Telomerase is regulated by protein kinase C-zeta in human nasopharyngeal cancer cells

N-(1-pyrenyl) maleimide induces bak oligomerization and mitochondrial dysfunction in Jurkat Cells

N-(1-pyrenyl) maleimide (NPM) is a fluorescent reagent that is frequently used as a derivatization agent for the detection of thio-containing compounds. NPM has been shown to display a great differential cytotoxicity against hematopoietic cancer cells. In this study, the molecular mechanism by which NPM induces apoptosis was examined. Here, we show that treatment of Jurkat cells with NPM leads to Bak oligomerization, loss of mitochondrial membrane potential (Δψm), and release of cytochrome C from mitochondria to cytosol. Induction of Bak oligomerization appears to play a critical role in NPM-induced apoptosis, as downregulation of Bak by shRNA significantly prevented NPM-induced apoptosis. Inhibition of caspase 8 by Z-IETD-FMK and/or depletion of Bid did not affect NPM-induced oligomerization of Bak. Taken together, these results suggest that NPM-induced apoptosis is mediated through a pathway that is independent of caspase-8 activation.

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.

Protein Kinase C (PKC) Isozymes and Cancer

Protein kinase C (PKC) is a family of phospholipid-dependent serine/threonine kinases, which can be further classified into three PKC isozymes subfamilies: conventional or classic, novel or nonclassic, and atypical. PKC isozymes are known to be involved in cell proliferation, survival, invasion, migration, apoptosis, angiogenesis, and drug resistance. Because of their key roles in cell signaling, PKC isozymes also have the potential to be promising therapeutic targets for several diseases, such as cardiovascular diseases, immune and inflammatory diseases, neurological diseases, metabolic disorders, and multiple types of cancer. This review primarily focuses on the activation, mechanism, and function of PKC isozymes during cancer development and progression.

The role of the JAK-STAT pathway and related signal cascades in telomerase activation during the development of hematologic malignancies

Telomerase, comprising a reverse transcriptase protein (TERT) and an RNA template, plays a critical role during senescence and carcinogenesis; however, the mechanisms by which telomerase is regulated remain to be elucidated. Several signaling pathways are involved in the activation of TERT at multistep levels. The JAK-STAT pathway is indispensable for mediating signals through growth factor and cytokine receptors during the development of hematopoietic cells, and its activity is frequently upregulated in hematological malignancies. Here, we review the role of the JAK-STAT pathway and related signaling cascades in the regulation of telomerase in hematological malignancies.

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.

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.

EGFR overexpression induces activation of telomerase via PI3K/AKT-mediated phosphorylation and transcriptional regulation through Hif1-alpha in a cellular model of oral-esophageal carcinogenesis

Telomerase plays an important role during immortalization and malignant transformation as crucial steps in the development of human cancer. In a cellular model of oral-esophageal carcinogenesis, recapitulating the human disease, immortalization occurred independent of the activation of telomerase but through the recombination-based alternative lengthening of telomeres (ALT). In this stepwise model, additional overexpression of EGFR led to in vitro transformation and activation of telomerase with homogeneous telomere elongation in already immortalized oral squamous epithelial cells (OKF6-D1_dnp53). More interestingly, EGFR overexpression activated the PI3K/AKT pathway. This strongly suggested a role for telomerase in tumor progression in addition to just elongating telomeres and inferring an immortalized state. Therefore, we sought to identify the regulatory mechanisms involved in this activation of telomerase and in vitro transformation induced by EGFR. In the present study we demonstrate that telomerase expression and activity are induced through both direct phosphorylation of hTERT by phospho-AKT as well as PI3K-dependent transcriptional regulation involving Hif1-alpha as a key transcription factor. Furthermore, EGFR overexpression enhanced cell cycle progression and proliferation via phosphorylation and translocation of p21. Whereas immortalization was induced by ALT, in vitro transformation was associated with telomerase activation, supporting an additional role for telomerase in tumor progression besides elongating telomeres.

