-h standard (51) Cr-release assay and an ELISPOT assay were used for both in vitro and ex vivo analyses. Results demonstrated that targeting hTERT with an adenovector was the most effective way to stimulate a CD8(+) T cell response. When compared with linear hTERT epitopes, MAP could trigger stronger hTERT-specific CTL responses against tumor cells expressing hTERT and HLA-A0201. In contrast, the activated CTL could neither kill the hTERT-negative tumor cells, such as U2OS cells, nor kill HLA-A0201 negative cells, such as HepG2 cells. We also found that these peptide-specific CTL could not kill autologous lymphocytes and DC with low telomerase activity. Our results indicate that MAP from hTERT can be exploited for cancer immunotherapy.

A small molecule inhibitor of Pot1 binding to telomeric DNA

Chromosome ends are complex structures, consisting of repetitive DNA sequence terminating in an ssDNA overhang with many associated proteins. Because alteration of the regulation of these ends is a hallmark of cancer, telomeres and telomere maintenance have been prime drug targets. The universally conserved ssDNA overhang is sequence-specifically bound and regulated by Pot1 (protection of telomeres 1), and perturbation of Pot1 function has deleterious effects for proliferating cells. The specificity of the Pot1/ssDNA interaction and the key involvement of this protein in telomere maintenance have suggested directed inhibition of Pot1/ssDNA binding as an efficient means of disrupting telomere function. To explore this idea, we developed a high-throughput time-resolved fluorescence resonance energy transfer (TR-FRET) screen for inhibitors of Pot1/ssDNA interaction. We conducted this screen with the DNA-binding subdomain of Schizosaccharomyces pombe Pot1 (Pot1pN), which confers the vast majority of Pot1 sequence-specificity and is highly similar to the first domain of human Pot1 (hPOT1). Screening a library of ∼20 000 compounds yielded a single inhibitor, which we found interacted tightly with sub-micromolar affinity. Furthermore, this compound, subsequently identified as the bis-azo dye Congo red (CR), was able to competitively inhibit hPOT1 binding to telomeric DNA. Isothermal titration calorimetry and NMR chemical shift analysis suggest that CR interacts specifically with the ssDNA-binding cleft of Pot1, and that alteration of this surface disrupts CR binding. The identification of a specific inhibitor of ssDNA interaction establishes a new pathway for targeted telomere disruption.

hTERT mutations associated with idiopathic pulmonary fibrosis affect telomerase activity, telomere length, and cell growth by distinct mechanisms

Telomerase is a ribonucleoprotein reverse transcriptase (RT) that synthesizes specific DNA repeats, or telomeric DNA, at the ends of chromosomes. Telomerase is minimally composed of a protein subunit, TERT, and an RNA component, TR. Aberrant telomerase activity has been associated with most human cancers and several premature aging diseases, such as idiopathic pulmonary fibrosis (IPF), a chronic, progressive, and fatal lung disease characterized by alveolar epithelial cell damage and fibrosis. Our study focuses on three hTERT mutations that were identified in a subset of patients with IPF, in which these patients also exhibited shorter telomeres compared with age-matched controls. We characterized how three IPF-associated hTERT mutations, V144M, R865C, and R865H, affected telomerase function both in vitro and in human cells. We demonstrated that the R865 residue is crucial for repeat addition processivity and thus telomere synthesis in telomerase-positive 293 cells and telomerase-negative BJ cells, consistent with its location in the hTERT nucleotide-binding motif. In contrast, while the V144M mutant did not exhibit any biochemical defects, this mutant was unable to elongate telomeres in human cells. As a result, our studies have identified hTERT V144 and R865 as two critical residues required for proper telomerase function in cells. Together, this may explain how inherited hTERT mutations can lead to shortened telomeres in patients with IPF and, thus, provide further insight into the role of naturally occurring telomerase mutations in the pathophysiology of certain age-related disease states.

Inhibition of cell proliferation and induction of apoptosis by oleanane triterpenoid (CDDO-Me) in pancreatic cancer cells is associated with the suppression of hTERT gene expression and its telomerase activity

Methyl-2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oate (CDDO-Me) is a multifunctional oleanane synthetic triterpenoid with potent anti-inflammatory and antitumorigenic properties. The mechanisms of the antisurvival and apoptosis-inducing activities of CDDO-Me and related derivatives of oleanolic acid have been defined; however, to date, no study has been carried out on the effect of CDDOs on human telomerase reverse transcriptase (hTERT) gene or telomerase activity. Here we report for the first time that inhibition of cell proliferation and induction of apoptosis by CDDO-Me in pancreatic cancer cell lines is associated with the inhibition of hTERT gene expression, hTERT telomerase activity and a number of proteins that regulate hTERT expression and activity. Furthermore, abrogation or overexpression of hTERT protein altered the susceptibility of tumor cells to CDDO-Me. These findings suggest that telomerase (hTERT) is a relevant target of CDDO-Me in pancreatic cancer cells.

