351
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Abstract
There has been a vast increase in telomerase research over the past several years, with many different pre-clinical approaches being tested for inhibiting the activity of this enzyme as a novel therapeutic modality to treat malignancy. In this review, we will provide some basic background information about telomeres and telomerase and then discuss the pros, cons and challenges of the approaches that are currently under investigation, and what we might expect in the future of this emerging field.
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Affiliation(s)
- L K White
- Depts of Internal Medicine and Cell Biology, The University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75390-9039, USA
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352
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Arthanari H, Bolton PH. Functional and dysfunctional roles of quadruplex DNA in cells. CHEMISTRY & BIOLOGY 2001; 8:221-30. [PMID: 11306347 DOI: 10.1016/s1074-5521(01)00007-2] [Citation(s) in RCA: 142] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
A number of biological roles have been proposed for quadruplex, also referred to as G4 or tetraplex, DNA. The presence of quadruplex DNA may lead to errors in some biological processes and be required in others. Proteins that interact with quadruplex DNA have been identified including those that cause Bloom's and Werner's syndromes. There are small molecules that specifically bind to quadruplex DNA, inhibit telomerase, and are cytotoxic towards tumor cells indicating a role for quadruplex DNA in telomere function. It is now possible to make testable proposals for the possible biological implications of quadruplex DNA in replication, transcription, and recombination as well as possible routes to therapeutic intervention.
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Affiliation(s)
- H Arthanari
- Chemistry Department, Wesleyan University, Middletown, CT 06459, USA
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353
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Bachand F, Autexier C. Functional regions of human telomerase reverse transcriptase and human telomerase RNA required for telomerase activity and RNA-protein interactions. Mol Cell Biol 2001; 21:1888-97. [PMID: 11238925 PMCID: PMC86762 DOI: 10.1128/mcb.21.5.1888-1897.2001] [Citation(s) in RCA: 97] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
Abstract
Telomerase is a specialized reverse transcriptase (RT) that is minimally composed of a protein catalytic subunit and an RNA component. The RNA subunit contains a short template sequence that directs the synthesis of DNA repeats at the ends of chromosomes. Human telomerase activity can be reconstituted in vitro by the expression of the human telomerase protein catalytic subunit (hTERT) in the presence of recombinant human telomerase RNA (hTR) in a rabbit reticulocyte lysate (RRL) system. We analyzed telomerase activity and binding of hTR to hTERT in RRL by expressing different hTERT and hTR variants. hTRs containing nucleotide substitutions that are predicted to disrupt base pairing in the P3 helix of the pseudoknot weakly reconstituted human telomerase activity yet retained their ability to bind hTERT. Our results also identified two distinct regions of hTR that can independently bind hTERT in vitro. Furthermore, sequences or structures between nucleotides 208 and 330 of hTR (which include the conserved CR4-CR5 domain) were found to be important for hTERT-hTR interactions and for telomerase activity reconstitution. Human TERT carboxy-terminal amino acid deletions extending to motif E or the deletion of the first 280 amino acids abolished human telomerase activity without affecting the ability of hTERT to associate with hTR, suggesting that the RT and RNA binding functions of hTERT are separable. These results indicate that the reconstitution of human telomerase activity in vitro requires regions of hTERT that (i) are distinct from the conserved RT motifs and (ii) bind nucleotides distal to the hTR template sequence.
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Affiliation(s)
- F Bachand
- Department of Anatomy & Cell Biology, McGill University, Montréal, Québec, Canada H3A 2B2
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354
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Abstract
The maintenance of specialized nucleoprotein structures termed telomeres is essential for chromosome stability. Without new synthesis of telomeres at chromosome ends the chromosomes shorten with progressive cell division, eventually triggering either replicative senescence or apoptosis when telomere length becomes critically short. The regulation of telomerase activity in human cells plays a significant role in the development of cancer. Telomerase is tightly repressed in the vast majority of normal human somatic cells but becomes activated during cellular immortalization and in cancers. While the mechanisms for telomerase activation in cancers have not been fully defined, they include telomerase catalytic subunit gene (hTERT) amplification and trans-activation of the hTERT promoter by the myc oncogene product. Ectopic expression of hTERT is sufficient to restore telomerase activity in cells that lack the enzyme and can immortalize many cell types. Understanding telomerase biology will eventually lead to several clinically relevant telomerase-based therapies. These applications include inhibiting or targeting telomerase as a novel antineoplastic strategy and using cells immortalized by telomerase for therapeutic applications.
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Affiliation(s)
- W C Hahn
- Department of Adult Oncology, Dana-Farber Cancer Institute and Brigham and Women's Hospital, Boston, MA 02115, USA.
