Attenuation of telomerase activity does not increase sensitivity of human melanoma cells to anticancer agents

Telomerase antisense inhibition for the proliferation of endometrial cancer in vitro and in vivo

The objective of this study was to investigate the antitumor effect of antisense telomerase oligodeoxynucleotides to endometrial cancer cells in vitro and in vivo. Antisense oligodeoxynucleotides (ODNs) against the human telomerase transcripatse (hTERT) synthesized to serve as telomerase inhibitors. Reverse transcription-polymerase chain reaction and 3-[4,5-dimethylthiazol-2yl]-2,5-diphenyltetrazolium bromide (MTT) assay were used to test the expression of hTERT messengerRNA (mRNA) and inhibition of cell proliferation in vitro. In vivo, antitumor effects of ODNs or combined with cisplatin were evaluated in endometrial cancer xenograft. Telomerase activity was tested by telomeric repeat amplification protocol. Antisense ODNs could inhibit proliferation of human endometrial cancer cells (HEC-1-A) in vitro, and downregulate the expression hTRET mRNA in a dose- and period-dependent manner. The tumor growth inhibitory rate of low- and high-dose ODNs were 34.20% and 89.21%, and combined group was 75.30%. Telomerase activity was downregulated to 87.32% compared to the control in the ODNs-treated xenograft tumors. Antisense oligonucleotides of hTERT effectively inhibit the growth of endometrial cancer cell line. Telomerase inhibitor might be a new strategy for chemotherapy or chemoprevention in endometrial cancer.

Mutated telomeres sensitize tumor cells to anticancer drugs independently of telomere shortening and mechanisms of telomere maintenance

Telomerase is a ribonucleoprotein complex that maintains the stability of chromosome ends and regulates replicative potential. Telomerase is upregulated in over 85% of human tumors, but not in adjacent normal tissues and represents a promising target for anticancer therapy. Most telomerase-based therapies rely on the inhibition of telomerase activity and require extensive telomere shortening before inducing any antiproliferative effect. Disturbances of telomere structure rather than length may be more effective in inducing cell death. Telomerase RNA subunits (hTRs) with mutations in the template region reconstitute active holoenzymes that incorporate mutated telomeric sequences. Here, we analysed the feasibility of an anticancer approach based on the combination of telomere destabilization and conventional chemotherapeutic drugs. We show that a mutant template hTR dictates the synthesis of mutated telomeric repeats in telomerase-positive cancer cells, without significantly affecting their viability and proliferative ability. Nevertheless, the mutant hTR increased sensitivity to anticancer drugs in cells with different initial telomere lengths and mechanisms of telomere maintenance and without requiring overall telomere shortening. This report is the first to show that interfering with telomere structure maintenance in a telomerase-dependent manner may be used to increase the susceptibility of tumor cells to anticancer drugs and may lead to the development of a general therapy for the treatment of human cancers.

Water-soluble benzoheterocycle triosmium clusters as potential inhibitors of telomerase enzyme

We have studied the ability of several bioorganometallic clusters [(mu-H)Os(3)(CO)(9)(L)(mu(3)-eta(2)-(Q-H))], where L = [P(C(6)H(4)SO(3)Na)(3)] or [P(OCH(2)CH(2)NMe(3)I)(3)], and Q = quinoline, 3-aminoquinoline, quinoxaline or phenanthridine, of inhibiting telomerase, a crucial enzyme for cancer progression. In general, quinolines have shown interesting biological properties, especially in inhibiting enzymes. For example, the 2,3,7-trichloro-5-nitroquinoxaline (TNQX) exhibited strong anti-telomerase activity in vitro. Among the quinoline-clusters under study, only the negatively charged ones (by virtue of the sulfonated phosphines) exhibited good anti-telomerasic activity on semi-purified enzyme in a cell-free assay, while they were ineffective in vitro on Taq, a different DNA-polymerase. On the contrary, the treatment of breast cancer MCF-7 cell line did not evidence any activity of these clusters, suggesting a low aptitude for crossing cell membrane. Furthermore, all clusters exhibited non-specific, acute cytotoxicy, probably due to accumulation on cell membranes by virtue of their amphiphilic character. A detailed study of Os uptake and accumulation in MCF-7 cells supported this hypothesis.

