• E-mail
• Print
• Comment
• Font Size
• Digg
• del.icio.us
• Discuss article

The evolving role of telomerase inhibitors in the treatment of cancer

Posted on: Saturday, 1 November 2003, 06:00 CST

Key words: Cancer - Inhibitors - Telomerase - Therapy

SUMMARY

Telomerase is a ribonucleoprotein that maintains telomeres and is essential for cellular immortality and tumour growth. The differential expression of telomerase in cancer cells makes it an attractive therapeutic target. Anti-sense oligonucleotides directed against the RMA template of hTR and small molecules that can interact and stabilise the G-quadruplex represent promising therapeutic strategies. Human trials investigating the potential role of the catalytic subunit hTERT as a universal cancer vaccine have already commenced. Alternative lengthening of telomeres (ALT) and efficacy delay remain important limitations to antitelomerase therapy.

Introduction

Telomerase is a multi-component ribonucleoprotein located within the nucleus. It is an RNA-dependant DNA polymerase, the function of which is to synthesise the repetitive nucleotide sequence (TTAGGG in humans) forming the telomeres at the end of chromosomes1. Without telomerase activity, each round of cellular division results in the shortening of telomeres and reaching a critical length seems to trigger off cellular apoptosis. Telomerase appears to stabilise telomeres, thus leading to cellular immortality2. The enzyme is active in most human cancers and immortal cell lines2,3. Most normal somatic cells have no detectable telomerase activity, with the exception of certain stem cells, lymphocytes and germline cells4. The activation of telomerase does not cause carcinogenesis but it does allow a cell to continue division and attain immortality, a necessary achievement for a cancerous cell to be successful. Although alternative methods of maintaining telomere stability have been identified5,6, the majority of tumours in all types investigated appear to employ telomerase activation as a means of maintaining proliferation at a high rate without becoming senescent.

Human telomerase consists of an RNA subunit - human telomerase RNA (hTR), a protein component (human telomerase associated protein 1 (hTEP1); chaperones such as hsp90, and the catalytic subunit, human telomerase reverse transcriptase known as hTERT5,7,8. The differential expression of telomerase in most malignant cells makes it an attractive target for cancer therapy. All telomerase components represent potential therapeutic targets.

Targeting hTERT

hTERT can be targeted by nucleoside analogues, antisense oligonucleotides, hammerhead ribozymes and phosphorylation inhibitors9-15. The nucleoside azidothymidine (AZT) has been shown to inhibit telomerase activity and reduce telomeric length in malignant cells, but this effect seems to be cell-type specific9. A potent nucleoside analogue inhibitor of hTERT, 6-thio-7-deaza-2'- deoxyguanosine 5'-triphosphate has been recently described10. The role of antisense nucleotides directed against hTERT mRNA has not been adequately investigated. Yokoyama et al. have demonstrated that a hammerhead ribozyme targeting the 5'-terminal of hTERT mRNA can suppress telomerase activity11. Inhibition of hTERT phosphorylation by phosphokinase C (PKC) inhibitors such as bis-indolylmaleimide I and H-7 is unlikely to be an effective therapeutic strategy due to potential interference with physiologically active PKCs12. hTERT can also be inhibited by drugs capable of down-regulating the upstream regulators of c-Myc13.

Targeting hTR

The RNA template of hTR is easily accessible to hybridisation with complementary oligonucleotides and serves a critical function. Inhibition of hTR RNA template does not seem to affect the function of hTR positive cells, but does affect telomerase negative cells. Herbert et al. have recently observed that a phosphoramidate oligonucleotide directed against the RNA template can inhibit telomerase in cell culture14. More recently, Chen et al. have demonstrated that a 2-methoxy-ethyl RNA oligonucleotide (ISIS 24691) is a potent growth inhibitor of cancer cell lines without the need for a lipid carrier. ISIS 24691 also reduced telomeric length and cellular proliferation with IC^sub 50^ values ranging from 0.3 to 1 micro-molar15. This agent is currently being investigated using xenograft animal models. Other phosphoramidate derivatives have been shown to inhibit telomerase activity in a sequence-specific and dose- dependent fashion with nano-molar IC^sub 50^ values16. Unlike ISIS 24691, these compounds require a lipid vector. Other antisense oligomers targeting hTR include 2',5'-oligoadenylate which has been found to exhibit anti-tumour activity in both in vitro and in vivo experimental models17. The hTR sub-unit can be also targeted by hammerhead ribozymes18, compounds that bind to the RNA/DNA heteroduplex19 and mutant RNA templates that can exert a dominant negative effect20.