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355
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Nielsen PE. Peptide nucleic acids as antibacterial agents via the antisense principle. Expert Opin Investig Drugs 2001; 10:331-41. [PMID: 11178345 DOI: 10.1517/13543784.10.2.331] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
Peptide nucleic acid (PNA) is a peptide-like DNA mimic that was introduced almost ten years ago. It was immediately predicted that PNA would have a bright future in gene therapeutic drug development, but progress in this direction has been rather modest thus far. This is predominantly due to inefficient uptake of PNA by most living cells. However, within the past couple of years a variety of methods have been devised to address this problem and the stage should now be set for more rapid progress. Several studies have demonstrated antisense effects ex vivo in cells in culture and two reports on direct injection of PNA into the brain of rats are also interesting. Only a few studies have addressed the possible exploitation of the antisense principle for development of antibacterial drugs. However, the first in vitro results using antiribosomal RNA PNAs and antisense PNAs targeted to the beta-lactamase gene on Escherichia coli cultures were quite promising. Most recently, these preliminary studies have been extended to demonstrate in vivo efficacy of antibacterial PNAs in an E. coli peritonitis/sepsis mouse model. Therefore, PNA drug development again is rapidly picking up pace.
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Affiliation(s)
- P E Nielsen
- Center for Biomolecular Recognition, Department for Biochemistry and Genetics, Laboratory B, The Panum Institute, Blegdamsvej 3c, DK-2200 N, Copenhagen, Denmark.
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356
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Abstract
In normal somatic cells, the ends of chromosomes (the telomeres) shorten with each cell division. By contrast, in tumour cells, telomere length is maintained, generally through the reactivation of the reverse transcriptase enzyme, telomerase. At least three applications relating to telomeres and telomerase have been proposed: in cancer diagnosis and prognosis (especially through measurements of the catalytic component of telomerase, hTERT) and as a means of monitoring tumour response to therapy; as an aid to tissue engineering; and inhibition as a cancer therapeutic strategy. Mouse knockout, hTERT dominant negative, and antisense experiments suggest that telomerase inhibitors will confer anticancer activity, especially in tumours with short telomeres. Inhibitory strategies have focused on antisense molecules, inhibitors of reverse transcriptases, and small molecules able to interact with and stabilise four-stranded (G-quadruplex) structures formed by telomeres. Clinical trials involving telomerase inhibitors require careful consideration compared to those looking at conventional anticancer cytotoxic drugs.
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Affiliation(s)
- L R Kelland
- Cancer Research Campaign Centre for Cancer Therapeutics, Institute of Cancer Research, Sutton, UK.
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357
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Braasch DA, Corey DR. Synthesis, analysis, purification, and intracellular delivery of peptide nucleic acids. Methods 2001; 23:97-107. [PMID: 11181029 DOI: 10.1006/meth.2000.1111] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Peptide nucleic acids (PNAs) are nonionic DNA mimics. Their novel chemical properties may facilitate the development of selective and potent antisense and antigene strategies for regulating intracellular processes. Described herein are procedures for the synthesis, purification, handling, and characterization of PNAs. A simple protocol for the lipid-mediated introduction of PNAs into in vitro cultures of mammalian cells is provided.
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Affiliation(s)
- D A Braasch
- Department of Pharmacology, Department of Biochemistry, University of Texas Southwestern Medical Center at Dallas, 5323 Harry Hines Boulevard, Dallas, Texas 75390-9041, USA
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358
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Abstract
Telomerase is a ribonucleoprotein reverse transcriptase that extends the ends of chromosomes. The two telomerase subunits essential for catalysis in vitro are the telomerase reverse transcriptase (TERT) and the telomerase RNA. Using truncations and site-specific mutations, we identified sequence elements of TERT and telomerase RNA required for catalytic activity and protein-RNA interaction for Tetrahymena thermophila telomerase. We found that the TERT amino and carboxyl termini, although evolutionarily poorly conserved, are nonetheless important for catalytic activity. In contrast, high-affinity telomerase RNA binding requires only a small region in the amino terminus of TERT. Surprisingly, the TERT region necessary and sufficient for telomerase RNA binding is completely separable from the reverse transcriptase motifs. The minimal Tetrahymena TERT RNA binding domain contains two sequence motifs with ciliate-specific conservation and one TERT motif with conservation across all species. With human TERT, we demonstrate that a similar region within the TERT amino terminus is essential for human telomerase RNA binding as well. Finally, we defined the Tetrahymena telomerase RNA sequences that are essential for TERT interaction. We found that a four-nucleotide region 5' of the template is critical for TERT binding and that the 5' end of telomerase RNA is sufficient for TERT binding. Our results reveal at least one evolutionarily conserved molecular mechanism by which the telomerase reverse transcriptase is functionally specialized for obligate use of an internal RNA template.
