Ribozyme-mediated telomerase inhibition induces immediate cell loss but not telomere shortening in ovarian cancer cells

Use of ribozymes in validation of targets involved in tumor progression

The discovery over 20 years ago that RNA molecules called ribozymes are able to catalyze chemical reactions was a breakthrough in biology. Because of their high specificity, wide range of target selection and action before protein translation, ribozymes, mainly hammerhead ribozymes, have been largely used as specific suppressors of gene functions with the additional aim of validating disease-related genes as potential targets for new therapeutic interventions. However, the lack of suitable delivery systems still hampers the clinical development of ribozyme-based therapeutics. In this review, examples of ribozyme-based strategies to validate targets involved in tumor progression are reported together with a comparison of the advantages and disadvantages of ribozymes with respect to RNA interference technology.:

Emerging Technologies: Trendy RNA Tools for Aging Research

Aging is an inevitable biological phenomenon. Attempts to understand its mechanisms and, consequently, to therapeutically decelerate or even reverse the process are limited by its daunting complexity. Rapid and robust functional genomic tools suited to a wide array of experimental model systems are needed to dissect the interplay of individual genes during aging. In this article, we review principles that transcend the view of RNA, from a molecule merely mediating the flow of genetic information, into a unique molecular tool. In the form of catalytic molecular scissors (ribozymes), antibody-like antagonists (aptamers) and gene silencers (interfering RNAs, RNAi) can be effectively used to dissect biofunctions conserved throughout the evolution. In this review, application of recent RNA tools in aging research is discussed.

DNA and the chromosome - varied targets for chemotherapy

The nucleus of the cell serves to maintain, regulate, and replicate the critical genetic information encoded by the genome. Genomic DNA is highly associated with proteins that enable simple nuclear structures such as nucleosomes to form higher-order organisation such as chromatin fibres. The temporal association of regulatory proteins with DNA creates a dynamic environment capable of quickly responding to cellular requirements and distress. The response is often mediated through alterations in the chromatin structure, resulting in changed accessibility of specific DNA sequences that are then recognized by specific proteins. Anti-cancer drugs that target cellular DNA have been used clinically for over four decades, but it is only recently that nuclease specific drugs have been developed to not only target the DNA but also other components of the nuclear structure and its regulation. In this review, we discuss some of the new drugs aimed at primary DNA sequences, DNA secondary structures, and associated proteins, keeping in mind that these agents are not only important from a clinical perspective but also as tools for understanding the nuclear environment in normal and cancer cells.

DNA damage transiently increases TRF2 mRNA expression and telomerase activity

Telomerase activity transiently increases when HL60 cells are treated with the topoisomerase II inhibitor etoposide. A quantitative assessment revealed that telomerase is activated by etoposide treatment in a number of cell lines and that the increase is reversible after withdrawal of etoposide from the cell culture. Telomerase activation correlated with the occurrence of DNA damage but not with cell cycle arrest. We did not detect any transcriptional upregulation of hTERT mRNA, suggesting a post-transcriptional mechanism of telomerase activation. Furthermore, the mRNA expression of the telomere binding protein TRF2 was upregulated early and reversibly after etoposide treatment. TRF1 mRNA expression levels were unchanged after DNA damage, but increased when the cells accumulated in the G2/M phase. The data show that the telosome reacts after DNA damage by upregulating telomerase activity and TRF2 expression in malignant cells. It has previously been shown that overexpression of TRF2 can repress senescence signals arising from critically shortened telomeres. We show here that TRF2 is upregulated by undirected DNA damage that also affects the telomeric DNA. These data suggest that upregulation of telomerase activity and TRF2 expression might act as antiapoptotic mechanisms in the DNA-damage response of malignant cells.

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.

Alternative lengthening of telomeres, telomerase, and cancer

Telomere length may be maintained in cancer cells by telomerase or an alternative lengthening of telomeres (ALT) mechanism. Low levels of telomerase activity have been detected in some normal somatic cells and presumably some types of normal cells also have low levels of an ALT-like activity. It is hypothesized here that inherited abnormalities of these and other aspects of telomere maintenance may contribute to cancer and ageing. The telomere length maintenance mechanisms are similar in that activation of each is associated with immortalization. They may also confer other properties on cancer cells, however, and the nature of these additional properties may be different for telomerase and ALT. It is expected that these similarities and differences will have implications for prognosis and treatment.

