Regulation of telomerase by alternate splicing of human telomerase reverse transcriptase (hTERT) in normal and neoplastic ovary, endometrium and myometrium

The Opposing Effect of Hypoxia-Inducible Factor-2α on Expression of Telomerase Reverse Transcriptase

Hypoxia-inducible factor-1alpha (HIF-1alpha) has been implicated in the transcriptional regulation of the telomerase reverse transcriptase (hTERT) gene expression and telomerase activity, essential elements for cellular immortalization and transformation. However, controversial results were obtained in different studies. Moreover, it is totally unclear whether HIF-2alpha, the paralog of HIF-1alpha, plays a role in regulating hTERT expression. In the present study, we found that hypoxic treatment enhanced hTERT mRNA expression and telomerase activity in three renal cell carcinoma (RCC) cell lines with different genetic backgrounds. Both HIF-1alpha and HIF-2alpha were capable of significantly increasing the hTERT promoter activity in these cells. Moreover, depleting HIF-2alpha led to a down-regulation of hTERT mRNA level in RCC A498 cells expressing constitutive HIF-2alpha. It was found that HIF-2alpha bound to the hTERT proximal promoter and enhanced the recruitment of the histone acetyltransferase p300 and histone H3 acetylation locally in A498 cells treated with hypoxia. Increased levels of hTERT mRNA were observed in two of three hypoxia-treated malignant glioma cell lines. However, HIF-1alpha stimulated whereas HIF-2alpha inhibited the hTERT promoter activity in these glioma cell lines. Ectopic expression of HIF-2alpha resulted in diminished hTERT expression in glioma cells. Collectively, HIF-1alpha activates hTERT and telomerase expression in both RCC and glioma cells, and HIF-2alpha enhances hTERT expression in RCC cells, whereas it represses the hTERT transcription in glioma cells. These findings reveal a complex relationship between HIF-1alpha/2alpha and hTERT/telomerase expression in malignant cells, which may have both biological and clinical implications.

Telomerase activity of basal cell carcinoma in patients living in North Queensland, Australia

Basal cell carcinoma (BCC) is the most common skin cancer in North Queensland, Australia. The role of telomerase in BCC has not been investigated in this region. The objective of the study was to investigate the expression levels of human telomerase reverse transcriptase (hTERT) and telomerase activity in nodular BCCs and superficial BCCs from patients living in North Queensland. Matched tumor and adjacent nontumorous mucosa samples from 12 BCCs (7 nodular and 5 superficial) among 11 male patients were collected. In these samples, RNA was extracted and then transcribed to complementary DNA. Real-time polymerase chain reaction (PCR) was used to quantify the expression levels of hTERT messenger RNA (mRNA). In addition, telomeric repeat amplification protocol assay, a PCR-based method, was performed to detect telomerase activity in tissue extracts from these samples. The findings were correlated with the clinicopathologic features of patients with these tumors. All BCC samples expressed telomerase hTERT mRNA, and 75% (9/12) of the BCC samples showed telomerase activity. The mean level of expression among BCC samples was higher than that among matched nontumor samples (mean, 0.468 versus 0.140; P < .0001). Nodular BCC samples showed higher expression levels of hTERT mRNA as compared with superficial BCC samples (mean, 0.584 versus 0.305; P < .05). There was no significant association between expression of hTERT mRNA and tumor size. In conclusion, this study is the first on telomerase expression in BCC in Queensland as well as the first to analyze telomerase in BCC by quantitative PCR. The results indicate that telomerase alterations have an important role in the pathogenesis of BCCs. A unique finding is that the telomerase expression level in nodular BCCs is different from that in superficial BCCs.

Up-regulation of Mac-2 binding protein by hTERT in gastric cancer

Mac-2 binding protein (Mac-2BP) is a secreted tumor antigen that is elevated in many cancers and implicated in tumor metastasis, as well as cell adhesion and immune functions. We focused on the human telomerase reverse transcriptase (hTERT) induced Mac-2BP expression and the relationship between Mac-2BP expression and the progression of gastric cancer. A cDNA expression array analysis was performed on the telomerase-negative cell line, SW13, which was engineered to overexpress hTERT when compared with the parental SW13 cell. hTERT-induced Mac-2BP expression was confirmed via RT-PCR and Northern blotting. ELISA and flow cytometric analyses revealed that Mac-2BP protein was increased by 2- to 4-fold in hTERT-overexpressing cells compared with the mock control. Mac-2BP expression was significantly reduced when the overexpressed hTERT was neutralized by the introduction of hTERT-specific siRNA. These results suggest that Mac-2BP expression is modulated by hTERT. Mac-2BP levels in both gastric cancer cells and tumor tissues were determined via Northern blot analysis and immunohistochemistry. Mac-2BP protein was highly expressed in most gastric cancer cell lines, and gastric tumor tissues were stained more densely than normal tissues. The intracellular and secreted Mac-2BP levels were also evaluated via ELISA, indicating that Mac-2BP was expressed and secreted more abundantly in gastric cancer patients than in healthy donors. The elevated serum Mac-2BP level in gastric tumor patients was also significantly associated with distant metastasis (p = 0.05) and higher tumor stage (p = 0.04). Our findings suggest that Mac-2BP is induced by hTERT, and that it may prove to be a useful prognostic marker for the detection of malignant progression of metastatic stomach cancers.