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Affiliation(s)
- C K Lai
- Division of Biochemistry and Molecular Biology, Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3204, USA
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359
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Fletcher TM, Cathers BE, Ravikumar KS, Mamiya BM, Kerwin SM. Inhibition of human telomerase by 7-deaza-2'-deoxyguanosine nucleoside triphosphate analogs: potent inhibition by 6-thio-7-deaza-2'-deoxyguanosine 5'-triphosphate. Bioorg Chem 2001; 29:36-55. [PMID: 11300694 DOI: 10.1006/bioo.2000.1194] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We have examined analogs of the previously reported 7-deaza-2'-deoxypurine nucleoside triphosphate series of human telomerase inhibitors. Two new telomerase-inhibiting nucleotides are reported: 6-methoxy-7-deaza-2'-deoxyguanosine 5'-triphosphate (OMDG-TP) and 6-thio-7-deaza-2'-deoxyguanosine 5'-triphosphate (TDG-TP). In particular, TDG-TP is a very potent inhibitor of human telomerase with an IC(50) of 60 nM. TDG-TP can substitute for dGTP as a substrate for telomerase, but only at relatively high concentrations. Under conditions in which TDG-TP is the only available guanosine substrate, telomerase becomes nonprocessive, synthesizing short products that appear to contain only one to three TDG residues. Similarly, the less potent telomerase inhibitor OMDG-TP gives rise to short telomerase products, but less efficiently than TDG-TP. We show here that TDG-TP, and to a lesser extent OMDG-TP, can serve as substrates for both templated (Klenow exo) and nontemplated (terminal transferase) DNA polymerases. For either polymerase, the products arising from TDG-TP are relatively short, and give rise to bands of unusual mobility under PAGE conditions. These anomalous bands revert, under treatment with DTT, to normal mobility bands, indicating that these products may contain thiol-labile disulfide linkages involving the incorporated TDG residues. This observation of potential TDG-crosslinks may have bearing on the mechanism of telomerase inhibition by this nucleotide analog.
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Affiliation(s)
- T M Fletcher
- College of Pharmacy, University of Texas, Austin, Texas 78712-1074, USA
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360
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Ren JG, Xia HL, Tian YM, Just T, Cai GP, Dai YR. Expression of telomerase inhibits hydroxyl radical-induced apoptosis in normal telomerase negative human lung fibroblasts. FEBS Lett 2001; 488:133-8. [PMID: 11163759 DOI: 10.1016/s0014-5793(00)02397-8] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
In tumor cells telomerase activity is associated with resistance to apoptosis and the introduction of the human telomerase reverse transcriptase (hTERT) subunit into normal human cells is associated with life span extension of the cells. To determine the role of telomerase in regulating apoptosis, telomerase negative human embryo lung fibroblasts were transfected with the hTERT gene. Unlike the control fibroblasts, the telomerase-expressing cells had elongated telomeres and were resistant to apoptosis induced by hydroxyl radicals. The results indicate that expression of telomerase and, thus, the maintenance of telomere length in normal human somatic cells caused resistance to not only cellular senescence but also apoptosis. Moreover, we found that hydroxyl radical-induced apoptosis in telomerase-expressing and control fibroblasts was caspase-3 independent. These findings have revealed a new type of interrelation between telomerase and caspase-3, which may indicate that in this case the expressed telomerase may inhibit apoptosis at a site not related to the caspase-3 cascade.
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Affiliation(s)
- J G Ren
- Department of Biological Sciences and Biotechnology, Tsinghua University, Beijing, PR China
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361
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Ren JG, Xia HL, Just T, Dai YR. Hydroxyl radical-induced apoptosis in human tumor cells is associated with telomere shortening but not telomerase inhibition and caspase activation. FEBS Lett 2001; 488:123-32. [PMID: 11163758 DOI: 10.1016/s0014-5793(00)02377-2] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
Reactive oxygen species (ROS) have been found to trigger apoptosis in tumor cells. At the same time, telomerase is found to be associated with malignancy and reduced apoptosis. However little is known about the linkage between ROS such as *OH and telomerase/telomere. To address the interrelations between *OH and telomerase/telomere in tumor cell killing, HeLa, 293 and MW451 cells were induced to undergo apoptosis with *OH radicals generated via Fe(2+)-mediated Fenton reactions (0.1 mM FeSO(4) plus 0.3-0.9 mM H2O2) and telomerase activity, telomere length were measured during apoptosis. We found that during *OH-induced apoptosis, telomere shortening took place while no changes in telomerase activity were observed. Our results suggest that *OH-induced telomere shortening is not through telomerase inhibition but possibly a direct effect of *OH on telomeres themselves indicating that telomere shortening but not telomerase inhibition is the primary event during *OH-induced apoptosis. Strikingly, we also found that *OH-induced apoptosis in HeLa cells is caspase-3-independent but is associated with reduction of mitochondrial transmembrane potential. Our results indicate that *OH triggers apoptotic tumor cell death through a telomere-related, caspase-independent pathway.
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Affiliation(s)
- J G Ren
- Department of Biological Sciences and Biotechnology, Tsinghua University, Beijing, PR China
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362
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O'Hare MJ, Bond J, Clarke C, Takeuchi Y, Atherton AJ, Berry C, Moody J, Silver AR, Davies DC, Alsop AE, Neville AM, Jat PS. Conditional immortalization of freshly isolated human mammary fibroblasts and endothelial cells. Proc Natl Acad Sci U S A 2001; 98:646-51. [PMID: 11209060 PMCID: PMC14642 DOI: 10.1073/pnas.98.2.646] [Citation(s) in RCA: 166] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
Reports differ as to whether reconstitution of telomerase activity alone is sufficient for immortalization of different types of human somatic cells or whether additional activities encoded by other "immortalizing" genes are also required. Here we show that ectopic expression of either the catalytic subunit of human telomerase (hTERT) or a temperature-sensitive mutant (U19tsA58) of simian virus 40 large-tumor antigen alone was not sufficient for immortalization of freshly isolated normal adult human mammary fibroblasts and endothelial cells. However, a combination of both genes resulted in the efficient generation of immortal cell lines irrespective of the order in which they were introduced or whether they were introduced early or late in the normal proliferative lifespan of the cultures. The order and timing of transduction, however, did influence genomic stability. Karyotype analysis indicated that introduction of both transgenes at early passage, with hTERT first, yielded diploid cell lines. Temperature-shift experiments revealed that maintenance of the immortalized state depended on continued expression of functional U19tsA58 large-tumor antigen, with hTERT alone unable to maintain growth at nonpermissive temperatures for U19tsA58 large-tumor antigen. Such conditional diploid lines may provide a useful resource for both cell engineering and for studies on immortalization and in vitro transformation.