Vascular dysfunction by myofibroblast activation in patients with idiopathic pulmonary fibrosis and prognostic significance

In this study, we demonstrated the importance of telomerase protein expression and determined the relationships among telomerase, endothelin-1 (ET-1) and myofibroblasts during early and late remodeling of parenchymal and vascular areas in usual interstitial pneumonia (UIP) using 27 surgical lung biopsies from patients with idiopathic pulmonary fibrosis (IPF). Telomerase+, myofibroblasts α-SMA+, smooth muscle cells caldesmon+, endothelium ET-1+ cellularity, and fibrosis severity were evaluated in 30 fields covering normal lung parenchyma, minimal fibrosis (fibroblastic foci), severe (mural) fibrosis, and vascular areas of UIP by the point-counting technique and a semiquantitative score. The impact of these markers was determined in pulmonary functional tests and follow-up until death from IPF. Telomerase and ET-1 expression was significantly increased in normal and vascular areas compared to areas of fibroblast foci. Telomerase and ET-1 expression was inversely correlated with minimal fibrosis in areas of fibroblast foci and directly associated with severe fibrosis in vascular areas. Telomerase activity in minimal fibrosis areas was directly associated with diffusing capacity of the lung for oxygen/alveolar volume and ET-1 expression and indirectly associated with diffusing capacity of the lungs for carbon monoxide and severe fibrosis in vascular areas. Cox proportional hazards regression revealed a low risk of death for females with minimal fibrosis displaying high telomerase and ET-1 expression in normal areas. Vascular dysfunction by telomerase/ET-1 expression was found earlier than vascular remodeling by myofibroblast activation in UIP with impact on IPF evolution, suggesting that strategies aimed at preventing the effect of these mediators may have a greater impact on patient outcome.

Combinatorial assembly and function of chromatin regulatory complexes

The introduction of new methods for genome-wide analyses of the chromatin state, together with the power of refined techniques for mass spectrometry and biochemistry, has provided an unprecedented view on the complexity of eukaryotic gene regulation. Chromatin structure, the state of histone modifications and DNA methylation are highly dynamic and subject to various levels of regulation. In addition, the subunit compositions of the protein complexes that bring about these changes appear to be assembled in a combinatorial manner that is specific for the cell type and developmental stage, providing increased specificity to these complexes. Here we discuss recent evidence regarding the combinatorial control of chromatin regulatory complexes.

Long telomeres bypass the requirement for telomere maintenance in human tumorigenesis

Despite the importance of telomere maintenance in cancer cell survival via the elongation of telomeres by telomerase reverse transcriptase (TERT) or alternative lengthening of telomeres (ALT), it had not been tested directly whether telomere maintenance is dispensable for human tumorigenesis. We engineered human tumor cells containing loxP-flanked hTERT to enable extensive telomere elongation prior to complete hTERT excision. Despite unabated telomere erosion, hTERT-excised cells formed tumors in mice and proliferated in vitro for up to 1 year. Telomerase reactivation or ALT was not observed, and the eventual loss of telomeric signal coincided with loss of tumorigenic potential and cell viability. Crisis was averted via the reintroduction of active but not inactive hTERT. Thus, telomere maintenance is dispensable for human tumorigenesis when telomere reserves are long. Yet, despite telomere instability and the presence of oncogenic RAS, human tumors remain susceptible to crisis induced by critically short telomeres.

A role for sister telomere cohesion in telomere elongation by telomerase

Telomere length homeostasis is achieved by a balance of telomere shortening caused by DNA replication and nucleolytic attack and telomere lengthening by telomerase. The importance of telomere length maintenance to human health is best illustrated by dyskeratosis congenita (DC) a disease of telomere shortening caused by mutations in telomerase subunits. DC patients suffer stem cell depletion and die of bone marrow stem cell failure. Recently a new class of particularly severe DC patients was found to harbor mutations in the shelterin subunit TIN2. The DC-TIN2 mutations were clustered in small domain of unknown function. In a recently published study we showed that the DC mutation cluster in TIN2 harbored a binding site for heterochromatin protein 1 (HP1) and further, that HP1 binding to TIN2 was required for sister telomere cohesion in S phase and for telomere length maintenance by telomerase. We briefly review and discuss the implications of our findings in this Extra View, and present some new data that may shed light on how sister telomere cohesion could influence telomere elongation by telomerase.

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.

Twisted epithelial-to-mesenchymal transition promotes progression of surviving bladder cancer T24 cells with hTERT-dysfunction

Background

Human cancer cells maintain telomeres to protect cells from senescence through telomerase activity (TA) or alternative lengthening of telomeres (ALT) in different cell types. Moreover, cellular senescence can be bypassed by Epithelial-to-mesenchymal transition (EMT) during cancer progression in diverse solid tumors. However, it has not been elucidated the characteristics of telomere maintenance and progression ability after long-term culture in bladder cancer T24 cells with hTERT dysfunction.

Methodology/Principal Findings

In this study, by using a dominant negative mutant human telomerase reverse transcriptase (hTERT) vector to inhibit TA in bladder cancer T24 cells, we observed the appearance of long phenotype of telomere length and the ALT-associated PML body (APB) complex after the 27^th passage, indicating the occurrence of ALT-like pathway in surviving T24/DN868A cells with telomerase inhibition. Meanwhile, telomerase inhibition resulted in significant EMT as shown by change in cellular morphology concomitant with variation of EMT markers. Consistently, the surviving T24/DN868A cells showed increased progression ability in vitro and in vivo. In addition, we found Twist was activated to mediate EMT in surviving T24/DN868A samples.

Conclusions/Significance

Taken together, our findings indicate that bladder cancer T24 cells may undergo the telomerase-to-ALT-like conversion and promote cancer progression at advanced stages through promoting EMT, thus providing novel possible insight into the mechanism of resistance to telomerase inhibitors in cancer treatment.