Telomerase and tumorigenesis

The unique biology of telomeres and telomerase plays important roles in many aspects of mammalian cell physiology. Over the past decade, several lines of evidence have confirmed that the maintenance of telomeres and telomerase participate actively in the pathogenesis of human cancer. Specifically, activation of telomerase is strongly associated with cancer, and recent observations confirm that telomeres and telomerase perform important roles in both suppressing and facilitating malignant transformation by regulating genomic stability and cell lifespan. In addition, recent evidence suggests that telomerase activation contributes to tumorigenesis independently of its role in maintaining telomere length. Here we review recent developments in our understanding of the relationships among telomeres, telomerase, and cancer.

Inhibition of telomerase activity by a distamycin derivative: effects on cell proliferation and induction of apoptosis in human cancer cells

In this study, we evaluated the potential of the distamycin derivative MEN 10716 as a telomerase inhibitor. Exposure of human melanoma cell extracts to MEN 10716 induced a dose-dependent inhibition of telomerase activity, with an IC50 of 24+/-3 microM. When intact JR8 melanoma cells were chronically exposed to the drug (200 microM every other day for 50 days), a marked inhibition (>80%) of the enzyme's catalytic activity was consistently observed starting from day 1. At later points in time, MEN 10716 inhibited melanoma cell proliferation and induced apoptosis. Cells surviving MEN 10716 exposure were characterised by a higher melanin content and a greater expression of p16(INK4A) protein than control cells. The effects of MEN 10716 were subsequently evaluated in different tumour cell systems. In particular, even in the H460 non-small cell lung cancer cell line, chronic exposure of the cells to the drug (100 microM every other day for 50 days) induced a consistent inhibition (>85%) of telomerase activity, a reduction of cell proliferation potential, and apoptosis. Conversely, MEN 10716 treatment did not appreciably inhibit cell proliferation in the U2-OS telomerase-negative human osteogenic sarcoma cell line. Interestingly, no variation in the mean telomere length was observed in MEN 10716-treated JR8 melanoma cells, whereas an appreciable increase in the mean telomere length was found in H460 lung cancer cells after drug exposure. Overall, the results of the study indicate that MEN 10716 is a possible telomerase inhibitor and suggest that abrogation of telomerase activity can affect cell proliferation even through pathways that are not dependent on telomere erosion.

2-5A antisense therapy directed against human telomerase RNA inhibits telomerase activity and induces apoptosis without telomere impairment in cervical cancer cells

Human telomerase RNA (hTR), an important component of telomerase, is a possible target of telomerase-based cancer gene therapy. The present study was undertaken to assess the efficacy of antisense hTR therapy using newly developed 2-5A (5'-phosphorylated 2'-5'-linked oligoadenylate)-linked oligonucleotides against cervical cancer cells. ME180 and SiHa cells were treated with 2-5A-linked antisense hTR designed to complement the region of hTR between residues 76 and 94. The hTR expression, telomerase activity, cell viability, and apoptosis were then examined. The 2-5A anti-hTR effectively degraded hTR and inhibited telomerase activity. The 2-5A mutant anti-hTR and the anti-hTR without 2-5A were not capable of inhibiting telomerase activity. Inhibition of telomerase by 2-5A anti-hTR rapidly decreased cell viability only in telomerase-positive cells within 3-6 days after the treatment, when telomere length has not yet been shortened. This inhibition was associated with apoptosis, possibly through activation of caspase family members. These findings suggest that 2-5A-linked antisense-hTR therapy has a potent telomerase-inhibitory effect associated with a cytocidal effect from caspase-induced apoptosis, and may therefore be a potential tool in telomerase-based gene therapy against cervical cancers.

TERT regulates cell survival independent of telomerase enzymatic activity

Human telomerase reverse transcriptase (hTERT), the catalytic subunit of telomerase, plays a pivotal role in the maintenance of telomeres and cell proliferation. Here we report that down-regulation of hTERT induces apoptosis independently of telomerase enzymatic activity in human breast cancer cells. Expression of a hTERT mutant lacking telomerase activity rescues the cells with lowered telomerase without inducing cell death. With similar patterns of subcellular distribution to that of the tumor suppressor protein p53 during mitosis, hTERT interacts with p53 and poly(ADP-ribose) polymerase (PARP). Decreasing p53 expression in intact cells worsens, and increasing p53 prevents, cell death induced by lowering hTERT. Thus, hTERT maintains cell survival and proliferation via both telomerase enzymatic activity-dependent telomere lengthening and enzymatic activity-independent intermolecular interactions involving p53 and PARP.