Human telomerase activity regulation

Telomerase has been recognized as a relevant factor distinguishing cancer cells from normal cells. Thus, it has become a very promising target for anticancer therapy. The cell proliferative potential can be limited by replication end problem, due to telomeres shortening, which is overcome in cancer cells by telomerase activity or by alternative telomeres lengthening (ALT) mechanism. However, this multisubunit enzymatic complex can be regulated at various levels, including expression control but also other factors contributing to the enzyme phosphorylation status, assembling or complex subunits transport. Thus, we show that the telomerase expression targeting cannot be the only possibility to shorten telomeres and induce cell apoptosis. It is important especially since the transcription expression is not always correlated with the enzyme activity which might result in transcription modulation failure or a possibility for the gene therapy to be overcome. This review summarizes the current state of knowledge of numerous telomerase regulation mechanisms that take place after telomerase subunits coding genes transcription. Thus we show the possible mechanisms of telomerase activity regulation which might become attractive anticancer therapy targets.

Wogonin, an active compound in Scutellaria baicalensis, induces apoptosis and reduces telomerase activity in the HL-60 leukemia cells

Crude extract of Scutellaria baicalensis (S. baicalensis) has cytotoxic effect on human myelogenous leukemia cells (HL-60). We invesigated which compound from the crude extract is responsible for the cytotoxic effect on HL-60 cells. We identified 29 compounds from the crude extract using high performance liquid chromatography mass spectrometry (HPLC/MS). Two of the compounds, baicalin and wogonoside, are converted to baicalein and wogonin, respectively, after treatment with beta-glucuronidase. We observed a dose-dependent reduction in cell viability when cells with either wogonin or aqueous extract of S. baicalensis. Several of the apoptotic features including deoxyribonucleic acid (DNA) fragmentation and increased caspase-3 activity were found in cells treated with wogonin and aqueous extract. The changes were associated with down-regulation of Bcl-2, and not Bax. Furthermore, treatment of HL-60 cells with wogonin or S. baicalensis led to the inhibition of human telomerase reverse transcriptase (hTERT), human telomerase-associated protein 1 (hTP1) and c-myc messenger ribonucleic acid (m-RNA) expression. Wogonin and S. baicaleisis down-regulated the telomerase activity. Our findings suggest that wogonin may be the major compound in S. baicalensis responsible for HL-60 growth inhibition in vitro. The inhibition of HL-60 cell growth is mediated partly through the induction of Bax/Bcl-2 apoptosis and by telomerase inhibition through suppression of c-myc, which is a promoter of hTERT.

Telomerase prevents accelerated senescence in glucose-6-phosphate dehydrogenase (G6PD)-deficient human fibroblasts

Fibroblasts derived from glucose-6-phosphate dehydrogenase (G6PD)-deficient patients display retarded growth and accelerated cellular senescence that is attributable to increased accumulation of oxidative DNA damage and increased sensitivity to oxidant-induced senescence, but not to accelerated telomere attrition. Here, we show that ectopic expression of hTERT stimulates telomerase activity and prevents accelerated senescence in G6PD-deficient cells. Stable clones derived from hTERT-expressing normal and G6PD-deficient fibroblasts have normal karyotypes, and display no sign of senescence beyond 145 and 105 passages, respectively. Activation of telomerase, however, does not prevent telomere attrition in earlier-passage cells, but does stabilize telomere lengths at later passages. In addition, we provide evidence that ectopic expression of hTERT attenuates the increased sensitivity of G6PD-deficient fibroblasts to oxidant-induced senescence. These results suggest that ectopic expression of hTERT, in addition to acting in telomere length maintenance by activating telomerase, also functions in regulating senescence induction.

