Regulation of telomerase towards tumor therapy

Cancer is an aging-related disease, while aging plays an important role in the development process of tumor, thus the two are inextricably associated. Telomere attrition is one of the recognized hallmark events of senescence. Hence, targeting telomerase which could extends telomere sequences to treat tumors is widely favored. Cancer cells rely on high activity of telomerase to maintain a strong proliferative potential. By inhibiting the expression or protein function of telomerase, the growth of cancer cells can be significantly suppressed. In addition, the human immune system itself has a defense system against malignant tumors. However, excessive cell division results in dramatic shortening on telomeres and decline in the function of immune organs that facilitates cancer cell evasion. It has been shown that increasing telomerase activity or telomere length of these immune cells can attenuate senescence, improve cellular viability, and enhance the immunosuppressive microenvironment of tumor. In this paper, we review the telomerase-targeting progress using different anti-tumor strategies from the perspectives of cancer cells and immune cells, respectively, as well as tracking the preclinical and clinical studies of some representative drugs for the prevention or treatment of tumors.

Prevalence of telomerase activity in human cancer

Telomerase activity has been measured in a wide variety of cancerous and non-cancerous tissue types, and the vast majority of clinical studies have shown a direct correlation between it and the presence of cancerous cells. Telomerase plays a key role in cellular immortality and tumorigenesis. Telomerase is activated in 80-90% of human carcinomas, but not in normal somatic cells, therefore, its detection holds promise as a diagnostic marker for cancer. Measurable levels of telomerase have been detected in malignant cells from various samples: tissue from gestational trophoblastic neoplasms; squamous carcinoma cells from oral rinses; lung carcinoma cells from bronchial washings; colorectal carcinoma cells from colonic luminal washings; bladder carcinoma cells from urine or bladder washings; and breast carcinoma or thyroid cancer cells from fine needle aspirations. Such clinical tests for telomerase can be useful as non-invasive and cost-effective methods for early detection and monitoring of cancer. In addition, telomerase activity has been shown to correlate with poor clinical outcome in late-stage diseases such as non-small cell lung cancer, colorectal cancer, and soft tissue sarcomas. In such cases, testing for telomerase activity can be used to identify patients with a poor prognosis and to select those who might benefit from adjuvant treatment. Our review of the latest medical advances in this field reveals that telomerase holds great promise as a biomarker for early cancer detection and monitoring, and has considerable potential as the basis for developing new anticancer therapies.

The potential utility of telomere-related markers for cancer diagnosis

The role telomeres and telomerase play in the initiation and progression of human cancers has been extensively evaluated. Telomeres are nucleoprotein complexes comprising the hexanucleotide DNA repeat sequence, TTAGGG and numerous telomere-associated proteins, including the six member Shelterin complex. The main function of the telomere is to stabilize the ends of the chromosomes. However, through multiple mechanisms, telomeres can become dysfunctional, which may drive genomic instability leading to the development of cancer. The majority of human cancers maintain, or actively lengthen, telomeres through up-regulation of the reverse transcriptase telomerase. Because there are significant differences in telomere length and telomerase activity between malignant and non-malignant tissues, many investigations have assessed the potential to utilize these molecular markers for cancer diagnosis. Here, we critically evaluate whether measurements of telomere lengths and telomerase levels may be clinically utilized as diagnostic markers in solid tumours, with emphasis on breast and prostate cancer as representative examples. Future directions focusing on the direct detection of dysfunctional telomeres are explored. New markers for telomere dysfunction may eventually prove clinically useful.

Telomerase detection in the diagnosis and prognosis of cancer

Telomerase, a critical enzyme responsible 'for cellular immortality, is usually repressed in somatic cells except for lymphocytes and self-renewal cells, but is activated in approximately 85% of human cancer tissues. The human telomerase reverse transcriptase (hTERT) is the catalytic component of human telomerase. In cancers in which telomerase activation occurs at the early stages of the disease, telomerase activity and hTERT expression are useful markers for the detection of cancer cells. In other cancers in which telomerase becomes upregulated upon tumor progression, they are useful as prognostic indicators. However, careful attention should be paid to false-negative results caused by the instability of telomerase and of the hTERT mRNA and the presence of PCR inhibitors, as well as to false-positive results caused by the presence of alternatively spliced hTERT mRNA and normal cells with telomerase activity. Recently, methods for the in situ detection of the hTERT mRNA and protein have been developed. These methods should facilitate the unequivocal detection of cancer cells, even in tissues containing a background of normal telomerase-positive cells.