Miscellaneous targets

TEP1 is not essential for telomerase activity and it has not therefore been investigated for anti-cancer therapeutic targets. Inhibition of the chaperone protein hsp90 is unlikely to be a successful therapeutic strategy due to interference with normal physiological processes.

There are also indirect therapeutic approaches against telomerase. These include suicide gene therapy21 and immunotherapy22.

Koga et al. demonstrated that hTERT/caspase 8 constructs exhibited anti-tumour activity in vivo in animal studies21. hTERT is a widely expressed tumour associated antigen that can be recognised by cytotoxic T-lymphocytes. Cytotoxic T-cells induced by hTERT have been reported to lyse melanoma cells and exhibit anti-tumour activity in animal studies22. Ayyoub et al. have recently reported lack of tumour recognition by hTERT peptides 540-548-specific CDS +ve T cells from melanoma patients, thus raising the possibility of inadequate antigen processing by the proteosome23.

Small molecules interacting and stabilising the four-stranded telomeric G-quadruplex have been shown to be potent inhibitors of telomerase and human cancer cell line growth24. Such compounds (e.g. cationic porphyrins, pentacyclic acridines) may also interact with telomere-like DNA regions, guanine rich RNA or DNA enzymes, thus interfering with normal cellular function.

Future prospects

Although inhibition of telomerase and telomeres offers exciting therapeutic possibilities in the fight against human cancer, further basic scientific research is required in order to overcome two important limitations to this approach. Firstly, alternative lengthening of telomeres (ALT) by copy switching and homologous recombination occurs in up to 10% of human cancers. The process is suppressed in telomerase-positive tumours and is reactivated in telomerase-negative cancer.

Theoretically ALT can lead to anti-telomerase resistance. Such resistance is least likely with anti-G-quadruplex small molecules. Secondly, anti-telomerase therapy is associated with efficacy delay which seems to be dependant on the tumour doubling time and the size distribution of telomeres25. This observation suggests that cancer cells will be affected before telomerase positive normal cells (long telomeres, long doubling time) by anti-telomerase therapy.

Furthermore normal cells are likely to recover rapidly after the completion of treatment.

Phase I and II clinical trials evaluating indirect inhibitors of telomerase such as hTERT immunotherapy have already commenced. However, careful animal studies are required to evaluate anti-sense oligonucleotides and anti-G-quadruplex small molecules before planning clinical trials in humans.

CURRENT MEDICAL RESEARCH AND OPINION(R)

VOL. 19, NO. 6, 2003, 470-472

(C) 2003 LIBRAPHARM LIMITED

CrossRef links are available in the online published version of this paper: http://www.cmrojournal.com

Paper CMRO-2342, Accepted for publication: 30 March 2003

Published Online: 11 July 2003

doi: 10.1185/030079903125002081

References

1. Mokbel K. The role of telomerase in breast cancer. Eur J Surg Oncol 2000;26:509-14

2. Zhu J, Wang H, Bishop J, Blackburn E. Telomerase extends the lifespan of virus-transformed human cells without net telomere lengthening. Proc Natl Acad Sci USA 1999;96:3723-8

3. Mokbel K, Parris CN, Ghilchik M, Williams G, Newbold RF. The association between telomerase, histopathological parameters, and KI- 67 expression in breast cancer. Am J Surg 1999;178:69-72

4. Liu K, Schoonmaker MM, Levine BL, June CH, Hodes RJ, Weng NP. Constitutive and regulated expression of telomerase reverse transcriptase (hTERT] in human lymphocytes. Proc Natl Acad Sci USA 1999;96:5147-52

5. Kirkpatrick KL, Mokbel K. The significance of human telomerase reverse transcriptase (hTERT) in cancer. Eur J Surg Oncol 2001;27:754-60

6. Le S, Moore J, Haber J, Greider C. RAD50 and RAD51 define two pathways that collaborate to maintain telomeres in the absence of telomerase. Genetics 1999;152:143-52

7. Feng J, Funk WD, Wang SS, Weinrich SL, Avilion AA, Chiu CP, et al. The RNA component of human telomerase. Science 1995;269:1236-41