Activation of telomerase by ionizing radiation: differential response to the inhibition of DNA double-strand break repair by abrogation of poly (ADP-ribosyl)ation, by LY294002, or by Wortmannin

PURPOSE

Telomerase activity represents a radiation-inducible function, which may be targeted by a double-strand break (DSB)-activated signal transduction pathway. Therefore, the effects of DNA-PK inhibitors (Wortmannin and LY294002) on telomerase upregulation after irradiation were studied. In addition, the role of trans-dominant inhibition of poly(ADP-ribosyl)ation, which strongly reduces DSB rejoining, was assessed in comparison with 3-aminobenzamide.

METHODS AND MATERIALS

COM3 rodent cells carry a construct for the dexamethasone-inducible overexpression of the DNA-binding domain of PARP1 and exhibit greatly impaired DSB rejoining after irradiation. Telomerase activity was measured using polymerase chain reaction ELISA 1 h after irradiation with doses up to 10 Gy. Phosphorylation status of PKB/Akt and of PKCalpha/beta(II) was assessed by western blotting.

RESULTS

No telomerase upregulation was detectable for irradiated cells with undisturbed DSB rejoining. In contrast, incubation with LY294002 or dexamethasone yielded pronounced radiation induction of telomerase activity that could be suppressed by Wortmannin. 3-Aminobenzamide not only was unable to induce telomerase activity but also suppressed telomerase upregulation upon incubation with LY294002 or dexamethasone. Phospho-PKB was detectable independent of irradiation or dexamethasone pretreatment, but was undetectable upon incubations with LY294002 or Wortmannin, whereas phospho-PKC rested detectable.

CONCLUSIONS

Telomerase activation postirradiation was triggered by different treatments that interfere with DNA DSB processing. This telomerase upregulation, however, was not reflected by the phosporylation status of the putative mediators of TERT activation, PKB and PKC. Although an involvement of PKB in TERT activation is not supported by the present findings, a respective role of PKC isoforms other than alpha/beta(II) cannot be ruled out.

Immortalization of epithelial progenitor cells mediated by resveratrol

Within the hierarchy of epithelial stem cells, normal progenitor cells may express regulated telomerase during renewal cycles of proliferation and differentiation. Discontinuous telomerase activity may promote increased renewal capacity of progenitor cells, while deregulated/continuous telomerase activity may promote immortalization when differentiation and/or senescent pathways are compromised. In the present work, we show that resveratrol activates, while progesterone inactivates, continuous telomerase activity within 24 h in subpopulations of human Li-Fraumeni syndrome-derived breast epithelial cells. Resveratrol results in immortalization of mixed progenitor cells with mutant p53, but not human epithelial cells with wild type p53. Our results demonstrate the potential for renewing progenitor cells with mutant p53 to immortalize after continuous telomerase expression when exposed to certain environmental compounds. Understanding the effects of telomerase modulators on endogenous telomerase activity in progenitor cells is relevant to the role of immortalization in the initiation and progression of cancer subtypes.

Suramin increased telomerase activity in the c6 glioma/wistar experimental brain tumor model

Glioblastoma multiforme (GBM) is the most treatment-resistant glioma variant. Significant roles for telomerase in etiology, recurrence and drug resistance of GBM have been highlighted. Suramin (Bayer, Leverkusen, Germany) is an antineoplastic agent that affects many cellular mechanisms including growth factor, purinergic receptor, cytokine and key cellular enzymes signaling. The aim of this study was to investigate whether suramin, 40 mg/kg, i.p., inhibits telomerase activity in a subcutaneous C6 glioma/Wistar experimental brain tumor model using PCR based telomeric repeat amplification assay. In comparison to the control group, suramin increased tumor volume and telomerase activity. We also used transmission electron microscopy to evaluate the alterations of cell morphology. Apoptosis was seen markedly in electron micrographs of the control group and anti-apoptotic activity of telomerase was verified in the electron micrographs of suramin-applied group. The in vitro inhibitory effects of suramin on telomerase activity in several cell lines except for brain tumors have been reported. Contrary to in vitro reports, our results were the first to demonstrate that suramin increased telomerase activity in a C6 glioma/Wistar experimental brain tumor. Large numbers of drugs exhibited apparent hormetic effects on cultured cancer cells and in vivo cancer growth. Several drug examples for their hormetic effects in vivo were listed as resveratrol, suramin, and tamoxifen. The action of suramin in the present study could be evaluated as one of the hormetic examples of suramin in vivo.