Epigenetic-based companion diagnostics

In current medical practice, when a patient is diagnosed with cancer the treating physician generally follows a standard protocol, assigning the treatment that gives a favorable response in the largest proportion of patients. However, in many individual instances this approach may not be the most effective solution and, typically, treatment is only initiated or altered once the cancer has actually started progressing. During this process, patients will lose treatment time waiting to start chemotherapy or will endure severe side effects associated with toxic chemotherapeutic treatments. While some patients are undertreated because current diagnostic methods cannot provide accurate enough information regarding the aggressiveness or drug response of their disease, others with nonaggressive forms of cancer are overtreated and unnecessarily undergo the side effects associated with chemotherapeutic treatment. Epigenetic markers have been widely investigated and are considered key regulators of cellular transcription. Histone modifications and DNA methylation have been demonstrated to play key roles in maintaining stem-cell-like states, cellular differentiation and cancer. In particular, DNA methylation is a frequent, abundant and stable cancer mark, with an inherent role in oncogenesis and tumor progression. In this article, the potential of DNA methylation as a companion diagnostic is assessed, illustrated by exploring some development paths. Epigenetic silencing of MGMT is a key example of how biomarker development, biological pathways and clinical utility come together, serving as a hallmark of epigenetic companion diagnostics.

Genomic profiling of glioblastoma: convergence of fundamental biologic tenets and novel insights

With advances in genomic profiling and sequencing technology, we are beginning to understand the landscape of the genetic events that accumulated during the neoplastic process. The insights gleamed from these genomic profiling studies with regards to glioblastoma etiology has been particularly satisfying because it cemented the clinical pertinence of major concepts in cancer biology-concepts developed over the past three decades. This article will review how the glioblastoma genomic data set serves as an illustrative platform for the concepts put forward by Hanahan and Weinberg on the cancer phenotype. The picture emerging suggests that most glioblastomas evolve along a multitude of pathways rather than a single defined pathway. In this context, the article will further provide a discussion of the subtypes of glioblastoma as they relate to key principles of developmental neurobiology.

Molecular regulation of telomerase activity in aging

The process of aging is mitigated by the maintenance and repair of chromosome ends (telomeres), resulting in extended lifespan. This review examines the molecular mechanisms underlying the actions and regulation of the enzyme telomerase reverse transcriptase (TERT), which functions as the primary mechanism of telomere maintenance and regulates cellular life expectancy. Underpinning increased cell proliferation, telomerase is also a key factor in facilitating cancer cell immortalization. The review focuses on aspects of hormonal regulations of telomerase, and the intracellular pathways that converge to regulate telomerase activity with an emphasis on molecular interactions at protein and gene levels. In addition, the basic structure and function of two key telomerase enzyme components-the catalytic subunit TERT and the template RNA (TERC) are discussed briefly.

A single-molecule platform for investigation of interactions between G-quadruplexes and small-molecule ligands

Ligands that stabilize the formation of telomeric DNA G-quadruplexes have potential as cancer treatments, because the G-quadruplex structure cannot be extended by telomerase, an enzyme over-expressed in many cancer cells. Understanding the kinetic, thermodynamic and mechanical properties of small-molecule binding to these structures is therefore important, but classical ensemble assays are unable to measure these simultaneously. Here, we have used a laser tweezers method to investigate such interactions. With a force jump approach, we observe that pyridostatin promotes the folding of telomeric G-quadruplexes. The increased mechanical stability of pyridostatin-bound G-quadruplex permits the determination of a dissociation constant K(d) of 490 ± 80 nM. The free-energy change of binding obtained from a Hess-like process provides an identical K(d) for pyridostatin and a K(d) of 42 ± 3 µM for a weaker ligand RR110. We anticipate that this single-molecule platform can provide detailed insights into the mechanical, kinetic and thermodynamic properties of liganded bio-macromolecules, which have biological relevance.

MMP-9 silencing regulates hTERT expression via β1 integrin-mediated FAK signaling and induces senescence in glioma xenograft cells

In more than 90% of cancers including glioma, telomere elongation reverse transcriptase (hTERT) is overexpressed. In the present study, we sought to explore whether matrix metalloproteinase-9 (MMP-9) shRNA could alter hTERT-mediated proliferation in glioma cells. MMP-9 shRNA induced senescence and apoptosis in glioma cells by inhibiting hTERT expression and telomere activity. MMP-9 silencing decreased oncogenic c-Myc expression (hTERT activator), whereas the expression of the c-Myc antagonist MAD increased drastically (hTERT repressor); both c-Myc and MAD are transcription factors for hTERT. In addition, MMP-9 suppression turns the switch from c-Myc/MAX to MAD/MAX heterodimer binding to the hTERT promoter as determined by chromatin immunoprecipitation assay. We also show that silencing MAD via siRNA restored hTERT expression and inhibited senescence in glioma cells. MMP-9 transcriptional suppression decreased the expression of FAK, phospho FAK and β1 integrin in glioma xenograft cells. Further, MMP-9 suppression decreased the interaction of β1 integrin/FAK and also MMP-9/β1 integrin as confirmed by immunoprecipitation analysis. Studies with either function blocking β1 integrin or FAK shRNA indicate that suppression of MMP-9 decreased β1 integrin-mediated induction of FAK, which led to decreased hTERT expression. Moreover, 4910 and 5310 glioma xenograft tissue sections from mice treated with MMP-9 shRNA showed reduced expression of FAK/c-Myc and elevated MAD levels. Decreased co-localization of β1 integrin and MMP-9 was associated with MMP-9-suppressed tumor sections. Further, immunoprecipitation analysis showed decreased association of proteins involved in telomere end repair in MMP-9 shRNA-treated glioma cells. Elevated levels of p73 and TRAIL and the results of the FACS analysis show induction of apoptosis in MMP-9-silenced glioma cells. Taken together, these data provide new insights into the mechanisms underlying MMP-9-mediated hTERT expression in glioma proliferation.