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Affiliation(s)
- M J O'Hare
- Ludwig Institute for Cancer Research-University College London Breast Cancer Laboratory, Department of Surgery, Royal Free and University College School of Medicine, 67-73 Riding House Street, London W1W 7EJ, United Kingdom
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363
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Herbert BS, Wright AC, Passons CM, Wright WE, Ali IU, Kopelovich L, Shay JW. Effects of chemopreventive and antitelomerase agents on the spontaneous immortalization of breast epithelial cells. J Natl Cancer Inst 2001; 93:39-45. [PMID: 11136840 DOI: 10.1093/jnci/93.1.39] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023] Open
Abstract
BACKGROUND Activation of telomerase is an early event in the development of breast and other cancers that may lead to cell immortalization, a critical and rate-limiting step in cancer progression. Breast epithelial cells from women with Li-Fraumeni syndrome (LFS) immortalize spontaneously and reproducibly in culture. We, therefore, tested whether immortalization of these cells could be prevented by treating them with chemopreventive agents and by inhibiting telomerase activity. METHODS Noncancerous, preimmortal breast epithelial cells derived from a patient with LFS were treated for 3 months with nontoxic concentrations of the chemopreventive agents oltipraz, difluoromethylornithine, tamoxifen, and retinoic acid or with two different telomerase inhibitors. The frequency of spontaneous immortalization of LFS-derived cells was estimated by an approach based on fluctuation analyses. Statistical analyses were two-sided. RESULTS The frequency of spontaneous immortalization events of LFS-derived breast epithelial cells was reduced by long-term treatment with retinoic acid (P<0.001) or tamoxifen (P<0.05) compared with solvent-treated cells. The frequency of immortalization was also reduced by treating LFS-derived cells with an antitelomerase antisense oligonucleotide (P<0.001) or by inducing the cells to express a dominant negative mutant of telomerase (P<0.025) compared with cells treated with a control oligonucleotide or with empty vector, respectively. CONCLUSIONS Treatment of preimmortal LFS breast epithelial cells with chemopreventive and antitelomerase agents decreased the frequency of spontaneous immortalization in vitro. These studies validate the application of a new cell culture model system to screen the effects of novel chemopreventive agents by use of cell immortalization as an end point. The results also suggest that the telomerase ribonucleoprotein complex may be an important molecular target for breast cancer prevention.
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Affiliation(s)
- B S Herbert
- Department of Cell Biology, The University of Texas Southwestern Medical Center, Dallas, TX 75390-9039, USA
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364
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Abstract
Maintenance of telomere stability is required for cells to escape from replicative senescence and proliferate indefinitely. Telomere length is maintained by a balance between processes that lengthen telomeres (telomerase) and processes that shorten telomeres (the end-replication problem). Telomerase is a cellular ribonucleoprotein reverse transcriptase which stabilizes telomere length by adding hexameric (TTAGGG) repeats to the telomeric ends of the chromosomes, thus compensating for the continued erosion of telomeres. Introduction of the telomerase catalytic protein component into normal telomerase-negative human cells results in restoration of telomerase activity and extension of cellular life span. Human cells with introduced telomerase maintain a normal chromosome complement and continue to grow in a normal manner. Telomerase-induced manipulations of telomere length may thus be important not only for cell and tissue engineering but also for dissecting the molecular mechanisms underlying inherited genetic diseases, as well as defining the genetic pathways leading to cancer. Because almost all human tumors express telomerase activity, inhibition of telomerase may result in gradual erosion of telomeres and eventual cessation of cell proliferation or induction of apoptosis. Thus telomerase may also be a promising target for cancer therapy.
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Affiliation(s)
- J W Shay
- The University of Texas Southwestern Medical Center, Department of Cell Biology, 5323 Harry Hines Boulevard, Dallas, Texas 75390-9039, USA
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365
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Braasch DA, Corey DR. Locked nucleic acid (LNA): fine-tuning the recognition of DNA and RNA. CHEMISTRY & BIOLOGY 2001; 8:1-7. [PMID: 11182314 DOI: 10.1016/s1074-5521(00)00058-2] [Citation(s) in RCA: 428] [Impact Index Per Article: 18.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Locked nucleic acid is an RNA derivative in which the ribose ring is constrained by a methylene linkage between the 2'-oxygen and the 4'-carbon. This conformation restriction increases binding affinity for complementarity sequences and provides an exciting new chemical approach for the control of gene expression and optimization of microarrays.