Chemosensitization of bladder cancer cell lines by human telomerase reverse transcriptase antisense treatment

PURPOSE

Responses of transitional cell carcinoma of the bladder (TCC) to commonly used chemotherapy agents such as mitomycin C (MMC), cisplatin and gemcitabine are often disappointing. Since human telomerase reverse transcriptase (hTERT) is tumor specifically expressed and contributes to the immortality and malignancy of the majority of tumors, it is regarded as a suitable antitumor target. We investigated whether combinations of hTERT antisense (AS)-oligonucleotides (ODNs) with common chemotherapy (CT) schedules may improve drug mediated antitumor effects.

MATERIALS AND METHODS

Initial screening for enhancement of the inhibitory effects of MMC, cisplatin and gemcitabine on viability by treatment with the 2 hTERT AS-ODNs ASt2206 and ASt2331 was performed in 4 TCC cell lines prior to CT. Apoptosis was assessed by annexin V staining and detection of activated caspase-3 using Western blot analysis. Nonsense (NS)-ODN served as a control in all experiments.

RESULTS

All cell lines responded to the anticancer agents tested. Treatment with AS plus CT resulted in a significantly stronger inhibition of viability than the NS plus CT control in the majority of combinations, indicating an AS specific enhancement effect. For example, ASt2331 plus MMC decreased viability to 17% in contrast to NS plus MMC (58%) in EJ28 cells. All ASt2331 plus CT combinations specifically increased the rate of apoptosis 1.3 to 3.0-fold compared with NS plus CT. Apoptosis induction was associated with caspase 3 activation.

CONCLUSIONS

Enhancement of cytotoxic drug effects on the growth of TCC cells by hTERT AS-ODNs presented herein allows a dose decrease in chemotherapy and confirms the suitability of hTERT as a target in a specific therapy approach.

Telomerase inhibition is a specific early event in salvicine-treated human lung adenocarcinoma A549 cells

The telomere and telomerase have been suggested as targets for anticancer drug discovery. However, the mechanisms by which conventional anticancer drugs affect these targets are currently unclear. The novel topoisomerase II inhibitor, salvicine, suppresses telomerase activity in leukemia HL-60 cells. To further determine whether this activity of salvicine is specific to the hematological tumor and distinct from those of other conventional anticancer agents, we studied its effects on telomere and telomerase in a solid lung carcinoma cell line, A549. Differences in telomerase inhibition and telomere erosion were observed between salvcine and other anticancer agents. All anticancer agents (except adriamycin) induced shortening of the telomere, which was identified independent of replication, but only salvicine inhibited telomerase activity in A549 cells under conditions of high concentration and short-term exposure. At the low concentration and long-term exposure mode, all the tested anticancer agents shortened the telomere and inhibited telomerase activity in the same cell line. Notably, salvicine inhibited telomerase activity more severely than the other agents examined. Moreover, the compound inhibited telomerase activity in A549 cells indirectly in a concentration- and time-dependent manner. Salvicine did not affect the expression of hTERT, hTP1, and hTR mRNA in A549 cells following 4 h of exposure. Okadaic acid protected telomerase from inhibition by salvicine. These results indicate specificity of salvicine and diversity of anticancer agents in the mechanism of interference with telomerase and the telomere system. Our data should be helpful for designing the study in the development of agents acting on telomere and/or telomerase.

Death receptor signaling regulatory function for telomerase: hTERT abolishes TRAIL-induced apoptosis, independently of telomere maintenance

Human telomerase has been implicated in cell immortalization and cancer. Recent works suggest that telomerase confers additional function required for tumorigenesis that does not depend on its ability to maintain telomeres. This new action may influence tumor therapy outcomes by yet unraveled mechanisms. Here, we show that overexpression of the catalytic subunit of telomerase (hTERT) protects a maturation-resistant acute promyelocytic leukemia (APL) cell line from apoptosis induced by the tumor necrosis factor (TNF) or TNF-related apoptosis-inducing ligand (TRAIL) and not from apoptosis induced by chemotherapeutic drugs such as etoposide or cisplatin. Conversely, in these cells, TRAIL-induced cell death is magnified by all-trans retinoic acid (ATRA) treatment, independently of telomerase activity on telomeres. Of note, this response is subordinated neither to maturation nor to telomere shortening. This work underlines that retinoids and death receptor signaling cross-talks offer new perspectives for antitumor therapy.