Potent inhibition of human telomerase by U-73122

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. In this study, we report that U-73122, an amphiphilic alkylating agent that is commonly used as an inhibitor for phospholipase C, is also a potent and selective inhibitor of human telomerase. The inhibition of telomerase by U-73122 was attributed primarily to the pyrrole-2,5-dione group, since its structural analog U-73343 did not inhibit telomerase. In confirmation, we observed that telomerase was inhibited by N-ethylmaleimide, but not N-ethylsuccinimide. The IC(50) value of U-73122 for the in vitro inhibition of telomerase activity is 0.2 microM, which is comparable to or slightly more sensitive than that for phospholipase C. The inhibitory action of U-73122 on telomerase appears to be rather selective since the presence of externally added proteins did not protect the inhibition and the IC(50) values for the other enzymes tested in this study were at least an order of magnitude higher than that for telomerase. Furthermore, we demonstrate that U-73122 can inhibit telomerase in hematopoietic cancer cells. The potent and selective inhibition of telomerase by U-73122 raises the potential exploitation of this drug and other alkylating agents as telomerase inhibitor.

hTERT phosphorylation by PKC is essential for telomerase holoprotein integrity and enzyme activity in head neck cancer cells

Telomerase activity is suppressed in normal somatic tissues but is activated in most cancer cells. We have previously found that all six telomerase subunit proteins, including hTERT and hsp90 are needed for full enzyme activity. Telomerase activity has been reported to be upregulated by protein kinase C (PKC), but the mechanism is not clear. In this study, we examined how PKC regulates telomerase activity in head and neck cancer cells. PKC inhibitor, bisindolylmaleimide I (BIS), inhibited telomerase activity but had no effect on the expressions of telomerase core subunits. RNA interference (RNAi) and in vitro phosphorylation studies revealed that PKC isoforms α, β, δ, ε, ζ specifically involved in telomerase regulation, and the phosphorylation target was on hTERT. Treatment with the hsp-90 inhibitor novobiocin dissociated hsp90 and hTERT as revealed by immunoprecipitation and immunoblot analysis and reduced telomerase activity. Treatment with the PKC activator SC-10 restored the association of hsp90 and hTERT and reactivate telomerase, suggesting that hTERT phosphorylation by PKC is essential for telomerase holoenzyme integrity and function. Analysis on clinical normal and tumour tissues reveal that the expressions of PKC α, β, δ, ε, ζ were higher in the tumour tissues, correlated with telomerase activity. Disruption of PKC phosphorylation by BIS significantly increased chemosensitivity to cisplatin. In conclusion, PKC isoenzymes α, β, δ, ε, ζ regulate telomerase activity in head and neck cancer cells by phosphorylating hTERT. This phosphorylation is essential for telomerase holoenzyme assembly, leading to telomerase activation and oncogenesis. Manipulation of telomerase activity by PKC inhibitors is worth exploring as an adjuvant therapeutic approach.

Telomeres and telomerase as targets for anticancer drug development

In most human cancers, the telomere erosion problem has been bypassed through the activation of a telomere maintenance system (usually activation of telomerase). Therefore, telomere and telomerase are attractive targets for anti-cancer therapeutic interventions. Here, we review a large panel of strategies that have been explored to date, from small inhibitors of the catalytic sub-unit of telomerase to anti-telomerase immunotherapy and gene therapy. The many positive results that are reported from anti-telomere/telomerase assays suggest a prudent optimism for a possible clinical application in a close future. However, we discuss some of the main limits for these approaches of antitumour drug development and why significant work remains before a clinically useful drug can be proposed to patients.