Real-Time RT-PCR Quantification of Human Telomerase Reverse Transcriptase Splice Variants in Tumor Cell Lines and Non–Small Cell Lung Cancer

Background: We developed and validated a real-time reverse transcription (RT)–PCR for the quantification of 4 individual human telomerase reverse transcriptase (TERT) splice variants (α+β+, α−β+, α+β−, α−β−) in tumor cell lines and non–small cell lung cancer (NSCLC).

Methods: We used in silico designed primers and a common TaqMan probe for highly specific amplification of each TERT splice variant, PCR transcript–specific DNA external standards as calibrators, and the MCF-7 cell line for the development and validation of the method. We then quantified TERT splice variants in 6 tumor cell lines and telomerase activity and TERT splice variant expression in cancerous and paired noncancerous tissue samples from 28 NSCLC patients.

Results: In most tumor cell lines, we observed little variation in the proportion of TERT splice variants. The α+β− splice variant showed the highest expression and α−β+ and α−β− the lowest. Quantification of the 4 TERT splice variants in NSCLC and surrounding nonneoplastic tissues showed the highest expression percentage for the α+β− variant in both NSCLC and adjacent nonneoplastic tissue samples, followed by α+β+, with the α−β+ and α−β− splice variants having the lowest expression. In the NSCLC tumors, the α+β+ variant had higher expression than other splice variants, and its expression correlated with telomerase activity, overall survival, and disease-free survival.

Conclusions: Real-time RT-PCR quantification is a specific, sensitive, and rapid method that can elucidate the biological role of TERT splice variants in tumor development and progression. Our results suggest that the expression of the TERT α+β+ splice variant may be an independent negative prognostic factor for NSCLC patients.

Human normal T lymphocytes and lymphoid cell lines do express alternative splicing variants of human telomerase reverse transcriptase (hTERT) mRNA

Alternative splicing of telomerase reverse transcriptase (hTERT) mRNA is known to contribute to regulation of telomerase activity in normal and cancerous cells, however, previous studies indicated that normal human T and B cells exhibited constitutive expression of full-length hTERT mRNA without splicing variants and that activation of telomerase upon stimulation of the cells was due to the shuttling of hTERT protein from cytoplasm to nucleus [Proc. Natl. Acad. Sci. USA 96 (1999) 5147; J. Immunol. 166 (2001) 4826]. We found that typical variants of hTERT mRNA were widespread in human lymphocyte-derived cell lines and normal stimulated T cells. In activated T cells, induction of the full-length hTERT mRNA was coupled with increased hTERT protein expression and telomerase activity. Collectively, human normal and malignant lymphocytes, like other human cells, express splicing variants of hTERT mRNA and require transcriptional activation of the hTERT gene to acquire telomerase activity.

Increased telomerase activity and human telomerase reverse transcriptase mRNA expression in the endometrium of patients with endometriosis

BACKGROUND

Endometriosis is considered a frequent, benign disease with the ability to undergo neoplastic processes. The aim of this study was to evaluate the limitless replication potential of the endometrium in patients with endometriosis by examining human telomerase reverse transcriptase (hTERT) mRNA expression and telomerase activity.

METHODS

Endometrium samples from 30 endometriosis patients and 30 patients without endometriosis were obtained via endometrial biopsy. The expression of hTERT mRNA was determined by real-time RT-PCR assay, and telomerase activity was measured by telomerase repeat amplification protocol (TRAP) assay.

RESULTS

The mean normalized hTERT (N hTERT) mRNA level was significantly higher in the endometriosis than in the control group (P = 0.013). The mean hTERT mRNA levels during the proliferative phase and during the secretory phase were higher in the endometriosis group than in the control group, although the difference was only significant for the secretory phase (P = 0.036). We found a prominent difference in endometrial telomerase activity between moderate-to-severe endometriosis and the control group (P = 0.048). The levels of hTERT mRNA and telomerase activity increased as the disease became more severe (P = 0.038, P = 0.016).