Dynamics of telomeric chromatin at the crossroads of aging and cancer

Telomeres are nucleoprotein structures that protect the ends of human chromosomes through the formation of a 'cap', thus preventing exonucleolytic degradation, inter- and intra-chromosomal fusion, and subsequent chromosomal instability. During aging, telomere shortening correlates with tissue dysfunction and loss of renewal capacity. In human cancer, telomere dysfunction is involved in early chromosome instability, long-term cellular proliferation, and possibly other processes related to cell survival and microenvironment. Telomeres constitute an attractive target for the development of novel small-molecule anti-cancer drugs. In particular, individual protein components of the core telomere higher-order chromatin structure (known as the telosome or 'shelterin' complex) are promising candidate targets for cancer therapy.

A cancer detection platform which measures telomerase activity from live circulating tumor cells captured on a microfilter

Circulating tumor cells (CTC) quantified in cancer patients' blood can predict disease outcome and response to therapy. However, the CTC analysis platforms commonly used cannot capture live CTCs and only apply to tumors of epithelial origin. To address these limitations, we have developed a novel cancer detection platform which measures telomerase activity from live CTCs captured on a parylene-C slot microfilter. Using a constant low-pressure delivery system, the new microfilter platform was capable of cell capture from 1 mL of whole blood in less than 5 minutes, achieving 90% capture efficiency, 90% cell viability, and 200-fold sample enrichment. Importantly, the captured cells retained normal morphology by scanning electron microscopy and could be readily manipulated, further analyzed, or expanded on- or off-filter. Telomerase activity--a well-recognized universal cancer marker--was reliably detected by quantitative PCR from as few as 25 cancer cells added into 7.5 mL of whole blood and captured on the microfilter. Moreover, significant telomerase activity elevation was also measured from patients' blood samples and from single cancer cells lifted off of the microfilter. Live CTC capture and analysis is fast and simple yet highly quantitative, versatile, and applicable to nearly all solid tumor types, making this a highly promising new strategy for cancer detection and characterization.

Transduction of E2F-1 TAT fusion proteins represses expression of hTERT in primary ductal breast carcinoma cell lines

Background

Telomerase expression is detectable in 81–95% of breast carcinomas and may serve as a therapeutic target. The objective of this study was to investigate repression of telomerase activity in primary ductal breast cancer cells through transcriptional regulation of the catalytic subunit hTERT. We hypothesized that inhibition of telomerase expression could be achieved via Tat mediated protein transduction of the repressor protein E2F-1.

Methods

Protein purification techniques were refined to yield biologically active Tat fusion proteins (TFPs) capable of transducing the breast cancer cell lines HCC1937 and HCC1599. Cell lines were treated with wildtype E2F-1 (E2F-1/TatHA), mutant E2F-1 (E132/TatHA) and a control Tat peptide (TatHA) for 24 hours. Total RNA was isolated from treated cells, reverse transcribed and fold changes in gene expression for hTERT determined via real-time RT-qPCR.

Results

Significant repression of the catalytic subunit of telomerase (hTERT) was present in both HCC1937 and HCC1599 cells following treatment with E2F-1/TatHA. In HCC1937 cells, hTERT was repressed 3.5-fold by E2F-1/TatHA in comparison to E132/TatHA (p < 0.0012) and the TatHA peptide controls (p < 0.0024). In HCC1599 cells, hTERT was also repressed with E2F-1/TatHA treatment by 4.0-fold when compared to the E132/TatHA control (p < 0.0001). A slightly lower hTERT repression of 3.3-fold was observed with E2F-1/TatHA in the HCC1599 cells when compared to the TatHA control (p < 0.0001).

Conclusion

These results suggest that transduction of E2F-1/TatHA fusion proteins in vitro is an effective repressor of hTERT expression in the primary ductal breast cancer cell lines HCC1937 and HCC1599.

Telomerase activity in fine needle aspiration biopsy samples: Application to diagnosis of human thyroid carcinoma

BACKGROUND

The diagnosis of thyroid follicular carcinoma by fine needle aspiration biopsy is a well known problem in thyroid pathology.

METHODS

We evaluated telomerase activity (TA) in 85 fine needle aspiration biopsy (FNAB) samples from patients with thyroid nodules. Surgery samples from patients with tumor or follicular adenomas were also analyzed.