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.

Telomerase as a tumor-associated antigen for cancer immunotherapy

Telomerase reverse transcriptase hTERT is an attractive target for cancer immunotherapy given its broad expression in human tumors and its demonstrated immunogenicity. Human and murine model systems demonstrate that CD8(+) cytotoxic T-lymphocytes (CTL) and CD4(+) helper T-lymphocytes can recognize dominant epitopes derived from TERT. CTL kill TERT-positive tumor cells of multiple histologies, although there is some disagreement regarding the level of processing and presentation of certain TERT peptides within the context of MHC class I molecules. CTL recognizing modified, low-affinity cryptic TERT epitopes have also been generated that protect against tumor challenge in a murine model. Several phase I clinical trials testing hTERT as a cancer vaccine target have shown the induction of T-cell immune responses but minimal toxicities, including bone marrow toxicity, in patients with multiple types of cancer. Several studies report some patients experiencing clinical benefit, including partial tumor regression, providing further encouragement for hTERT as broadly applicable target for cancer immunotherapy.

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.

Indole-3-carbinol downregulation of telomerase gene expression requires the inhibition of estrogen receptor-alpha and Sp1 transcription factor interactions within the hTERT promoter and mediates the G1 cell cycle arrest of human breast cancer cells

Indole-3-carbinol (I3C), a naturally occurring hydrolysis product of glucobrassicin from cruciferous vegetables such as broccoli, cabbage and Brussels sprouts, is an anticancer phytochemical that triggers complementary sets of antiproliferative pathways to induce a cell cycle arrest of estrogen-responsive MCF7 breast cancer cells. I3C strongly downregulated transcript expression of the catalytic subunit of the human telomerase (hTERT) gene, which correlated with the dose-dependent indole-mediated G(1) cell cycle arrest without altering the transcript levels of the RNA template (hTR) for telomerase elongation. Exogenous expression of hTERT driven by a constitutive promoter prevented the I3C-induced cell cycle arrest and rescued the I3C inhibition of telomerase enzymatic activity and activation of cellular senescence. Time course studies showed that I3C downregulated expression of estrogen receptor-alpha (ERα) and cyclin-dependent kinase-6 transcripts levels (which is regulated through the Sp1 transcription factor) prior to the downregulation of hTERT suggesting a mechanistic link. Chromatin immunoprecipitation assays demonstrated that I3C disrupted endogenous interactions of both ERα and Sp1 with an estrogen response element-Sp1 composite element within the hTERT promoter. I3C inhibited 17β-estradiol stimulated hTERT expression and stimulated the production of threonine-phosphorylated Sp1, which inhibits Sp1-DNA interactions. Exogenous expression of both ERα and Sp1, but not either alone, in MCF7 cells blocked the I3C-mediated downregulation of hTERT expression. These results demonstrate that I3C disrupts the combined ERα- and Sp1-driven transcription of hTERT gene expression, which plays a significant role in the I3C-induced cell cycle arrest of human breast cancer cells.

Molecular and cellular pathways associated with chromosome 1p deletions during colon carcinogenesis

Chromosomal instability is a major pathway of sporadic colon carcinogenesis. Chromosome arm 1p appears to be one of the "hot spots" in the non-neoplastic mucosa that, when deleted, is associated with the initiation of carcinogenesis. Chromosome arm 1p contains genes associated with DNA repair, spindle checkpoint function, apoptosis, multiple microRNAs, the Wnt signaling pathway, tumor suppression, antioxidant activities, and defense against environmental toxins. Loss of 1p is dangerous since it would likely contribute to genomic instability leading to tumorigenesis. The 1p deletion-associated colon carcinogenesis pathways are reviewed at the molecular and cellular levels. Sporadic colon cancer is strongly linked to a high-fat/low-vegetable/low-micronutrient, Western-style diet. We also consider how selected dietary-related compounds (eg, excess hydrophobic bile acids, and low levels of folic acid, niacin, plant-derived antioxidants, and other modulatory compounds) might affect processes leading to chromosomal deletions, and to the molecular and cellular pathways specifically altered by chromosome 1p loss.