CONCLUSIONS

This study showed the overexpression of hTERT mRNA and telomerase activity in the endometrium of endometriosis patients. These finding suggest that replication potential of endometrial cells may have an important role in the pathogenesis of endometriosis.

The expression of hTR and hTERT in human breast cancer: correlation with clinico-pathological parameters

BACKGROUND

Telomerase is a ribonucleoprotein enzyme that synthesizes telomeres after cell division and maintains chromosomal stability leading to cellular immortalization. Telomerase has been associated with negative prognostic indicators in some studies. The present study aims to detect any association between telomerase sub-units: hTERT and hTR and the prognostic indicators including tumour's size and grade, nodal status and patient's age.

METHODS

Tumour samples from 46 patients with primary invasive breast cancer and 3 patients with benign tumours were collected. RT-PCR analysis was used for the detection of hTR, hTERT, and PGM1 (as a housekeeping) genes expression.

RESULTS

The expression of hTR and hTERT was found in 31(67.4%) and 38 (82.6%) samples respectively. We observed a significant association between hTR gene expression and younger age at diagnosis (p = 0.019) when comparing patients < or = 40 years with those who are older than 40 years. None of the benign tumours expressed hTR gene. However, the expression of hTERT gene was revealed in 2 samples. No significant association between hTR and hTERT expression and tumour's grade, stage and nodal status was seen.

CONCLUSION

The expression of hTR and hTERT seems to be independent of tumour's stage. hTR expression probably plays a greater role in mammary tumourogenesis in younger women (< or = 40 years) and this may have therapeutic implications in the context of hTR targeting strategies.

Telomerase regulation and stem cell behaviour

Telomerase expression is restricted to a few cell types of the adult organism, most notably germ cells and stem/progenitor cells. Telomerase activity in germ cells is sufficient to prevent telomere shortening with age. Stem cells, however, do not have sufficient telomerase to prevent telomere shortening associated with continuous tissue renewal with increasing age. Indeed, telomerase levels in the adult organism are thought to be rate-limiting for longevity. This is supported by rare human syndromes caused by mutations in telomerase components, which are characterized by premature loss of tissue renewal and premature death. More recently, the role of telomerase and telomere length in stem cells is starting to be elucidated.

Detection of telomerase activity by the TRAP assay and its variants and alternatives

Telomerase activity is closely connected to problems of cellular immortality, proliferative capacity, differentiation, cancer and aging. Correspondingly, techniques for its detection have been essential for progress in telomere biology and are of still increasing importance in molecular diagnostics and therapy of cancer. This article reviews the development of the telomere repeat amplification protocol (TRAP) and its various modifications as the most widespread assay to detect and measure telomerase activity. Alternative possibilities of telomerase activity detection are also discussed which make it possible to omit the PCR-mediated amplification of telomerase products. These approaches are based on recent advances in highly sensitive detection systems.

Interferon-induced sensitization to apoptosis is associated with repressed transcriptional activity of the hTERT promoter in multiple myeloma

The aim of the present study was to explore hTERT as a target for IFN-induced sensitization to apoptosis in multiple myeloma (MM). IFN-alpha and IFN-gamma downregulated telomerase activity in the IL-6-dependent MM cell line U-266-1970. In MM cells undergoing IFN-induced sensitization to Fas-mediated apoptosis, the repression of telomerase was increased as compared to IFN-alpha treatment alone. Similar to the sensitization effect of IFN, the use of a dominant negative IkappaBalpha vector inhibiting hTERT activity via transcriptional targeting resulted in augmentation of Fas-mediated apoptosis. The mechanism underlying the reduction of telomerase activity by IFN was shown to be transcriptional repression of the hTERT gene. The present study does not support a direct effect of IFN on NF-kappaB binding to the hTERT promoter as underlying the transcriptional repression. We conclude that one potential mechanism whereby IFNs induce apoptosis sensitization is by repressing hTERT transcription and telomerase activity, thereby constituting attractive targets for MM therapy.

Differential effects of TGF-β1 on telomerase activity in thyroid carcinoma cell lines

The aim of the present study was to investigate the effect of transforming growth factor-beta1 (TGF-beta1) on telomerase activity in a panel of human anaplastic thyroid carcinoma (ATC) cell lines. Addition of TGF-beta1 decreased the telomerase activity in HTh 74 and KTC-1 cells, while in C 643 and HTh 7 an increased activity was observed. The decreased telomerase activity appeared to be due to transcriptional repression of the hTERT promoter. Addition of a PI-3 kinase inhibitor (LY294002) abrogated the stimulatory effect of TGF-beta1 on the telomerase activity, indicating the possible involvement of hTERT activation via phosphorylation. Furthermore, the MEK-inhibitor U0126 had similar effects suggesting dual regulatory mechanisms. Interestingly, the cell lines differed genetically in that ATC cell lines responding with increased telomerase activity harbored a p53 mutation. In conclusion, TGF-beta1 exerts opposing effects on telomerase activity in ATC cell lines, possibly reflecting deregulation of TGF-beta1 signaling in a more malignant genotype.