RESULTS

Twenty of the FNAB samples corresponded to carcinomas and were positive to telomerase assay (TA >10 Units). Among them, 4 follicular carcinomas and 1 papillary carcinoma were labeled as indeterminate by FNAB cytological examination. Four percent false positive cases and no false negative cases for TA in FNABs were reported. FNAB samples from follicular adenomas were diagnosed as indeterminate by cytological examination, but they showed no detectable TA. Tumor tissues from patients with follicular or papillary thyroid carcinomas presented TA >10 Units, whereas follicular adenoma tissues (benign nodules) showed no TA.

CONCLUSION

Our results showed a good correlation between TA in FNAB samples and tumor/nodule thyroid tissue. This suggested that use of TA as a biological marker of malignancy might be a useful tool in the diagnosis of follicular thyroid carcinomas or follicular thyroid adenomas using FNAB samples.

The role of telomeres and telomerase in the pathology of human cancer and aging

Cellular senescence, the state of permanent growth arrest, is the inevitable fate of replicating normal somatic cells. Postulated to underlie this finite replicative span is the physiology of telomeres, which constitute the ends of chromosomes. The repetitive sequences of these DNA-protein complexes progressively shorten with each mitosis. When the critical length is bridged, telomeres trigger DNA repair and cell cycle checkpoint mechanisms that result in chromosomal fusions, cell cycle arrest, senescence and/or apoptosis. Should senescence be bypassed at such time, continued cell divisions in the face of dysfunctional telomeres and activated DNA repair machinery can result in the genomic instability favourable for oncogenesis. The longevity and malignant progression of the thus transformed cell requires coincident telomerase expression or other means to negate the constitutional telomeric loss. Practically then, telomeres and telomerase may represent plausible prognostic and screening cancer markers. Furthermore, if the argument is extended, with assumptions that telomeric attrition is indeed the basis of cellular senescence and that accumulation of the latter equates to aging at the organismal level, then telomeres may well explain the increased incidence of cancer with human aging.

Telomerase in breast cancer: a critical evaluation

Human chromosomes have highly specialized structures at their ends termed telomeres, repetitive, non-coding DNA sequences (5'-TTAGGG-3'), ranging in size from 5 to 20 kb in human cells. These highly specialized structures prevent chromosome ends from being recognized as double-strand DNA breaks, and they also provide protection from destabilizing agents. The mechanism for maintaining telomere integrity is controlled by telomerase, a ribonucleoprotein enzyme that specifically restores telomere sequences lost during replication by using an intrinsic RNA component as a template for polymerization. Telomerase has two core functional components required for its activity: the catalytic subunit of human telomerase reverse transcriptase (hTERT) and a telomerase RNA template (hTR). Telomerase is activated in the majority of immortal cell lines in culture and in most malignant tumors. This review outlines our current understanding of telomerase in breast cancer development and critically evaluates potential utilities in diagnosis, prognosis, and therapy.

Correlation between the expression of telomerase reverse transcriptase and proliferative activity in breast cancer cells using an immunocytochemical restaining method

Telomerase activity is thought to contribute to the immortality of cancers. Recently, some investigators described a correlation between the activity of telomerase and the proliferative activity of cancer cells. The aim of this study was to evaluate the correlation between the expression of telomerase-associated protein and proliferative activity. Telomerase reverse transcriptase (TERT) is one of the proteins that correlates with telomerase activity. We investigated TERT protein and its mRNA, and examined the correlation between the TERT protein and Ki-67, which reflects proliferative activity with immunostaining, and its mRNA, which correlates with telomerase activity, using in situ hybridization. Imprint smears from 17 invasive ductal adenocarcinomas were investigated. In most cases positive for TERT mRNA, the percentage of TERT protein-positive cells was also high and was closely related to mRNA (P = 0.024). The positive rates of TERT for the cases with lymph node metastasis were significantly higher than those for the cases without metastasis (P = 0.046). The positivity of TERT protein also correlated significantly with the Ki-67-positive rate (r = 0.82). As the proliferation activity increased, the number of cells positive for both proteins also increased (r = 0.89). In conclusion, it was suggested that the expression of TERT protein is associated with the expression of Ki-67, and is concerned with maintenance of the high proliferative activity in cancer cells with aggressive proliferation.

