Telomerase activity: An attractive target for cancer therapeutics (original) (raw)
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Telomerase inhibition as cancer therapy
Cancer letters, 2003
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. q
Therapeutic Anticancer Approaches Targeting Telomerase and Telomeres
Multi-Targeted Approach to Treatment of Cancer, 2014
Telomeres and telomerase are attractive targets for anticancer therapy. This is evidenced with the facts that majority of human cancers express the enzyme telomerase which is utmost important to maintain the telomere length, thereby to ensure indefinite cell proliferation -a hallmark of cancer. In human cells, a structure referred to as telomere has been identified to cap the terminal regions of chromosomes which can protect the ends of DNA strands from degradation and fusion, whereas telomerase plays a pivotal role in cellular immortality and tumorigenesis. Henceforth, strategies have been made to induce telomerase inhibition target virtually all of the major components of ribonucleoprotein holoenzyme and related cell signal pathways that regulates its activity which includes telomerase reverse transcriptase (hTERT) catalytic subunit, the telomere RNA component (hTERC), and associated proteins. It is noteworthy here that most of the cancers have an alternative lengthening of telomere termed as ALT even in the absence of telomerase activity. In these cases, there is an urgent need to understand the cell signaling pathways for ALT mechanism which can be used as therapeutic targets. Other strategies have been developed to target the protein associated with telomerase at the telomeric ends of chromosomes such as tankyrase. Increasing evidences suggest that directly targeting telomeric DNA using agents directed against the shelterin complex may also have anticancer activity. The limitations of strategies remain to be resolved to facilitate the clinical applications. In this chapter, recent development of strategies against these targets shall be discussed.
Telomeres and Telomerase: Pharmacological Targets for New Anticancer Strategies?
Current Cancer Drug Targets, 2006
Telomeres are located at the ends of eukaryotic chromosomes. Human telomerase, a cellular reverse transcriptase, is a ribonucleoprotein enzyme that catalyzes the synthesis and extension of telomeric DNA. It is composed of at least, a template RNA component (hTR; human Telomerase RNA) and a catalytic subunit, the telomerase reverse transcriptase (hTERT). The absence of telomerase is associated with telomere shortening and aging of somatic cells, while high telomerase activity is observed in over 85% of human cancer cells, strongly indicating its key role during tumorigenesis. Several details regarding telomere structure and telomerase regulation have already been elucidated, providing new targets for therapeutic exploitation. Further support for anti-telomerase approaches comes from recent studies indicating that telomerase is endowed of additional functions in the control of growth and survival of tumor cells that do not depend only on the ability of this enzyme to maintain telomere length. This observation suggests that inhibiting telomerase or its synthesis may have additional anti-proliferative and apoptosis inducing effect, independently of the reduction of telomere length during cell divisions. This article reviews the basic information about the biology of telomeres and telomerase and attempts to present various approaches that are currently under investigation to inhibit its expression and its activity. We summarize herein distinct anti-telomerase approaches like antisense strategies, reverse transcriptase inhibitors, and G-quadruplex interacting agents, and also review molecules targeting hTERT expression, such as retinoids and evaluate them for their therapeutic potential.
Targeting human telomerase for cancer therapeutics
Cytotechnology, 2004
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.
TELOMERASE TARGETING IN CANCER
International Research Journal of Modernization in Engineering Technology and Science, 2020
Telomerase is a terminal transferase ribonucleo protein that is responsible for maintaining telomere length. This is a feature of nearly 85% of the cancers where cell immortality is achieved by increased expression of telomerase enzyme responsible for the addition of telomere repeat sequences at the 3' end of the chromosomes after every replication cycle thereby maintaining telomere length. Telomerase is generally active in germ cells and stem cells but is undetectable in normal cells. This makes telomerase inhibitors a potential target in cancer immunotherapy. The telomerase holoenzyme contains two major subunits, the telomerase reverse transcriptase (hTERT) which is the catalytical subunit, and the telomerase RNA (hTR) which provides a template RNA with its sequence complementary to the telomere repeat sequence ("5'-dTTAGGG-3'"). These two subunits are considered important in the telomerase inhibitors studies. Other telomerase inhibitor targets are tankyrase 1 and 2 which is responsible for telomere homeostasis, HSP 90 inhibitors, and 3' telomere overhang sequence (T-oligos), etc. Inhibition of telomerase will lead to loss of telomeres in cancer cells thereby disabling its immortality feature and enabling replicative senescence and hence acting as a tumor suppressor mechanism. In this review paper, we discuss the recent advances in telomerase inhibition and focus on the future perspectives.