Human telomerase catalytic subunit (hTERT) suppresses p53-mediated anti-apoptotic response via induction of basic fibroblast growth factor

Although human telomerase catalytic subunit (TERT) has several cellular functions including telomere homeostasis, genomic stability, cell proliferation, and tumorigenesis, the molecular mechanism underlying anti-apoptosis regulated by TERT remains to be elucidated. Here, we show that ectopic expression of TERT in spontaneously immortalized human fetal fibroblast (HFFS) cells, which are a telomerase- and p53-positive, leads to increases of cell proliferation and transformation, as well as a resistance to DNA damage response and inactivation of p53 function. We found that TERT and a mutant TERT (no telomerase activity) induce expression of basic fibroblast growth factor (bFGF), and ectopic expression of bFGF also allows cells to be resistant to DNA-damaging response and to suppress activation of p53 function under DNA-damaging induction. Furthermore, loss of TERT or bFGF markedly increases a p53 activity and DNA-damage sensitivity in HFFS, HeLa and U87MG cells. Therefore, our findings indicate that a novel TERT-bFGF axis accelerates the inactivation of p53 and consequent increase of resistance to DNA-damage response.

Regulation of senescence in cancer and aging

Senescence is regarded as a physiological response of cells to stress, including telomere dysfunction, aberrant oncogenic activation, DNA damage, and oxidative stress. This stress response has an antagonistically pleiotropic effect to organisms: beneficial as a tumor suppressor, but detrimental by contributing to aging. The emergence of senescence as an effective tumor suppression mechanism is highlighted by recent demonstration that senescence prevents proliferation of cells at risk of neoplastic transformation. Consequently, induction of senescence is recognized as a potential treatment of cancer. Substantial evidence also suggests that senescence plays an important role in aging, particularly in aging of stem cells. In this paper, we will discuss the molecular regulation of senescence its role in cancer and aging. The potential utility of senescence in cancer therapeutics will also be discussed.

Cellular senescence as a target in cancer control

Somatic cells show a spontaneous decline in growth rate in continuous culture. This is not related to elapsed time but to an increasing number of population doublings, eventually terminating in a quiescent but viable state termed replicative senescence. These cells are commonly multinucleated and do not respond to mitogens or apoptotic stimuli. Cells displaying characteristics of senescent cells can also be observed in response to other stimuli, such as oncogenic stress, DNA damage, or cytotoxic drugs and have been reported to be found in vivo. Most tumors show unlimited replicative potential, leading to the hypothesis that cellular senescence is a natural antitumor program. Recent findings suggest that cellular senescence is a natural mechanism to prevent undesired oncogenic stress in somatic cells that has been lost in malignant tumors. Given that the ultimate goal of cancer research is to find the definitive cure for as many tumor types as possible, exploration of cellular senescence to drive towards antitumor therapies may decisively influence the outcome of new drugs. In the present paper, we will review the potential of cellular senescence to be used as target for anticancer therapy.

Dynamics of telomeric chromatin at the crossroads of aging and cancer

Telomeres are nucleoprotein structures that protect the ends of human chromosomes through the formation of a 'cap', thus preventing exonucleolytic degradation, inter- and intra-chromosomal fusion, and subsequent chromosomal instability. During aging, telomere shortening correlates with tissue dysfunction and loss of renewal capacity. In human cancer, telomere dysfunction is involved in early chromosome instability, long-term cellular proliferation, and possibly other processes related to cell survival and microenvironment. Telomeres constitute an attractive target for the development of novel small-molecule anti-cancer drugs. In particular, individual protein components of the core telomere higher-order chromatin structure (known as the telosome or 'shelterin' complex) are promising candidate targets for cancer therapy.

Telomeric recombination induced by dysfunctional telomeres

Telomeric recombination has been observed in telomerase-negative alternative lengthening of telomeres in human cancer cells and following telomerase inhibition or gene deletion. This study shows that telomeric recombination mechanisms can also be activated by dysfunctional telomeres without telomerase inhibition in telomerase-positive cells.

 Telomere maintenance is essential for cellular immortality, and most cancer cells maintain their telomeres through the enzyme telomerase. Telomeres and telomerase represent promising anticancer targets. However, 15% of cancer cells maintain their telomeres through alternative recombination-based mechanisms, and previous analyses showed that recombination-based telomere maintenance can be activated after telomerase inhibition. We determined whether telomeric recombination can also be promoted by telomere dysfunction. We report for the first time that telomeric recombination can be induced in human telomerase-positive cancer cells with dysfunctional telomeres.

Functional organization of the Sm core in the crystal structure of human U1 snRNP

U1 small nuclear ribonucleoprotein (snRNP) recognizes the 5'-splice site early during spliceosome assembly. It represents a prototype spliceosomal subunit containing a paradigmatic Sm core RNP. The crystal structure of human U1 snRNP obtained from natively purified material by in situ limited proteolysis at 4.4 Å resolution reveals how the seven Sm proteins, each recognize one nucleotide of the Sm site RNA using their Sm1 and Sm2 motifs. Proteins D1 and D2 guide the snRNA into and out of the Sm ring, and proteins F and E mediate a direct interaction between the Sm site termini. Terminal extensions of proteins D1, D2 and B/B', and extended internal loops in D2 and B/B' support a four-way RNA junction and a 3'-terminal stem-loop on opposite sides of the Sm core RNP, respectively. On a higher organizational level, the core RNP presents multiple attachment sites for the U1-specific 70K protein. The intricate, multi-layered interplay of proteins and RNA rationalizes the hierarchical assembly of U snRNPs in vitro and in vivo.