Lack of telomerase activity in lung carcinoids is dependent on human telomerase reverse transcriptase transcription and alternative splicing and is associated with long telomeres

PURPOSE

Preliminary evidence indicates that telomerase activity is significantly less expressed in typical carcinoids than in large cell neuroendocrine carcinomas or in small cell lung cancers. Knowledge of the mechanisms by which telomerase is differentially regulated in neuroendocrine lung tumors is important for a better understanding of the pathogenesis of these malignancies.

EXPERIMENTAL DESIGN

We investigated telomerase activity in 86 neuroendocrine lung tumors and correlated the enzyme activity with the expression of the enzyme subunits [human RNA component (hTR), human telomerase reverse transcriptase (hTERT), and alternatively spliced hTERT variants], with the telomere-associated protein human protection of telomere-1, and with the telomere length pattern.

RESULTS

A significantly (P = 0.0001) lower frequency of telomerase-positive cases was found in typical carcinoids (14%) than in large cell neuroendocrine carcinomas (87%) and small cell lung cancers (92%). hTR was constitutively expressed in all carcinoids. Telomerase-negative carcinoids were characterized by the absence of any hTERT transcript, only displayed the beta(-) alternatively spliced variant, or concomitantly expressed the alpha(+)beta(+) full-length message with different combinations of alternatively spliced variants. However, in these tumors, a more abundant level of alternatively spliced transcripts than that of the alpha(+)beta(+) full-length transcript was generally found. No significant difference was observed in human protection of telomere-1 expression between telomerase-negative and telomerase-positive carcinoids. Telomeres were significantly (P < 0.05) longer in telomerase-negative carcinoids than in telomerase-positive carcinoids (median value, 9.15 versus 4.47 kb). However, alternative lengthening of telomeres, as shown by associated promyelocytic leukemia bodies, was not observed in these tumors.

CONCLUSIONS

Our results indicate that telomerase is repressed in most lung carcinoids and that hTERT transcription and alternative splicing play a role in such a negative regulation. Moreover, the absence of any telomerase maintenance mechanism may contribute to the favorable prognosis of this malignancy.

Differential Expression of Full-length Telomerase Reverse Transcriptase mRNA and Telomerase Activity between Normal and Malignant Renal Tissues

Activation of telomerase, a key event during immortalization and malignant transformation, requires expression of the telomerase reverse transcriptase (hTERT). Consistently, lack of telomerase activity and hTERT expression occurs in most normal human somatic cells. However, it has been observed that both normal and cancerous renal tissues express hTERT whereas only the latter exhibits telomerase activity. The mechanism underlying the dissociation between hTERT expression and telomerase activity is unclear. In the present study, we examined telomerase activity and alternative splicing of hTERT transcripts in renal cell carcinoma (RCC) specimens and adjacent normal tissues from 33 patients with RCC. Telomerase activity was detectable in 27 of 33 (82%) RCC samples but none in their normal counterparts. Thirty-two of 33 tumors expressed overall hTERT mRNA and 27 of them contained full-length hTERT transcripts, all with telomerase activity. Although 42% (14 of 33) of normal renal samples expressed hTERT mRNA, none of them had full-length hTERT transcripts, coinciding with lack of telomerase activity. The presence of full-length hTERT mRNA and telomerase activity was significantly associated with c-MYC induction. In tumors, absence of full-length hTERT mRNA or telomerase activity defines a subgroup of nonmetastatic, early-stage RCCs. Taken together, telomerase repression in normal renal tissues is attributed to the absence of full-length hTERT transcripts, whereas telomerase activation is achieved via induction of or switch to expression of full-length hTERT mRNA during the oncogenic process of kidneys, and associated with aggressive RCCs.

A novel transgenic mouse model reveals humanlike regulation of an 8-kbp human TERT gene promoter fragment in normal and tumor tissues