Might telomerase enzyme be a possible target for trans-Pt(II) complexes?

Telomerase is a ribonucleoprotein polymerase that synthesizes telomeric DNA (TTAGGG) repeats. Previously, we have studied the effect on telomerase enzyme of several cis-platinum(II) complexes bearing aromatic amines as bulky carrier groups. All these complexes possess cis-geometry, according to the Cleare and Hoschele's rule. Since recent reports have dealt with the anti-cancer activity of trans-platinum compounds, in this study we have investigated the Farrell's prototypical trans-[Pt(Cl)2(pyridine)2], hereafter called trans-PtPy, in order to understand whether it may possess any anti-telomerase activity. The trans-PtPy has low water solubility and requires dimethyl sulfoxide (DMSO) as co-solvent, thus making the biological tests problematic. The effect of trans-PtPy on MCF-7 cell line concerning log-term telomerase inhibition, telomerase-related gene expression, viability, and apoptosis was evaluated. In a cell-free biochemical assay, trans-PtPy showed significant and dose-dependent inhibition of semi-purified telomerase. The bulk of data indicate that trans-PtPy acts as a non-properly selective anti-proliferative agent, although it shows an initial telomerase inhibitory effect. Telomere length reduction seems not to be the main mechanism causing the observed cell apoptosis. For comparison purpose, results on cis-[Pt(Cl)2(pyridine)2], hereafter cis-PtPy, are reported.

Telomerase Inhibition Using Azidothymidine in the HT-29 Colon Cancer Cell Line

BACKGROUND

We investigated the effects of telomerase inhibition by using the reverse transcriptase inhibitor azidothymidine (AZT) in the human colorectal cancer cell line HT-29 in the presence and absence of 5-fluorouracil (5-FU).

METHODS

HT-29 cells were cultured in the presence of AZT. Telomerase activity was measured by using the telomerase repeat amplification protocol. Telomere length was determined by Southern analysis. The colorimetric microtiter assay was performed to determine the cytotoxic effects of AZT, alone and in combination with 5-FU.

RESULTS

The presence of 3'-azido-3'-deoxythymidine triphosphate (AZT-TP) effectively inhibited telomerase extracted from HT-29 cells. HT-29 cells cultured with 125 microM of AZT underwent fewer total population doublings over 91 days. Southern analysis revealed that telomere attrition occurred within this period. Exposure to 125 microM of AZT resulted in slightly reduced viability (10%) of HT-29 cells. However, the presence of AZT increased 5-FU cytotoxicity, suggesting that the effects of these two drugs are synergistic.

CONCLUSIONS

The data are consistent with telomerase inhibition having growth-inhibitory effects in addition to those predicted to accompany loss of telomere function. Further studies using specific small-molecule inhibitors will confirm whether the growth-inhibitory and 5-FU-sensitivity effects seen here are a direct result of telomerase inhibition.

Telomerase inhibition as cancer therapy

A number of different approaches have been developed to inhibit telomerase activity in human cancer cells. Different components and types of inhibitors targeting various regulatory levels have been regarded as useful for telomerase inhibition. Most methods, however, rely on successive telomere shortening. This process is very slow and causes a long time lag between the onset of inhibition and the occurrence of senescence or apoptosis as a reversal of the immortal phenotype. Many telomerase inhibitors seem to be most efficient when combined with conventional chemotherapeutics. There are some promising approaches that seem to circumvent the slow way of telomere shortening and induce fast apoptosis in treated tumor cells. It has been demonstrated that telomerase may be involved in triggering apoptosis, but the underlying molecular mechanism remains unclear.