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Affiliation(s)
- D A Braasch
- Departments of Pharmacology and Biochemistry, University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75390-9041, USA
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366
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Grobelny JV, Godwin AK, Broccoli D. ALT-associated PML bodies are present in viable cells and are enriched in cells in the G(2)/M phase of the cell cycle. J Cell Sci 2000; 113 Pt 24:4577-85. [PMID: 11082050 DOI: 10.1242/jcs.113.24.4577] [Citation(s) in RCA: 99] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
Telomere maintenance is essential for the unlimited proliferative potential of human cells, and hence immortalization. However, a number of tumors, tumor-derived cell lines and in vitro immortalized cell lines have been described that do not express detectable telomerase activity. These lines utilize a mechanism, termed Alternative Lengthening of Telomeres (ALT), to provide telomere maintenance. A subset of the cells in each ALT cell line contain a novel form of the promyelocytic leukemia nuclear body (PML NB) in which telomeric DNA and the telomere binding proteins TRF1 and TRF2 co-localize with the PML protein, termed ALT-associated PML bodies (AA-PBs). In contrast, in non-ALT, telomerase-positive cell lines these telomeric proteins and the PML NB occupy distinct and separate subnuclear domains. PML NBs have been implicated in terminal differentiation, growth suppression and apoptosis. The role, if any, of AA-PBs in telomere maintenance or culture viability in telomerase negative cell lines is unclear, but it has been suggested that cells containing these structures are no longer viable and are marked for eventual death. We utilized a series of human ovarian surface epithelium (HOSE) cell lines that use ALT for telomere maintenance to determine if AA-PBs are indeed markers of cells in these cultures that are no longer cycling. We show that AA-PB positive cells incorporate BrdU and thus are able to carry out DNA replication. In addition, AA-PBs are present in mitotic cells and the frequency of cells containing these structures is increased when cultures are enriched for cells in the G(2)/M phase of the cell cycle suggesting that the formation of AA-PBs is coordinately regulated with the cell cycle. Finally, we demonstrate that the majority of the AA-PB positive cells in the culture are not destined for immediate apoptosis. Taken together the data argue against AA-PBs marking cells destined for death and, instead, raise the possibility that these structures may be actively involved in telomere maintenance via the ALT pathway.
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Affiliation(s)
- J V Grobelny
- Fox Chase Cancer Center, Philadelphia PA 1911, USA
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367
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Abstract
Telomeres, the eukaryotic chromosome termini, are deoxyribonucleoprotein structures that distinguish natural chromosome ends from broken DNA. In most organisms, telomeres are extended by a reverse transcriptase (RT) with an integrated RNA template, telomerase; in Drosophila melanogaster, however, telomere-specific retrotransposons, HeT-A and TART, transpose specifically to chromosome ends. Whether telomeres are extended by a telomerase or by retrotransposons, an RT is a key component. RT has been studied extensively, both for its important role in converting RNA genomes to DNA, which has great evolutionary impact, and as a therapeutic target in human retroviral diseases. Here we discuss a few important aspects of RT usage during retrotransposition and telomere elongation.Key words: telomeres, telomerase, retrotransposons, reverse transcriptase.
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368
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369
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Ford LP, Suh JM, Wright WE, Shay JW. Heterogeneous nuclear ribonucleoproteins C1 and C2 associate with the RNA component of human telomerase. Mol Cell Biol 2000; 20:9084-91. [PMID: 11074006 PMCID: PMC86561 DOI: 10.1128/mcb.20.23.9084-9091.2000] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Here we demonstrate that heterogeneous nuclear ribonucleoproteins (hnRNPs) C1 and C2 can associate directly with the integral RNA component of mammalian telomerase. The binding site for hnRNPs C1 and C2 maps to a 6-base uridylate tract located directly 5' to the template region in the human telomerase RNA (TR) and a 4-base uridylate tract directly 3' to the template in the mouse TR. Telomerase activity is precipitated with antibodies specific to hnRNPs C1 and C2 from cells expressing wild-type human TR but not a variant of the human TR lacking the hnRNPs C1 and C2 binding site, indicating that hnRNPs C1 and C2 require the 6-base uridylate tract within the human TR to associate with the telomerase holoenzyme. In addition, we demonstrate that binding of hnRNPs C1 and C2 to telomerase correlates with the ability of telomerase to access the telomere. Although correlative, these data do suggest that the binding of hnRNPs C1 and C2 to telomerase may be important for the ability of telomerase to function on telomeres. The C proteins of the hnRNP particle are also capable of colocalizing with telomere binding proteins, suggesting that the C proteins may associate with telomeres in vivo. Therefore, human telomerase is capable of associating with core members of the hnRNP family of RNA binding proteins through a direct and sequence-specific interaction with the human TR. This is also the first account describing the precise mapping of a sequence in the human TR that is required to associate with an auxiliary component of the human telomerase holoenzyme.