Telomerase activity is repressed in most human somatic tissues during differentiation processes but strongly up-regulated in most human tumors. Regulation of human telomerase activity primarily occurs at the level of transcriptional initiation of the TERT gene, which encodes the catalytic subunit of telomerase. We have generated a novel transgenic mouse model to study the regulation of the human TERT gene promoter in an in vivo system. For this purpose, we have cloned an 8.0-kbp human TERT promoter fragment in front of the bacterial lacZ reporter gene (hTERTp-lacZ), which encodes the beta-galactosidase enzyme. Expression of the reporter gene was monitored by reverse transcription-PCR analysis, 5-bromo-4-chloro-3-indolyl-beta-d-galactopyranoside staining of whole mount preparations, and histologic sections. We find that the activity of the human TERT promoter in most normal mouse tissues recapitulates the expression of the hTERT gene in normal human tissues and is under tighter control when compared with the endogenous mouse TERT gene expression. In testis, where highest lacZ expression was observed, the expression of the reporter gene was restricted to the spermatogonial stem cells and the spermatocytes. Intriguingly, we find increased levels of lacZ expression in mammary tumors of hTERTp-lacZ x p53(+/-) bitransgenic mouse mammary tumor model. Thus, this transgenic mouse model provides a suitable in vivo system to analyze the expression of the human TERT gene under physiologic conditions and during tumorigenesis.

Dysregulated expression of the major telomerase components in leukaemic stem cells

Telomere loss is rapid during the progression of chronic myeloid leukaemia (CML) and correlates with prognosis. We therefore sought to measure expression of the major telomerase components (hTR and hTERT) in CD34+ cells from CML patients and normal controls, to determine if their altered expression may contribute to telomere attrition in vivo. High-purity (median 94.1%) BCR-ABL+ CD34+ cells from CML (n=16) and non-CML (n=14) patients were used. CML samples had a small increase in telomerase activity (TA) compared to normal samples (approximately 1.5-fold, P=0.004), which was inversely correlated with the percentage of G0 cells (P=0.02) suggesting TA may not be elevated on a cell-to-cell basis in CML. Consistent with this, hTERT mRNA expression was not significantly elevated; however, altered mRNA splicing appeared to play a significant role in determining overall full length, functional hTERT levels. Interestingly, Q-RT-PCR for hTR demonstrated a mean five-fold reduction in levels in the chronic phase (CP) CML samples (P=0.002), raising the possibility that telomere homeostasis is disrupted in CML. In summary, the molecular events regulating telomerase gene expression and telomere maintenance during the CP of CML may influence the disease progression observed in these patients.

Techniques in plant telomere biology

The role model systems have played in understanding telomere biology has been enormous, and understanding has rapidly transferred to human telomere research. Most work using model organisms to study telomerase and nontelomerase-based telomere-maintenance systems has centered on yeasts, ciliates, and insects. But it is now timely to put considerably more effort into plant models for a number of reasons: (i) the rice and Arabidopsis genome sequencing projects make data mining possible; (ii) extensive collections of insertion mutants of Arabidopsis thaliana enable phenotypic effects of protein gene knockouts to be analyzed, including for those genes involved in telomere structure, function (including, for example, in meiosis), and maintenance; and (iii) the variability of plant telomeres is considerable and ranges from the telomerase-mediated synthesis of the Arabidopsis-type (TTTAGGG) and vertebrate-type (TTAGGG) repeats to sequences synthesized by telomerase-independent mechanism(s) that are still to be discovered. Here we describe how the understanding of telomere biology has been advanced by methods used to isolate telomeric sequences and prove that the putative sequences isolated are indeed telomeric. We show how assays designed to prove the activity of telomerase [e.g., telomeric repeat amplification protocol (TRAP)] lead not only to an understanding of telomere structure and function, but also to the understanding of cell activity in development and in the cell cycle. We review how assays designed to reveal protein/protein and protein/nucleic acid interactions promote understanding of the structure and activities of plant telomeres. Together, the data are making significant contributions to telomere biology in general and could have medical implications.

Antisense oligonucleotide-mediated inhibition of hTERT, but not hTERC, induces rapid cell growth decline and apoptosis in the absence of telomere shortening in human prostate cancer cells

Recent evidence points to a novel function of human telomerase reverse transcriptase (hTERT) in promoting tumour cell survival, which might be independent of the telomere-elongating activity of the enzyme. To test this hypothesis, we evaluated comparatively the effects of telomerase inhibition, accomplished through antisense oligonucleotide-mediated interference with hTERT or human telomerase RNA component (hTERC), on the proliferative potential of DU145 human prostate cancer cells. Exposure of cells to a 2'-O-methyl-RNA phosphorothioate oligonucleotide targeting a splicing site within hTERT pre-mRNA induced almost complete inhibition of telomerase activity as a consequence of a marked reduction of the hTERT mRNA expression level, an early decline of DU145 cell growth and apoptotic cell death without any appreciable telomere shortening. Conversely, exposure of DU145 cells to a 2'-O-methyl-RNA phosphorothioate oligonucleotide targeting the template region of hTERC failed to interfere with cell proliferation in spite of the almost complete abrogation of telomerase activity. These results extend and corroborate earlier evidence in favour of an enzymatic activity-independent mechanism by which hTERT maintains tumour cell survival and proliferation.