Telomerase as tumor marker

Telomerase, a critical enzyme responsible for continuous cell growth, is repressed in most somatic cells except proliferating progenitor cells and activated lymphocytes, and activated in approximately 85% of human cancer tissues. Telomerase activity is a useful cancer-cell detecting marker in some types of cancers in which almost all cases show telomerase activation. In other types in which telomerase becomes upregulated according to tumor progression, it is a useful prognostic indicator. Detection of human telomerase reverse transcriptase (hTERT) mRNA or protein in various clinical samples is also applicable. However, careful attention should be paid to the false negative results due to the instability of this enzyme or hTERT mRNA and the existence of polymerase chain reaction inhibitors as well as the false-positive results due to the contamination by normal cells with telomerase activity. If these pitfalls are avoided, in situ detection of hTERT mRNA or protein will facilitate the reliability of telomerase as a tumor marker.

Analysis of telomerase activity and detection of its catalytic subunit, hTERT

The discovery of the enzyme telomerase and its subunits has led to major advances in understanding the mechanisms of cellular proliferation, immortalization, aging, and neoplastic transformation. The expression of telomerase in more than 85% of tumors provides an excellent tool for the diagnosis, prognosis, and treatment of cancer. However, the techniques employed in its detection appear to play a significant role in the interpretation of the results. The telomeric repeat amplification protocol (TRAP assay) has been the standard assay in the detection of telomerase activity and many variations of this technique have been reported. Recent advances in the development of the TRAP assay and the incorporation of techniques that provide a quantitative and qualitative estimate of telomerase activity are assessed in this review. In addition to histological and cytological examination of tissues, distribution patterns of the catalytic subunit of telomerase, hTERT, are frequently used in the prognosis of tumors. The methods involved in the detection of hTERT as a biomarker of cellular transformation are also analyzed.

Telomeres, telomerase, and telomerase inhibition: clinical implications for cancer

Telomeres are located at the ends of eukaryotic chromosomes. The enzyme telomerase synthesized them, and they are responsible for maintaining the lengths of chromosomes. Absence of telomerase is associated with telomere shortening and aging of somatic cells, but high telomerase activity is observed in over 90% of human cancer cells. Although the disappearance of telomerase with aging is considered a natural defense against development of cancer, it is not known what triggers the reappearance of telomerase in cancer cells. Telomerase activity is directly correlated with the expression of its active catalytic component, the human telomerase reverse transcriptase (hTERT), which is controlled primarily at the level of transcription. An earlier paper discussed the relationship of telomerase with aging. In this article, the contemporary literature is reviewed to explore the associations between telomerase, telomerase inhibition, and cancer. Because most cancers occur in old age, with the aging of the population, the number of people suffering from cancer is expected to increase in the coming decades. It is not known what roles telomerase and hTERT play in the complex relationship between aging and cancer. Data from experimental studies suggest that telomerase assay could potentially play a role in the diagnosis and prognosis of cancers. There is also evidence that telomerase inhibitors might be used as anticancer agents. As the knowledge of the relationships between telomerase and cancer and between telomerase and aging advances, it is hoped that more about the interacting relationships between telomerase, aging, and cancer will be learned.

Clinical utility of telomerase in cancer

This review will focus on the clinical utilities of telomerase for human cancer diagnosis. Much attention has been focused on detection of telomerase activity and its essential components (hTR and hTERT) in cancer and noncancerous tissues. Expression of hTR and hTERT is upregulated in almost all human malignant tumors but not in benign or normal tissues with the exception of germline cells, proliferative stem cells, activated lymphocytes, and certain benign tumors. Thus, telomerase is a useful marker for cancer diagnosis and in some instance as a prognostic indicator of outcome. Telomerase detection in cells derived from breast fine needle aspirates, bronchial washes, and pancreatic juices show high sensitivity and specificity for cancer detection. In tissue samples, the level of telomerase activity is a useful prognostic indicator in certain adult cancers such as gastric and colon cancers and in neuroblastomas. Immunohistochemical detection of hTERT will facilitate exact diagnosis of the telomerase positive cells and expand the application of telomerase in cancer diagnosis.