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Affiliation(s)
- L P Ford
- Department of Cell Biology, The University of Texas Southwestern Medical Center, Dallas, Texas 75390-9039, USA
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370
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Cong YS, Bacchetti S. Histone deacetylation is involved in the transcriptional repression of hTERT in normal human cells. J Biol Chem 2000; 275:35665-8. [PMID: 10986277 DOI: 10.1074/jbc.c000637200] [Citation(s) in RCA: 119] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Trancriptional regulation of the human telomerase reverse transcriptase (hTERT) gene, encoding the catalytic protein of human telomerase, plays a critical role in the activation of the enzyme during cell immortalization and tumorigenesis. However, the molecular mechanisms involved in the regulation of hTERT expression are still not fully understood. We have previously cloned and characterized the genomic sequences and promoter of the hTERT gene. Here, we provide evidence that histone deacetylation is involved in the repression of hTERT in human cells. Inhibition of histone deacetylases by trichostatin A in telomerase-negative cells resulted in activation of telomerase activity and up-regulation of hTERT mRNA. Transient transfection experiments with a reporter under control of the hTERT promoter indicated that this promoter can be activated by trichostatin A. Finally, our results show that repression of the hTERT promoter by the Mad protein requires histone deacetylase activity, whereas de-repression by trichostatin A is independent of the E-boxes located in its core region.
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Affiliation(s)
- Y S Cong
- Department of Pathology and Molecular Medicine, McMaster University, 1200 Main Street West, Hamilton, Ontario L8N 3Z5, Canada
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371
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Liu Y, Snow BE, Hande MP, Yeung D, Erdmann NJ, Wakeham A, Itie A, Siderovski DP, Lansdorp PM, Robinson MO, Harrington L. The telomerase reverse transcriptase is limiting and necessary for telomerase function in vivo. Curr Biol 2000; 10:1459-62. [PMID: 11102810 DOI: 10.1016/s0960-9822(00)00805-8] [Citation(s) in RCA: 173] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
Mammalian telomerase is essential for the maintenance of telomere length [1-5]. Its catalytic core comprises a reverse transcriptase component (TERT) and an RNA component. While the biochemical role of mammalian TERT is well established [6-11], it is unknown whether it is sufficient for telomere-length maintenance, chromosome stability or other cellular processes. Cells from mice in which the mTert gene had been disrupted showed progressive loss of telomere DNA, a phenotype similar to cells in which the gene encoding the telomerase RNA component (mTR) has been disrupted [1,12]. On prolonged growth, mTert-deficient embryonic stem (ES) cells exhibited genomic instability, aneuploidy and telomeric fusions. ES cells heterozygous for the mTert disruption also showed telomere attrition, a phenotype that differs from heterozygous mTR cells [12]. Thus, telomere maintenance in mammals is carried out by a single, limiting TERT.
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Affiliation(s)
- Y Liu
- Ontario Cancer Institute/Amgen Institute, Department of Medical Biophysics, University of Toronto, 620 University Avenue, ON M5G 2C1, Toronto, Canada
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372
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Abstract
Almost 40 years ago, Leonard Hayflick discovered that cultured normal human cells have limited capacity to divide, after which they become senescent -- a phenomenon now known as the 'Hayflick limit'. Hayflick's findings were strongly challenged at the time, and continue to be questioned in a few circles, but his achievements have enabled others to make considerable progress towards understanding and manipulating the molecular mechanisms of ageing.
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Affiliation(s)
- J W Shay
- University of Texas Southwestern Medical Center at Dallas, Department of Cell Biology, 5323 Harry Hines Boulevard, Dallas, Texas 75390-9039, USA.
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373
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Colgin LM, Wilkinson C, Englezou A, Kilian A, Robinson MO, Reddel RR. The hTERTalpha splice variant is a dominant negative inhibitor of telomerase activity. Neoplasia 2000; 2:426-32. [PMID: 11191109 PMCID: PMC1507985 DOI: 10.1038/sj.neo.7900112] [Citation(s) in RCA: 161] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
The telomerase catalytic subunit (hTERT) is an essential component of the holoenzyme complex that adds telomeric repeats to the ends of human chromosomes. Maintenance of telomeres by telomerase or another mechanism is required for cell immortalization, and loss of telomeric DNA has been proposed as a trigger for cellular senescence. Available evidence suggests that regulation of telomerase activity primarily depends on transcriptional control of hTERT. However, several human tissues as well as some normal cell strains have been shown to express low levels of hTERT mRNA even though they lack telomerase activity. We have previously identified six splice variants of hTERT, including a "deletion" variant (hTERTalpha) that is missing conserved residues from the catalytic core of the protein. Several of the deletion variants have been detected in normal and developing human tissues. We now show that hTERTalpha inhibits endogenous telomerase activity, which results in telomere shortening and chromosome end-to-end fusions. Telomerase inhibition induced a senescence-like state in HT1080 cells and apoptosis in a jejunal fibroblast cell line. These results suggest a possible role for hTERT splice variants in the regulation of telomerase activity.