Telomerase inhibition is a specific early event in salvicine-treated human lung adenocarcinoma A549 cells

The telomere and telomerase have been suggested as targets for anticancer drug discovery. However, the mechanisms by which conventional anticancer drugs affect these targets are currently unclear. The novel topoisomerase II inhibitor, salvicine, suppresses telomerase activity in leukemia HL-60 cells. To further determine whether this activity of salvicine is specific to the hematological tumor and distinct from those of other conventional anticancer agents, we studied its effects on telomere and telomerase in a solid lung carcinoma cell line, A549. Differences in telomerase inhibition and telomere erosion were observed between salvcine and other anticancer agents. All anticancer agents (except adriamycin) induced shortening of the telomere, which was identified independent of replication, but only salvicine inhibited telomerase activity in A549 cells under conditions of high concentration and short-term exposure. At the low concentration and long-term exposure mode, all the tested anticancer agents shortened the telomere and inhibited telomerase activity in the same cell line. Notably, salvicine inhibited telomerase activity more severely than the other agents examined. Moreover, the compound inhibited telomerase activity in A549 cells indirectly in a concentration- and time-dependent manner. Salvicine did not affect the expression of hTERT, hTP1, and hTR mRNA in A549 cells following 4 h of exposure. Okadaic acid protected telomerase from inhibition by salvicine. These results indicate specificity of salvicine and diversity of anticancer agents in the mechanism of interference with telomerase and the telomere system. Our data should be helpful for designing the study in the development of agents acting on telomere and/or telomerase.

Nuclear telomerase is less accessible to antibody probing than known nuclear antigens: retrieval with new immunostaining buffer

Telomerase is an important tumor marker but few antibodies to the enzyme have been described or used without difficulty in histochemical detection. Here we report specific detection of the enzyme in cell and tissue preparations using a new monoclonal antibody (mAb 476) and a new antigen-retrieval buffer (Enhancing buffer). When used to detect telomerase under normal immunostaining conditions in HL-60 cells or tissue sections of hepatocellular carcinoma or metastatic choriocarcinoma, unexpectedly, the antibody stained the cytoplasm rather than the nucleus. Nuclear staining, however, was revealed using the Enhancing buffer. Since other nuclear antigens in the HL-60 cell could be stained both ordinarily and in the Enhancing buffer, nuclear telomerase appears to be shrouded by the nuclear matrix or blocked by accessory proteins. The cytoplasmic activity seen in normal buffer but absent largely from the Enhancing buffer may be an artifact or the nascent, "naked" enzyme. With a known cytoplasmic antigen (proteinase-3) chosen arbitrarily for comparison, the antigenicity was found enhanced, instead, by the Enhancing buffer. The mode of action of the Enhancing buffer differs from that of microwave irradiation or the signal amplification (CSA) used by some investigators. The latter was found to enhance the cytoplasmic reactivity rather than the nuclear reactivity of mAb 476.

The tamoxifen-induced suppression of telomerase activity in the human hepatoblastoma cell line HepG2: a result of post-translational regulation

PURPOSE

Patients with advanced hepatocellular carcinoma (HCC) have shown to benefit from tamoxifen treatment. The mechanisms of tamoxifen action in HCC, however, are not yet clearly understood. Results from studies on the human hepatoblastoma cell line HepG2 provide evidence that estrogen-receptor-alpha-independent antiproliferative actions of tamoxifen in HCC are mediated by the suppression of telomerase activity [5].

MATERIALS AND METHODS

We investigate the pathway of the tamoxifen-induced down-regulation of telomerase activity, using HepG2 cells incubated over 24 h or 48 h in the presence of 20 microM tamoxifen.

RESULTS

The transcriptional levels of the three telomerase core components-human telomerase RNA (hTR), human telomerase reverse transcriptase (hTERT) (all variants), and telomerase-associated protein (TP1)-did not change during tamoxifen treatment, as revealed by RT-PCR analysis. Furthermore, the hTERT splice pattern was not shifted from the active full-length variant (+alpha/+beta) to the inactive deletion variants (-alpha; -beta; -alpha/-beta) and the level of the 120 kDa hTERT full-length protein remained constant, as shown by Western blot analysis. Protein kinase C (PKC) activity has been suggested to be crucial for post-translational up-regulation of telomerase activity. In HepG2 cells, we observed a tamoxifen-induced suppression of the total protein kinase C (PKC) activity (cytosolic and membrane-bound). Inhibition of PKC with bisindolylmaleimide I resulted in a reduction of telomerase activity, as revealed by TRAP-assay. Alpha-tocopherol (vitamin E) diminished the effects of tamoxifen on PKC-activity as well as on telomerase activity.