Biochimie, 2008
Telomerase enzyme is a ribonucleoprotein maintaining the length of the telomeres by adding G-rich repeats to the end of the eukaryotic chromosomes. Normal human somatic cells, cultured in vitro, have a strictly limited proliferative potential undergoing senescence after about 50e70 population doublings. In contrast, most of the tumor cells have unlimited replicative potential. Although the mechanisms of immortalization are not understood completely at a genetic level, the key role of the telomere/telomerase system in the process is clear. The DNA replication machinery is not able to replicate fully the DNA at the very end of the chromosomes; therefore, about 50e200 nucleotides are lost during each of the replication cycles resulting in a gradual decrease of telomere length. Critically short telomere induces senescence, subsequent crisis and cell death. In tumor cells, however, the telomerase enzyme prevents the formation of critically short telomeres, adding GGTTAG repeats to the 3 0 end of the chromosomes immortalizing the cells. Immortality is one of the hallmarks of cancer. Besides the catalytic activity dependent telomere maintenance, catalytic activity-independent effects of telomerase may also be involved in the regulation of cell cycle. The telomere/telomerase system offers two possibilities to intervene the proliferative activity of the cell: (1) inhibition the telomere maintenance by inhibiting the telomerase activity; (2) activating the residual telomerase enzyme or inducing telomerase expression. Whilst the former approach could abolish the limitless replicative potential of malignant cells, the activation of telomerase might be utilized for treating degenerative diseases. Here, we review the current status of telomerase therapeutics, summarizing the activities of those pharmacological agents which either inhibit or activate the enzyme. We also discuss the future opportunities and challenges of research on pharmacological intervention of telomerase activity.
Simultaneous Targeting of Telomeres and Telomerase as a Cancer Therapeutic Approach1
Telomeres, which are important for maintaining chromosome integrity and functions, shorten with each cell division. Telomerase, responsible for telomere synthesis, is expressed in ϳ90% of human tumor cells but seldom in normal somatic cells. This study evaluated the hypothesis that simultaneous shortening of telomeres and inhibition of telomerase results in synergistic and tumor-selective cytotoxicity. In telomerase-positive human pharynx FaDu tumor cells, paclitaxel caused telomere erosion (first detected at 1 h) and apoptosis. Expression of antisense to the RNA component of human telomerase (hTR) inhibited telomerase activity, shortened telomere length, reduced cell growth rate, and resulted in a significant higher sensitivity to paclitaxel. Another telomerase inhibitor, 3-azido-3deoxythymidine (AZT), at a concentration that produced little or no cell detachment or apoptosis, inhibited the telomerase activity and enhanced the paclitaxel-induced cell detachment and apoptosis. AZT also enhanced the activity of paclitaxel in mice bearing well-established s.c. FaDu xenograft tumors (i.e., reduced residual tumor size, enhanced apoptotic cell fraction, and prolonged survival time), without enhancing host toxicity. In contrast, AZT did not enhance the paclitaxel activity in the telomerasenegative osteosarcoma Saos-2 cells nor in FaDu cells where telomerase was already suppressed by antisense hTR, confirming that the AZT effect in parent FaDu cells is mediated through telomerase inhibition. These results demonstrate that combined use of agents targeting both telomere and telomerase yielded synergistic activity selective for tumors that depend on telomerase for telomere maintenance. . The abbreviations used are: ALT, alternative telomere lengthening; AZT, 3Ј-azido-3Ј-deoxythymidine; FISH, fluorescence in situ hybridization; Flow-FISH, quantification of fluorescence signal from FISH using flow cytometry; hTR, human telomerase RNA; IPTG, isopropyl-1-thio--D-galactopyranoside; TALA, telomere amount and length assay; TRAP, telomeric repeat amplification protocol.