The telomerase inhibitor imetelstat depletes cancer stem cells in breast and pancreatic cancer cell lines

Cancer stem cells (CSC) are rare drug-resistant cancer cell subsets proposed to be responsible for the maintenance and recurrence of cancer and metastasis. Telomerase is constitutively active in both bulk tumor cell and CSC populations but has only limited expression in normal tissues. Thus, inhibition of telomerase has been shown to be a viable approach in controlling cancer growth in nonclinical studies and is currently in phase II clinical trials. In this study, we investigated the effects of imetelstat (GRN163L), a potent telomerase inhibitor, on both the bulk cancer cells and putative CSCs. When breast and pancreatic cancer cell lines were treated with imetelstat in vitro, telomerase activity in the bulk tumor cells and CSC subpopulations were inhibited. Additionally, imetelstat treatment reduced the CSC fractions present in the breast and pancreatic cell lines. In vitro treatment with imetelstat, but not control oligonucleotides, also reduced the proliferation and self-renewal potential of MCF7 mammospheres and resulted in cell death after <4 weeks of treatment. In vitro treatment of PANC1 cells showed reduced tumor engraftment in nude mice, concomitant with a reduction in the CSC levels. Differences between telomerase activity expression levels or telomere length of CSCs and bulk tumor cells in these cell lines did not correlate with the increased sensitivity of CSCs to imetelstat, suggesting a mechanism of action independent of telomere shortening for the effects of imetelstat on the CSC subpopulations. Our results suggest that imetelstat-mediated depletion of CSCs may offer an alternative mechanism by which telomerase inhibition may be exploited for cancer therapy.

Direct involvement of the TEN domain at the active site of human telomerase

Telomerase is a ribonucleoprotein that adds DNA to the ends of chromosomes. The catalytic protein subunit of telomerase (TERT) contains an N-terminal domain (TEN) that is important for activity and processivity. Here we describe a mutation in the TEN domain of human TERT that results in a greatly increased primer K_d, supporting a role for the TEN domain in DNA affinity. Measurement of enzyme kinetic parameters has revealed that this mutant enzyme is also defective in dNTP polymerization, particularly while copying position 51 of the RNA template. The catalytic defect is independent of the presence of binding interactions at the 5′-region of the DNA primer, and is not a defect in translocation rate. These data suggest that the TEN domain is involved in conformational changes required to position the 3′-end of the primer in the active site during nucleotide addition, a function which is distinct from the role of the TEN domain in providing DNA binding affinity.

p21 WAF1 is involved in interferon-β-induced attenuation of telomerase activity and human telomerase reverse transcriptase (hTERT) expression in ovarian cancer

Telomerase activation is a key step in the development of human cancers. Interferon-β (IFN-β) signaling induces growth arrest in many tumors but the anticancer mechanism of IFN-β is poorly understood. In the present study, we show that IFN-β signaling represses telomerase activity and human telomerase reverse transcriptase (hTERT) transcription in ovarian cancer and suggest that this signaling is mediated by p21(WAF1). IFN-β triggered down-regulation of telomerase activity and hTERT mRNA expression and also induced p21 expression, independently of p53 induction. Ectopic expression of p21 attenuated hTERT promoter activity. Murine embryonic fibroblasts (MEFs) genetically deficient in p21 (p21-/-) showed elevated (> 15 times) hTERT promoter activity compared to wild-type MEFs. Overexpression of p21 reduced the hTERT promoter activity of p21-/- MEFs and hTERT mRNA expression in HCT119 p21(WAF1) null cell. These findings provide evidence that p21 is a potential mediator of IFN-β-induced attenuation of telomerase activity and tumor suppression.

Human aging and CD31+ T-cell number, migration, apoptotic susceptibility, and telomere length

CD31(+) T cells, or so-called "angiogenic T cells," have been shown to demonstrate vasculoprotective and neovasculogenic qualities. The influence of age on CD31(+) T-cell number and function is unclear. We tested the hypothesis that circulating CD31(+) T-cell number and migratory capacity are reduced, apoptotic susceptibility is heightened, and telomere length is shortened with advancing age in adult humans. Thirty-six healthy, sedentary men were studied: 12 young (25 ± 1 yr), 12 middle aged (46 ± 1 yr), and 12 older (64 ± 2 yr). CD31(+) T cells were isolated from peripheral blood samples by magnetic-activated cell sorting. The number of circulating CD31(+) T cells (fluorescence-activated cell sorting analysis) was lower (P < 0.01) in older (24% of CD3(+) cells) compared with middle-aged (38% of CD3(+) cells) and young (40% of CD3(+) cells) men. Migration (Boyden chamber) to both VEGF and stromal cell-derived factor-1α was markedly blunted (P < 0.05) in cells harvested from middle-aged [306.1 ± 45 and 305.6 ± 46 arbitrary units (AU), respectively] and older (231 ± 65 and 235 ± 62 AU, respectively) compared with young (525 ± 60 and 570 ± 62 AU, respectively) men. CD31(+) T cells from middle-aged and older men demonstrated greater apoptotic susceptibility, as staurosporine-stimulated intracellular caspase-3 activation was ∼ 40% higher (P < 0.05) than young. There was a progressive age-related decline in CD31(+) T-cell telomere length (young: 10,706 ± 220 bp; middle-aged: 10,179 ± 251 bp; and older: 9,324 ± 192 bp). Numerical and functional impairments in this unique T-cell subpopulation may contribute to diminished angiogenic potential and greater cardiovascular risk with advancing age.