CONCLUSIONS

We conclude that the tamoxifen-induced decrease of telomerase activity in HepG2 cells is mediated post-translationally via suppression of PKC-activity.

Effect of antineoplastic agents on the expression of human telomerase reverse transcriptase beta plus transcript in MCF-7 cells

OBJECTIVES

To evaluate the effect of antineoplastic agents on the expression of human telomerase reverse transcriptase (hTERT) splice variants in MCF-7 cells.

DESIGN AND METHODS

We have developed a luminometric hybridization assay for hTERT beta plus transcript. MCF-7 cells were isolated before and after treatment with antineoplastic agents. A combination of nested RT-PCR and the developed luminometric hybridization assay was used for the specific detection of hTERT beta plus transcript in treated and untreated MCF-7 cells. Amplification of all hTERT splicing variants by nested PCR in the same samples was also performed.

RESULTS

MCF-7 cells treated with taxol and etoposide were found positive for all hTERT splicing variants, while the expression of hTERT beta plus transcript did not differ significantly before and after exposure. MCF-7 cells treated with doxorubicin and 5-fluorouracil did not express any of hTERT splicing variants. In the presence of cisplatin, three splicing variants of hTERT were detected.

CONCLUSIONS

The developed hybridization assay is highly sensitive and specific for the detection of hTERT beta plus transcript in clinical samples.

Alternative splicing of human telomerase reverse transcriptase may not be involved in telomerase regulation during all-trans-retinoic acid-induced HL-60 cell differentiation

Alternative splicing of the human telomerase reverse transcriptase subunit (hTERT) suppresses telomerase activity during the development of human fetal kidney cells into mature cells. Tumor cell differentiation is the process of turning abnormal tumor cells into 'normal' cells accompanied by down-regulation of telomerase activity. However, the precise mechanism of the regulation of telomerase activity in differentiated cells is not fully understood. In this study, we observed the role of alternative splicing of hTERT in the regulation of telomerase activity in all-trans-retinoic acid (ATRA)-induced, differentiated HL-60 cells. ATRA-induced down-regulation of telomerase activity in differentiated HL-60 cells was associated with a decrease in hTERT and an increase in human telomerase-associated protein-1 (hTP1) transcription. Expression of full length variant hTERT alpha+ beta+ mRNA decreased in a dose- and time-dependent manner. The drop of hTERT beta- mRNA was time-dependent. hTERT alpha- and hTERT alpha- beta- mRNA were reduced dramatically after ATRA treatment. In the dose-effect study, hTERT alpha+ beta+ and hTERT beta- maintained a relatively stable ratio when telomerase activity decreased largely from treatment with 1 to 5 microM ATRA. Although the splicing pattern of hTERT mRNA was altered in time-effect research, the change was not related to the ATRA-treated decline of telomerase activity. The expression of alternative splicing variants of hTERT also decreased at the protein level. All these results suggested that alternative splicing of hTERT mRNA may not contribute to the suppression of telomerase activity during ATRA-induced HL-60 leukemia cell differentiation.

Elevated hTERT mRNA levels: A potential determinant of bronchial squamous cell carcinoma (in situ)

Expression levels of hTERT mRNA were investigated by RT-PCR in tissue specimens of patients with (Group A) and without (Group B) clinically overt bronchial carcinoma, respectively. Bronchial carcinoma (n = 9) and distant normal (n = 9) specimens were analyzed in Group A. The chance of carcinoma seemed to increase with increasing hTERT mRNA levels (OR = 6.04, 95% CI = 1.02-37). Group B was comprised of 21 patients who underwent autofluorescence bronchoscopy. After analysis of 66 bronchial biopsies the chance of prevalent carcinoma in situ or carcinoma increased with increasing hTERT mRNA levels (OR = 6.19, 95% CI = 1.55-25). Variables like age, gender, smoking history, history of cancer within the airways or the degree of lymphocyte infiltrate in the specimens did not modify this relation. In 7 Group B patients in whom bronchial cancer was diagnosed during follow-up, biopsies taken before cancer diagnosis from both the area of the newly developed tumor and distantly from this area had been analyzed for hTERT expression. The median hTERT mRNA level in the biopsies from the area of future cancer was significantly higher than in biopsies taken from distant sites (p < 0.03). These data indicate that elevated hTERT mRNA is associated with an increased relative risk of prevalent and incident bronchial squamous cell carcinoma (in situ).