A G-quadruplex-interactive potent small-molecule inhibitor of telomerase exhibiting in vitro and in vivo antitumor activity

The telomerase complex is responsible for telomere maintenance and represents a promising cancer therapeutic target. We describe herein the antitelomerase and antitumor properties of a small-molecule compound designed by computer modeling to interact with and stabilize human G-quadruplex DNA, a structure that may form with telomeric DNA, thereby inhibiting access to telomerase. The 3,6,9-trisubstituted acridine 9-[4-(N,N-dimethylamino)phenylamino]-3,6-bis(3-pyrrolodinopropionamido) acridine (BRACO19) represents one of the most potent cell-free inhibitors of human telomerase yet described (50% inhibitory concentration of 115 +/- 18 nM). Moreover, in contrast to G-quadruplex interactive agents described previously, BRACO19 did not cause nonspecific acute cytotoxicity at similar concentrations to those required to completely inhibit telomerase activity. There exists a 90-fold differential (mean 50% inhibitory concentration for acute cell kill across seven human tumor cell lines of 10.6 +/- 0.7 microM). The exposure of 21NT human breast cancer cells, which possess relatively short telomeres, to nonacute cytotoxic concentrations of BRACO19 (2 microM) resulted in a marked reduction in cell growth after only 15 days. This was concomitant with a reduction in intracellular telomerase activity and onset of senescence as indicated by an increase in the number of beta-galactosidase positive-staining cells. Intraperitoneal administration of nontoxic doses of BRACO19 (2 mg/kg) to mice bearing advanced stage A431 human vulval carcinoma subcutaneous xenografts and previously treated with paclitaxel induced a significant increase in antitumor effect compared with that observed with paclitaxel alone. BRACO19 thus represents the first of a "second generation" of G-quadruplex-mediated telomerase/telomere-interactive compounds. It possesses nanomolar potency against telomerase but low nonspecific cytotoxicity, growth inhibitory effects, and induction of senescence in a human breast cancer cell line and, moreover, significant antitumor activity in vivo when administered post paclitaxel to mice bearing a human tumor xenograft carcinoma.

Replicative aging, telomeres, and oxidative stress

Aging is a very complex phenomenon, both in vivo and in vitro. Free radicals and oxidative stress have been suggested for a long time to be involved in or even to be causal for the aging process. Telomeres are special structures at the end of chromosomes. They shorten during each round of replication and this has been characterized as a mitotic counting mechanism. Our experiments show that the rate of telomere shortening in vitro is modulated by oxidative stress as well as by differences in antioxidative defence capacity between cell strains. In vivo we found a strong correlation between short telomeres in blood lymphocytes and the incidence of vascular dementia. These data suggest that parameters that characterise replicative senescence in vitro offer potential for understanding of, and intervention into, the aging process in vivo.

Preferential maintenance of critically short telomeres in mammalian cells heterozygous for mTert

Prolonged growth of murine embryonic stem (ES) cells lacking the telomerase reverse transcriptase, mTert, results in a loss of telomere DNA and an increased incidence of end-to-end fusions and aneuploidy. Furthermore, loss of only one copy of mTert also results in telomere shortening intermediate between wild-type (wt) and mTert-null ES cells [Liu, Y., Snow, B. E., Hande, M. P., Yeung, D., Erdmann, N. J., Wakeham, A., Itie, A., Siderovski, D. P., Lansdorp, P. M., Robinson, M. O. & Harrington, L. (2000) Curr. Biol. 10, 1459-1462]. Unexpectedly, although average telomere length in mTert(+/-) ES cells declined to a similar level as mTert-null ES cells, mTert(+/-) ES cell lines retained a minimal telomeric DNA signal at all chromosome ends. Consequently, no end-to-end fusions and genome instability were observed in the latest passages of mTert(+/-) ES cell lines. These data uncover a functional distinction between the dosage-dependent function of telomerase in average telomere-length maintenance and the selective maintenance of critically short telomeres in cells heterozygous for mTert. In normal and tumor cells, we suggest that telomerase activity insufficient to maintain a given average telomere length may, nonetheless, provide a protective advantage from end-to-end fusion and genome instability.