Telomerase inhibition targets clonogenic multiple myeloma cells through telomere length-dependent and independent mechanisms

Background

Plasma cells constitute the majority of tumor cells in multiple myeloma (MM) but lack the potential for sustained clonogenic growth. In contrast, clonotypic B cells can engraft and recapitulate disease in immunodeficient mice suggesting they serve as the MM cancer stem cell (CSC). These tumor initiating B cells also share functional features with normal stem cells such as drug resistance and self-renewal potential. Therefore, the cellular processes that regulate normal stem cells may serve as therapeutic targets in MM. Telomerase activity is required for the maintenance of normal adult stem cells, and we examined the activity of the telomerase inhibitor imetelstat against MM CSC. Moreover, we carried out both long and short-term inhibition studies to examine telomere length-dependent and independent activities.

Methodology/Principal Findings

Human MM CSC were isolated from cell lines and primary clinical specimens and treated with imetelstat, a specific inhibitor of the reverse transcriptase activity of telomerase. Two weeks of exposure to imetelstat resulted in a significant reduction in telomere length and the inhibition of clonogenic MM growth both in vitro and in vivo. In addition to these relatively long-term effects, 72 hours of imetelstat treatment inhibited clonogenic growth that was associated with MM CSC differentiation based on expression of the plasma cell antigen CD138 and the stem cell marker aldehyde dehydrogenase. Short-term treatment of MM CSC also decreased the expression of genes typically expressed by stem cells (OCT3/4, SOX2, NANOG, and BMI1) as revealed by quantitative real-time PCR.

Conclusions

Telomerase activity regulates the clonogenic growth of MM CSC. Moreover, reductions in MM growth following both long and short-term telomerase inhibition suggest that it impacts CSC through telomere length-dependent and independent mechanisms.

Targeting telomerase in hematologic malignancy

Over the past two decades, it has become increasingly apparent that telomerase-mediated telomere maintenance plays a crucial role in hematopoiesis. Supporting evidence is underscored by recent findings of mutations in genes involved in telomerase-mediated telomere maintenance that contribute to the pathogenesis of bone marrow failure syndromes. More recently described telomere-independent functions of telomerase are also likely to contribute to both normal hematopoiesis and hematologic diseases. The high levels of telomerase detected in aggressive leukemias have fueled fervent investigation into diverse approaches to targeting telomerase in hematologic malignancies. Successful preclinical investigations that employed genetic strategies, oligonucleotides, small-molecule inhibitors and immunotherapy have resulted in a rapid translation to clinical trials. Further investigation of telomere-independent functions of telomerase and detailed preclinical studies of telomerase inhibition in both normal and malignant hematopoiesis will be invaluable for refining treatments to effectively and safely exploit telomerase as a therapeutic target in hematologic malignancies.

HDAC inhibitor, scriptaid, induces glioma cell apoptosis through JNK activation and inhibits telomerase activity

The present study identified a novel mechanism of induction of apoptosis in glioblastoma cells by scriptaid – a histone deacetylase (HDAC) inhibitor. Scriptaid reduced glioma cell viability by increasing Jun N-terminal kinase (JNK) activation. Although scriptaid induced activation of both p38MAPK and JNK, it was the inhibition of JNK that attenuated scriptaid-induced apoptosis significantly. Scriptaid also increased the expression of (i) p21 and p27 involved in cell-cycle regulation and (ii) γH2AX associated with DNA damage response in a JNK-dependent manner. Treatment with scriptaid increased Ras activity in glioma cells, and transfection of cells with constitutively active RasV12 further sensitized glioma cells to scriptaid-induced apoptosis. Scriptaid also inhibited telomerase activity independent of JNK. Taken together, our findings indicate that scriptaid (i) induces apoptosis and reduces glioma cell proliferation by elevating JNK activation and (ii) also decreases telomerase activity in a JNK-independent manner.

Bicyclic triterpenoid Iripallidal induces apoptosis and inhibits Akt/mTOR pathway in glioma cells

BACKGROUND

The highly resistant nature of glioblastoma multiforme (GBM) to chemotherapy prompted us to evaluate the efficacy of bicyclic triterpenoid Iripallidal against GBM in vitro.

METHODS

The effect of Iripallidal on proliferation and apoptosis in glioma cell lines was evaluated by MTS, colony formation and caspase-3 activity. The effect of iripallidal to regulate (i) Akt/mTOR and STAT3 signaling (ii) molecules associated with cell cycle and DNA damage was evaluated by Western blot analysis. The effect of Iripallidal on telomerase activity was also determined.

RESULTS

Iripallidal (i) induced apoptosis, (ii) inhibited Akt/mTOR and STAT3 signaling, (iii) altered molecules associated with cell cycle and DNA damage, (iv) inhibited telomerase activity and colony forming efficiency of glioma cells. In addition, Iripallidal displayed anti-proliferative activity against non-glioma cancer cell lines of diverse origin.

CONCLUSION

The ability of Iripallidal to serve as a dual-inhibitor of Akt/mTOR and STAT3 signaling warrants further investigation into its role as a therapeutic strategy against GBM.

Reptin is required for the transcription of telomerase reverse transcriptase and over-expressed in gastric cancer

BACKGROUND

Telomerase is activated in oncogenesis, which confers an immortal phenotype to cancer cells. The AAA + ATPase Reptin is required for telomerase biogenesis by maintaining telomerase RNA (hTER) stability and is aberrantly expressed in certain cancers. Given its role in chromatin remodeling and transcription regulation, we determined the effect of Reptin on the transcription of the telomerase reverse transcriptase (hTERT) gene, a key component of the telomerase complex and its expression in gastric cancer.