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Affiliation(s)
- L M Colgin
- Children's Medical Research Institute, Westmead, New South Wales, Australia
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374
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Yi X, White DM, Aisner DL, Baur JA, Wright WE, Shay JW. An alternate splicing variant of the human telomerase catalytic subunit inhibits telomerase activity. Neoplasia 2000; 2:433-40. [PMID: 11191110 PMCID: PMC1507981 DOI: 10.1038/sj.neo.7900113] [Citation(s) in RCA: 144] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023] Open
Abstract
Telomerase, a cellular reverse transcriptase, adds telomeric repeats to chromosome ends. In normal human somatic cells, telomerase is repressed and telomeres progressively shorten, leading to proliferative senescence. Introduction of the telomerase (hTERT) cDNA is sufficient to produce telomerase activity and immortalize normal human cells, suggesting that the repression of telomerase activity is transcriptional. The telomerase transcript has been shown to have at least six alternate splicing sites (four insertion sites and two deletion sites), and variants containing both or either of the deletion sites are present during development and in a panel of cancer cell lines we surveyed. One deletion (beta site) and all four insertions cause premature translation terminations, whereas the other deletion (alpha site) is 36 bp and lies within reverse transcriptase (RT) motif A, suggesting that this deletion variant may be a candidate as a dominant-negative inhibitor of telomerase. We have cloned three alternately spliced hTERT variants that contain the alpha, beta or both alpha and beta deletion sites. These alternate splicing variants along with empty vector and wild-type hTERT were introduced into normal human fibroblasts and several telomerase-positive immortal and tumor cell lines. Expression of the alpha site deletion variant (hTERT alpha-) construct was confirmed by Western blotting. We found that none of the three alternate splicing variants reconstitutes telomerase activity in fibroblasts. However, hTERT alpha- inhibits telomerase activities in telomerase-positive cells, causes telomere shortening and eventually cell death. This alternately spliced dominant-negative variant may be important in understanding telomerase regulation during development, differentiation and in cancer progression.
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MESH Headings
- Alternative Splicing
- Blotting, Western
- Carcinoma/enzymology
- Carcinoma/pathology
- Carcinoma, Non-Small-Cell Lung/enzymology
- Carcinoma, Non-Small-Cell Lung/pathology
- Catalytic Domain
- Cell Line/enzymology
- Cell Line, Transformed/enzymology
- Cell Transformation, Neoplastic/genetics
- Chromosome Aberrations
- Chromosomes, Human/ultrastructure
- DNA, Complementary/genetics
- DNA-Binding Proteins
- Fetal Proteins/chemistry
- Fetal Proteins/genetics
- Fibroblasts/cytology
- Fibroblasts/enzymology
- Genes, Dominant
- Genetic Vectors/genetics
- Humans
- Lung/cytology
- Lung Neoplasms/enzymology
- Lung Neoplasms/pathology
- Male
- Neoplasm Proteins/chemistry
- Neoplasm Proteins/genetics
- Peptide Chain Termination, Translational/genetics
- Prostatic Neoplasms/enzymology
- Prostatic Neoplasms/pathology
- Protein Subunits
- RNA
- Recombinant Fusion Proteins/physiology
- Retroviridae/genetics
- Sequence Deletion
- Skin/cytology
- Telomerase/antagonists & inhibitors
- Telomerase/chemistry
- Telomerase/genetics
- Telomerase/physiology
- Telomere/metabolism
- Telomere/ultrastructure
- Transfection
- Tumor Cells, Cultured/enzymology
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Affiliation(s)
- X Yi
- Department of Cell Biology, The University of Texas Southwestern Medical Center at Dallas, 75390-9039, USA
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375
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Wright WE, Shay JW. Telomere dynamics in cancer progression and prevention: fundamental differences in human and mouse telomere biology. Nat Med 2000; 6:849-51. [PMID: 10932210 DOI: 10.1038/78592] [Citation(s) in RCA: 277] [Impact Index Per Article: 11.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Cells from the telomerase knockout mouse immortalize with an approximately ten million-fold greater frequency than human cells. In this commentary, Wright and Shay discuss the implications of this difference between mice and men and its relationship to cancer.
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Affiliation(s)
- W E Wright
- Department of Cell Biology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75390-9039, USA
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376
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Abstract
In the past decade, a great deal has been learnt about the maintenance of telomeres in mammalian cells by the specialized reverse transcriptase, telomerase, and its associated proteins. The catalytic component of telomerase, hTERT, appears to be selectively activated in the vast majority of tumors relative to most somatic cells suggesting that its inhibition may result in antitumor effects. Although beset with some unusual issues as a drug target, recent 'target validation' studies using hTERT dominant-negative and antisense approaches strongly support the view that potent and selective telomerase inhibitors will induce inhibitory effects on tumors, especially in those possessing relatively short telomeres. Inhibitory strategies have focused on three main areas: antisense molecules (oligonucleotides, RNA molecules, ribozymes and peptide nucleic acids) directed against the hTR RNA component of telomerase, small molecule reverse transcriptase inhibitors (e.g. azidothymidine), and, probably most advanced, small molecules capable of interacting with and stabilizing four-stranded (G-quadruplex) structures formed by telomeres. G-quadruplex interactive agents that inhibit telomerase at sub-micromolar concentrations in cell-free assays have been described. Lead optimization and preclinical whole-cell and animal antitumor and pharmacology studies are now progressing which should result in the first generation of telomerase inhibitors being evaluated in the clinic within the next few years.