8. Harrington L, McPhail T, Mar V, Zhou W, Oulton R, Bass MB, et al. A mammalian telomerase-associated protein. Science 1997;275:973- 7

9. Murakami J, Nagai N, Shigemasa K, Ohama K. Inhibition of telomerase activity and cell proliferation by a reverse transcriptase inhibitor in gynaecological cancer cell lines. Eur J Cancer 1999;35:1027-34

10. Fletcher TM, Cathers BE, Ravikumar KS, Mamiya BM, Kerwin M. 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

11. Yokoyama Y, Takahashi Y, Shinohara A, et al. The 5'-end of hTERT mRNA is a good target for hammerhead ribozyme to suppress telomerase activity. Biochem Biophys Res Commun 2000;273:316-21

12. Goekjian PG, Jirousek MR. Protein kinase C inhibitors as novel anticancer drugs. Expert Opin Invest Drugs 2001;10:2117-40

13. Chou WC, Hawkins AL, Barret JF, Griffin CA, Dang CV Arsenic inhibition of telomerase transcription leads to genetic instability. J Clin Invest 2001;108:1541-7

14. Herbert BS, Pongracz K, Shay JW, Gryaznov SM. Oligonucleotide N3'-P5' phosphoramidate as efficient telomerase inhibitors. Oncogene 2002;21:401-10

15. Chen Z, Monia PB, Corey DR. Telomerase inhibition, telomere shortening, and decreased cell proliferation by cell permeable 2'-O- Methoxyethyl oligonucleotides. J Med Chem 2002;45:5423-5

16. Gryaznov SM, Pongracz K, Matray T, et al. Telomerase inhibitors: oligonucleotide phosphoramidates as potential therapeutic agents. Nucleosides Nucleotide Nucleic Acids 2001;20:401- 10

17. Kondo Y, Koga S, Komata T, Kondo S. Treatment of prostate cancer in vitro and in vivo with 2-5A-anti-telomerase RNA component. Oncogene 2000;19:2205-11

18. Yokoyama Y, Takahashi Y, Shinohara A, et al. Attenuation of telomerase activity by a hammerhead ribozyme targeting the template region of telomerase RNA in endometrial carcinoma cells. Cancer Res 1998;58:5406-10

19. Ren J, Qu X, Dattagupta N and Chaires JB. Molecular recognition of a RNA:DNA hybrid structure. J Am Chem Soc 2001;123:6742-3

20. Guiducci C, Cerone MA, Bacchetti S. Expression of mutant telomerase in immortal telomerase-negative human cells results in cell cycle deregulation, nuclear and chromosomal abnormalities and rapid loss of viability. Oncogene 2001;20:714-25

21. Koga S, Hirohata S, Kondo Y, et al. A novel telomerase- specific gene therapy: gene transfer of caspase-8 utilizing the human telomerase catalytic subunit gene promoter. Hum Gene Ther 2000;11:1397-1406

22. Nair SK, Heiser A, Boczkowski D, et al. Induction of cytotoxic T cell responses and tumor immunity against unrelated tumors using telomerase reverse transcriptase RNA transfected dendritic cells. Nature Med 2000;6:1011-17

23. Ayyoub M, Migliaccio M, Guillaume P, et al. Lack of tumor recognition by hTERT peptide 540-548-specific CD8(+) T cells from melanoma patients reveals inefficient antigen processing. Eur J Immunol 2001;31:2642-51

24. Gowan SM, Heald R, Stevens MF, Kelland LR. Potent inhibition of telomerase by small pentacyclic acridines capable of interacting with G-quadruplex. Mol Pharmacol 2001;60:981-8

25. Sidorov IA, Hirsch KS, Harley CB, Dimitrov DS. Cancer treatment by telomerase inhibitors: predictions by a kinetic model. Math Biosci 2003;181:209-21

Kefah Mokbel

Institute of Cancer Genetics and Pharmacogenomics, Brunel University, and St George's and the Princess Grace Hospitals, London, UK

Address for correspondence: Professor Kefah Mokbel, Consultant Breast and Endocrine Surgeon, The Princess Grace Hospital, Nottingham Place, London W1M 3FD, UK. Tel. +44 (0)207 908 2040; Fax +44 (0)207 908 2275; email kefahmokbel@hotmail.com

Copyright Librapharm 2003

More News in this Category