Telomere dysfunction increases cisplatin and ecteinascidin-743 sensitivity of melanoma cells

The aim of this study was to investigate the role of telomerase function on the chemosensitivity of melanoma cells. To this end, ecteinascidin-743 (ET-743) and cisplatin [cis-diamminedichloroplatinum(II) (CDDP)], two DNA-interacting drugs that invariably cause an arrest in the G(2)/M phase, and 1-(2,4-dichlorobenzyl)-1H-indazole-3-carboxylic acid (LND), a mitochondria-targeting drug inducing a G(1) block, were used. As experimental model, human melanoma clones showing reduced human telomerase reverse transcriptase (hTERT) expression and telomerase activity and characterized by telomere dysfunction were used. Reconstitution of telomerase activity by exogenous hTERT expression improved telomere function and reduced the sensitivity to CDDP and ET-743 without affecting LND susceptibility. The decreased sensitivity to CDDP and ET-743 was mainly caused by the ability of cells to recover from drug-induced damage, evaluated in terms of both chromosomal lesions and cell survival. The ability of hTERT-reconstituted cells to recover from drug-induced damage was attributable to the restoration of cell cycle progression. In fact, the cells without hTERT restoration remained for a prolonged time in the G(2)/M phase, and this cell cycle alteration made irreversible the drug-induced S-G(2)/M block and led to the activation of apoptotic program. On the contrary, the hTERT-reconstituted cells progressed quickly through the cell cycle, thus acquiring the capacity to recover from drug-induced block and to protect themselves from the G(2)/M phase-specific drug-triggered apoptosis.

Inhibition of telomerase increases resistance of melanoma cells to temozolomide, but not to temozolomide combined with poly (adp-ribose) polymerase inhibitor

In the present study, we have investigated the influence of telomerase inhibition in chemosensitivity of melanoma cells to temozolomide (TMZ), a methylating agent with promising antitumor activity against metastatic melanoma. In fact, telomerase, a ribonucleoprotein enzyme expressed in the majority of tumors, is presently considered an attractive target for anticancer therapy, with the double aim of reducing tumor growth and increasing chemosensitivity of cancer cells. Susceptibility to TMZ and to other antitumor agents used for treatment of metastatic melanoma was initially assessed in melanoma lines with different basal levels of telomerase activity. Thereafter, chemosensitivity was investigated after inhibition of telomerase by means of stable transfection of a catalytically inactive, dominant-negative mutant of hTERT (DN-hTERT). This study shows for the first time that: a) susceptibility to TMZ of melanoma lines derived from the same patient did not depend on basal telomerase activity; b) inhibition of telomerase by DN-hTERT resulted in reduced growth rate and increased resistance to TMZ and to the chloroethylating agent carmustine, increased sensitivity to cisplatin, and no change in response to tamoxifen or to a selective N3-adenine methylating agent; c) inhibition of poly(ADP-ribose) polymerase (PARP), an enzyme involved in the repair of N-methylpurines, restored sensitivity of DN-hTERT clones to TMZ. These results indicate that a careful selection of the antitumor agent has to be made when antitelomerase therapy is combined with chemotherapy. Moreover, the data presented here suggest that TMZ + PARP inhibitor combination is active against telomerase-suppressed and slowly growing tumors.

Tiptoeing to chromosome tips: facts, promises and perils of today's human telomere biology

The past decade has witnessed an explosion of knowledge concerning the structure and function of chromosome terminal structures-telomeres. Today's telomere research has advanced from a pure descriptive approach of DNA and protein components to an elementary understanding of telomere metabolism, and now to promising applications in medicine. These applications include 'passive' ones, among which the use of analysis of telomeres and telomerase (a cellular reverse transcriptase that synthesizes telomeres) for cancer diagnostics is the best known. The 'active' applications involve targeted downregulation or upregulation of telomere synthesis, either to mortalize immortal cancer cells, or to rejuvenate mortal somatic cells and tissues for cellular transplantations, respectively. This article reviews the basic data on structure and function of human telomeres and telomerase, as well as both passive and active applications of human telomere biology.

Targeted therapy for malignant melanoma

There has been much development in the field of targeted therapy for melanoma stemming from efforts to decrease treatment-related toxicities and enhance specific cytotoxicity. This review focuses on three modalities of targeted melanoma therapy based on the biology of the targeting mechanism. The first of these modalities is immunotherapy, which functions to generate a specific antimelanoma immunity. A second modality utilizes metabolic pathways of melanin synthesis to target melanoma cells specifically. A third modality ensues from recent advances in molecular biology and the identification of genes responsible for the malignant transformation of normal melanocytes to melanomas. This work has furthered our understanding of the basis of malignancy, as well as the development of novel strategies aimed at targeting aberrant growth in melanoma.