RESULTS

Knocking down Reptin or its partner Pontin using small interfering RNA in gastric and cervical cancer cells led to significant decreases in hTERT mRNA, but hTERT promoter activity was inhibited in only Reptin-depleted cells. Reptin interacted with the c-MYC oncoprotein and its stimulatory effect on the hTERTpromoter was significantly dependent on functional E-boxes in the promoter. Moreover, Reptin bound to the hTERT proximal promoter and the loss of the Reptin occupancy led to dissociation of c-MYC from the hTERT promoter in Reptin-depleted cells. Reptin inhibition dramatically impaired clonogenic potential of gastric cancer cells by inducing cell growtharrest and over-expression of Reptin was observed in primary gastric cancer specimens.

CONCLUSIONS

The hTERT gene is a direct target of Reptin, and hTERT transcription requires constitutive expression of Reptin and its cooperation with c-MYC. Thus, Reptin regulates telomerase at two different levels. This finding, together with the requirementof Reptin for the clonogenic potential of cancer cells and its over-expression in gastriccancer and other solid tumors, suggests that Reptin may be a putative therapeutic target.

Expression of (NES-)hTERT in cancer cells delays cell cycle progression and increases sensitivity to genotoxic stress

Telomerase is a reverse transcriptase associated with cellular immortality through telomere maintenance. This enzyme is activated in 90% of human cancers, and inhibitors of telomerase are currently in clinical trials to counteract tumor growth. Many aspects of telomerase biology have been investigated for therapy, particularly inhibition of the enzyme, but little was done regarding its subcellular shuttling. We have recently shown that mutations in the nuclear export signal of hTERT, the catalytic component of telomerase, led to a mutant ((NES-)hTERT) that failed to immortalize cells despite nuclear localization and catalytic activity. Expression of (NES-)hTERT in primary fibroblast resulted in telomere-based premature senescence and mitochondrial dysfunction. Here we show that expression of (NES-)hTERT in LNCaP, SQ20B and HeLa cells rapidly and significantly decreases their proliferation rate and ability to form colonies in soft agar while not interfering with endogenous telomerase activity. The cancer cells showed increased DNA damage at telomeric and extra-telomeric sites, and became sensitive to ionizing radiation and hydrogen peroxide exposures. Our data show that expression of (NES-)hTERT efficiently counteracts cancer cell growth in vitro in at least two different ways, and suggest manipulation with the NES of hTERT or its subcellular shuttling as a new strategy for cancer treatment.

Hdm2 negatively regulates telomerase activity by functioning as an E3 ligase of hTERT

In this study, we identified posttranslational regulation of human telomerase reverse-transcriptase (hTERT) by the E3 ligase Hdm2. The telomerase activity generated by exogenous hTERT in U2OS cells was reduced on adriamycin treatment. The overexpressed levels of hTERT were also decreased under the same conditions. These processes were reversed by treatment with a proteasome inhibitor or depletion of Hdm2. Furthermore, intrinsic telomerase activity was increased in HCT116 cells with ablation of Hdm2. Immunoprecipitation analyses showed that hTERT and Hdm2 bound to each other in multiple domains. Ubiquitination analyses showed that Hdm2 could polyubiquitinate hTERT principally at the N-terminus, which was further degraded in a proteasome-dependent manner. An hTERT mutant with all five lysine residues at the N-terminus of hTERT that mutated to arginine became resistant to Hdm2-mediated ubiquitination and degradation. In U2OS cells, depletion of Hdm2 or addition of the Hdm2-resistant hTERT mutant strengthened the cellular protective effects against apoptosis. Similar results were obtained with the Hdm2-stable H1299 cell line. These observations indicate that Hdm2 is an E3 ligase of hTERT.

Imetelstat (GRN163L)--telomerase-based cancer therapy

Telomeres and telomerase play essential roles in the regulation of the lifespan of human cells. While normal human somatic cells do not or only transiently express telomerase and therefore shorten their telomeres with each cell division, most human cancer cells typically express high levels of telomerase and show unlimited cell proliferation. High telomerase expression allows cells to proliferate and expand long-term and therefore supports tumor growth. Owing to the high expression and its role, telomerase has become an attractive diagnostic and therapeutic cancer target. Imetelstat (GRN163L) is a potent and specific telomerase inhibitor and so far the only drug of its class in clinical trials. Here, we report on the structure and the mechanism of action of imetelstat as well as about the preclinical and clinical data and future prospects using imetelstat in cancer therapy.

Telomeres and telomerase: from discovery to clinical trials

Telomeres are the ends of linear chromosomes. They cannot be fully replicated by standard polymerases and are maintained by the ribonucleoprotein telomerase. Telomeres and telomerase stand at a junction of critical processes underlying chromosome integrity, cancer, and aging, and their importance was recognized by the 2009 Nobel Prize in Physiology or Medicine to Elizabeth Blackburn, Jack Szostak, and Carol Greider. Where will the field go now? What are the prospects for antitelomerase agents as drugs? Nearly 30 years after Szostak and Blackburn's pioneering manuscript on telomere ends, the challenges of discovery remain.