Potent inhibition of human telomerase by helenalin

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. Inhibition of telomerase in cancer cells has been shown to limit the growth of human cancer cells in culture. In this study, we report that helenalin, a natural sesquiterpene lactone, is a potent and selective inhibitor for human telomerase. In vitro studies indicate that this drug can inactivate telomerase directly in a manner that is dependent on concentration and time. The inhibitory action of this drug on telomerase is selective since the presence of excessive externally added proteins did not protect the inhibition and all of the other enzymes tested in this study were not inhibited by this drug. Furthermore, we demonstrated that helenalin can inhibit the expression of hTERT and telomerase in hematopoietic cancer cells. Therefore, the anti-tumor activity of helenalin is attributed, at least in part, to the inhibition of telomerase.

Cdc42/Rac1 participates in the control of telomerase activity in human nasopharyngeal cancer cells

Telomerase, a specialized ribonucleoprotein reverse transcriptase that directs the synthesis of telomeric DNA, is repressed in normal human somatic cells, but is activated in most cancers. Little is known concerning how telomerase activity is activated and maintained in cancer cells. We have previously shown that protein kinase C-zeta (PKC zeta) controls telomerase activity in nasopharyngeal cancer (NPC) cells. Since PKC zeta activity is known to be modulated by Cdc42/Rac1, we investigated the effects of inhibiting Cdc42 and Rac1 on the telomerase activity of NPC-076 cells. Treatment of NPC cells with antisense oligonucleotides against Cdc42 or Rac1 produced an inhibition of telomerase activity. Similarly, transient expression of dominant-negative mutants of Cdc42 or Rac1, but not the wild-type Cdc42 or Rac1, also produced an inhibition of telomerase activity in NPC cells. This inhibition of telomerase activity is not associated with a transcriptional down-regulation of hTERT, the key regulator of telomerase. We suggest that Cdc42/Rac1 participates in the posttranscriptional control of telomerase activity in NPC cells.

Telomere inhibition and telomere disruption as processes for drug targeting

The components and cofactors of the holoenzyme telomerase and its substrate telomeric DNA are attractive targets for anticancer agents that act by inhibiting the activity of telomerase. This review outlines recent advances in telomerase inhibition that have been achieved using antisense oligonucleotides and ribozymes that target the telomerase mRNA or its hTR RNA template. Although these are potent catalytic inhibitors of telomerase, they are challenging to implement in the clinic due to their delayed effectiveness. Drugs that directly bind to the telomeres, the complex structures that are associated at the telomeric ends, and stabilize secondary DNA structures such as G-quadruplexes are also potent inhibitors of telomerase. Special focus is given here to the telomeres, the biological machinery that works in tandem with telomerase to elongate telomeres, the causes of telomere disruption or dysfunction, and the consequences of disruption/dysfunction on the activity and design of anticancer agents.

Human telomerase and its regulation

The telomere is a special functional complex at the end of linear eukaryotic chromosomes, consisting of tandem repeat DNA sequences and associated proteins. It is essential for maintaining the integrity and stability of linear eukaryotic genomes. Telomere length regulation and maintenance contribute to normal human cellular aging and human diseases. The synthesis of telomeres is mainly achieved by the cellular reverse transcriptase telomerase, an RNA-dependent DNA polymerase that adds telomeric DNA to telomeres. Expression of telomerase is usually required for cell immortalization and long-term tumor growth. In humans, telomerase activity is tightly regulated during development and oncogenesis. The modulation of telomerase activity may therefore have important implications in antiaging and anticancer therapy. This review describes the currently known components of the telomerase complex and attempts to provide an update on the molecular mechanisms of human telomerase regulation.

Natural and pharmacological regulation of telomerase

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