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Affiliation(s)
- L R Kelland
- CRC Centre for Cancer Therapeutics, The Institute of Cancer Research, Sutton, Surrey, UK.
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377
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Weng NP, Hodes RJ. The role of telomerase expression and telomere length maintenance in human and mouse. J Clin Immunol 2000; 20:257-67. [PMID: 10939713 DOI: 10.1023/a:1017223602293] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
The molecular regulation of telomere length has been well elucidated by a series of elegant studies over the past decade. More recently, experimental evidence has accrued that addresses the challenging question of if and how telomere length regulation may contribute to normal human aging or to human disease. Recent studies in mice have provided a mammalian precedent indicating that telomerase deficiency can lead to in vivo dysfunction, most probably as a consequence of progressive telomere shortening. In humans, the evidence that telomere shortening might lead to in vivo dysfunction is far less direct, although the recent description of telomerase deficiency and telomere shortening associated with the DKC syndrome is suggestive of such a link. Methodologies exist and continue to be developed that are increasingly capable of manipulating telomerase activity and telomere length in human cells. It remains to be determined whether scientifically rigorous and (equally important) medically ethical approaches will emerge to directly assess the ability of telomere length modulation to correct functional disorders of human cellular function ex vivo or more challenging still, in vivo.
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Affiliation(s)
- N P Weng
- National Institute on Aging and National Cancer Institute, Bethesda, Maryland 20892, USA
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378
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Abstract
Shortening of the telomeric DNA at chromosome ends is postulated to limit the lifespan of human cells. In contrast, activation of telomerase, the enzyme that synthesizes telomeric DNA, is proposed to be an essential step in cancer cell immortalization and cancer progression. This review discusses the structure and function of telomeres and telomerase, the role of telomerase in cell immortalization, and the effects of telomerase inactivation on normal and cancer cells. Moreover, data on the experimental use of telomerase assays for cancer detection and diagnosis are reviewed. Finally, the review considers the evidence regarding whether telomerase inhibitors could be used to treat human cancers.
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Affiliation(s)
- M Meyerson
- Department of Adult Oncology, Dana-Farber Cancer Institute, Boston, MA 02115, USA.
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379
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Faraoni I, Bonmassar E, Graziani G. Clinical applications of telomerase in cancer treatment. Drug Resist Updat 2000; 3:161-170. [PMID: 11498381 DOI: 10.1054/drup.2000.0139] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Telomerase activity has been found in most cancer cells, but not in the majority of normal differentiated tissues. Therefore, telomerase has been considered a relatively selective and widely expressed tumor marker to be used as a diagnostic tool, and in some cases, as a potential prognostic indicator. Telomerase activity can also be used to evaluate chemosensitivity of neoplastic cells obtained from cancer patients, by measuring residual telomerase activity after drug treatment. Finally, telomerase has been considered to represent a suitable target for designing new anticancer strategies. This review focuses on present and future clinical applications of telomerase studies in cancer management. Copyright 2000 Harcourt Publishers Ltd.
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Affiliation(s)
- Isabella Faraoni
- Section of Pharmacology, Medical Oncology, Department of Neuroscience, University of Rome 'Tor Vergata', Rome, Italy
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380
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Abstract
New features of mammalian telomeres and telomerase have been identified. Telomeres form t-loops, which engage the 3' single-stranded DNA overhang in an interaction with double-stranded telomeric repeats. Mammalian telomerases contain an RNA H/ACA motif and associated protein(s) shared with H/ACA family of small nucleolar ribonucleoproteins. Essential roles for telomerase in the sustained viability of cultured tumor cells and in the normal proliferative capacity of human somatic cells have been demonstrated.
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Affiliation(s)
- K Collins
- Division of Biochemistry and Molecular Biology, Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720-3204, USA.
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381
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Abstract
As normal humans age, telomeres shorten in tissues that contain dividing cells, and this has been proposed both as a cause of ageing and as a tumor-suppressor mechanism. The surprising finding that cells from individuals with the rare inherited disorder dyskeratosis congenita (DKC) have reduced levels of telomerase and shortened telomeres might provide the first direct genetic test of the function of telomeres in intact humans.
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Affiliation(s)
- R A Marciniak
- Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
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382
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Affiliation(s)
- C H Buys
- University of Groningen, The Netherlands
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383
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Abstract
Cancer as a disease in the human population is becoming a larger health problem, and the medicines used as treatments have clear limitations. In the past 20 years, there has been a tremendous increase in our knowledge of the molecular mechanisms and pathophysiology of human cancer. Many of these mechanisms have been exploited as new targets for drug development in the hope that they will have greater antitumor activity with less toxicity to the patient than is seen with currently used medicines. The fruition of these efforts in the clinic is just now being realized with a few encouraging results.
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Affiliation(s)
- J B Gibbs
- Department of Cancer Research, Merck Research Laboratories, West Point, PA 19486, USA.
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