Telomerase in cancer and aging

The telomere-telomerase hypothesis is the science of cellular aging (senescence) and cancer. The ends of chromosomes, telomeres, count the number of divisions a cell can undergo before entering permanent growth arrest. As divisions are being counted, events occur on the cellular and molecular level, which may either delay or hasten this arrest. As humans age, a particular concern is the accumulation of events that lead to the progression of cancer. Telomerase is a mechanism that most normal cells do not possess, but almost all cancer cells acquire, to overcome their mortality and extend their lifespan. This review aims to provide a comprehensive understanding of the role of telomerase in cancer development, progression, diagnosis, and in the future, treatment. The ultimate goal of telomerase research is to use our understanding to develop anti-telomerase therapies, an almost universal tumor target.

Telomerase: diagnostics, cancer therapeutics and tissue engineering

The enzyme telomerase has a key role in controlling the lifespan of human cells. It is absent from most somatic tissues but is reactivated in more than 85% of cancers, making the enzyme ideal as a marker of cancer cells and as a therapeutic target. In the context of normal human cells, the enzyme can extend cellular lifespan without causing cancer-associated changes or altering phenotypic properties. This capability could solve a major obstacle in the use of normal human cells for tissue engineering, that is, the induction of cellular senescence.

Telomerase activity in periampullary tumors correlates with aggressive malignancy

OBJECTIVE

To determine the presence of telomerase activity in a variety of periampullary malignancies and pancreatic diseases and quantify its activity to establish any association with the stage or aggressiveness of malignancy.

SUMMARY BACKGROUND DATA

Progressive shortening of telomeres, repetitive DNA sequences at the ends of chromosomes, plays a role in cell senescence. Telomerase catalyzes conservation of telomeric repeats and may promote cell immortality and hence malignancy. It is absent in normal tissues but upregulated in more than 80% of cancers.

METHODS

Fresh specimens of 62 periampullary tumors were snap-frozen in liquid nitrogen and adjacent tissue was formalin-fixed for histopathology. The telomerase repeat amplification protocol (TRAP) was used to obtain telomerase DNA products. These were separated with gel electrophoresis, stained with SYBR green, and quantified by densitometry. Findings were confirmed with a fluorometric TRAP assay in which fluorescent primers specific for telomerase were selectively amplified in its presence.

RESULTS

Telomerase activity was upregulated in 26 of 33 periampullary malignancies (79%): 17 of 21 pancreatic adenocarcinomas (81%), 2 of 2 cholangiocarcinomas, 2 of 2 duodenal carcinomas, and 5 of 8 ampullary carcinomas (63%). Poorly differentiated periampullary tumors had significantly higher telomerase activity than well-differentiated tumors, and tumors larger than 2 cm had significantly higher telomerase activity than those 2 cm or smaller. Pancreatic ductal adenocarcinomas with lymph node metastases had significantly greater activity than node-negative cancers. Two of 11 intraductal papillary mucinous tumors were positive for telomerase activity, but only in foci of invasive carcinoma. Chronic pancreatitis (n = 7), serous cystadenomas (n = 5), benign mucinous cystic neoplasms (n = 4), neuroendocrine cancer (n = 1), and acinar cell carcinoma (n = 1) had no detectable telomerase activity.

CONCLUSION

Telomerase activity is common in periampullary carcinomas. The magnitude of activity correlates with aggressiveness in pancreatic adenocarcinoma and may prove useful as a molecular index for biologic staging.

Telomerase in endocrine and endocrine-dependent tumors

Telomerase, the ribonucleoprotein enzyme that elongates chromosomal ends, or telomeres, is repressed in most normal somatic cells but reactivated in transformed cells to compensate for the progressive erosion of the telomeres during cell divisions. In accordance with this hypothesis, the presence of telomerase activity has been reported in more than 90% of human cancers, whereas most normal tissues or benign tumors contain low or undetectable telomerase activity. Reactivation of telomerase has also been widely reported in endocrine neoplasms and in hormone-related cancers. In the present study, we review the most recent publications on telomerase in these types of tumors. The hormonal regulation of telomerase activity and the possible strategies for cancer therapy based on the inhibition of telomerase has also been discussed.

Telomerase and breast cancer

Current therapies for breast cancer include treatments that are toxic and often result in drug resistance. Telomerase, a cellular reverse transcriptase that maintains the ends of chromosomes (telomeres), is activated in the vast majority of breast cancers (over 90% of breast carcinomas) but not in normal adjacent tissues. Telomerase is thus an attractive target for both diagnosis and therapy because of its distinct pattern of expression. We address the use of telomerase in the diagnostics of breast pathology, as well as the use of telomerase inhibitors in the treatment and prevention of breast cancer.