Hypermethylation of the hTERT promoter inhibits the expression of telomerase activity in normal oral fibroblasts and senescent normal oral keratinocytes

Telomerase activity in human cells closely correlates with the expression of its catalytic subunit, telomerase reverse transcriptase (hTERT). Previously, we reported the lack of telomerase activity in normal human oral fibroblasts (NHOF) and the diminution of telomerase activity during senescence in normal human oral keratinocytes (NHOK). To investigate the underlying mechanisms of telomerase regulation in both cell types, we analysed the expression, promoter activity, and methylation status of the hTERT gene. The expression of hTERT mRNA diminished in senescent NHOK, but was not detected in NHOF at any stage of replication. An exogenous hTERT promoter was active in NHOF and in senescing NHOK, indicating that the lack of hTERT gene expression resulted from alteration of the endogenous hTERT promoter. Since methylation is involved in the silencing of numerous genes, we carried out DNA methylation assays. The assay revealed that the hTERT promoter was hypermethylated in NHOF and was gradually methylated during senescence in NHOK. Treatment of NHOF and senescent NHOK with the demethylating agent 5-aza-2′-deoxycytidine restored the expression of endogenous hTERT mRNA. Our results suggest that hypermethylation of the hTERT promoter plays a critical role in the negative regulation of telomerase activity in normal human oral cells.

Differential expression of telomerase activity in neuroendocrine lung tumours: correlation with gene product immunophenotyping

Telomerase activity was examined by the telomeric repeat amplification protocol (TRAP) assay in 38 neuroendocrine (NE) lung tumours. A significantly (p = 0.001) different frequency of telomerase positivity was observed among different histological tumour types. Specifically, a positive TRAP signal was observed in 14 of 15 (93%) small cell lung cancers (SCLCs), 7 of 8 (87%) large-cell NE carcinomas (LCNECs), and only 1 of 15 (7%) typical carcinoid tumours. When telomerase activity was correlated with the gene product-based immunophenotypic profile of individual tumours, it was found that the absence of telomerase activity was associated with a lack of bcl-2, p53, and c-kit expression, and characterized by a low proliferation index consistent with the absence of cdk-4 expression and the presence of the cdk inhibitor Rb. Such a phenotype was appreciable in most of the carcinoid tumours. Conversely, telomerase-positive tumours generally showed an immunophenotype consistent with gene product alterations (including high expression of bcl-2, p53, and c-kit, and loss of Rb) and were characterized by a high proliferation index. These telomerase data support the previously reported evidence for two genetically unrelated groups of NE lung tumours (SCLC, and to some extent LCNEC, versus carcinoid tumours) that have distinct phenotypic profiles.

Significance of immunological detection of human telomerase reverse transcriptase: re-evaluation of expression and localization of human telomerase reverse transcriptase

Human telomerase reverse transcriptase (hTERT) is a catalytic subunit of telomerase and is a potentially useful diagnostic marker for cancers. There have been few studies in which immunological detection of hTERT has been attempted and its subcellular localization has not been precisely defined. In the present study, we re-evaluated expression and localization of hTERT in cancer and normal cells using a newly developed antibody. Immunohistochemistry revealed that hTERT is expressed in approximately 80% of gynecological cancers, but some premalignant lesions exhibited weak expression of hTERT. Interestingly, not only nuclei but also cytoplasm of cancer cells were positive for hTERT staining. This finding was supported by the results of Western blot analysis of cell lines, in which both nuclear and cytoplasmic extracts exhibited significant hTERT bands. Cytoplasmic hTERT in cancer cells may be functional because the telomeric repeat amplification protocol assay of cytoplasmic extracts showed high levels of telomerase activity. Unexpectedly, not all normal primary cells and telomerase-negative cancer cell lines lacked hTERT expression; some exhibited weak TERT signals. In Western analysis, hTERT signals did not always correlate with telomerase activity of the various cell types. These findings suggest that functional hTERT is expressed in both the nucleus and cytoplasm of cancer cells and that hTERT expression does not strictly reflect telomerase activity. Further analysis is needed to clarify the biological significance of cytoplasmic hTERT.

hTERT gene amplification and increased mRNA expression in central nervous system embryonal tumors

High-level gains at 5p15, a chromosomal region including the human telomerase catalytic protein subunit (hTERT) gene, have been documented in several medulloblastomas. We therefore analyzed hTERT gene dosage in a group of medulloblastomas and other embryonal brain tumors using differential PCR. Amplification of the hTERT locus was detected in 15 of 36 (42%) tumors examined. To correlate gene amplification with message level, we used real-time quantitative PCR to measure hTERT mRNA in 50 embryonal brain tumors. hTERT mRNA was detected in all but one of these cases, and mRNA level correlated significantly with gene dosage (r = 0.82). Log-rank analysis of survival data revealed a trend toward poor clinical outcomes in patients with medulloblastomas containing high hTERT mRNA levels, but clinical follow-up was relatively short and the association was not statistically significant (P = 0.078). Comparative genomic hybridization was used to further analyze the tumor with the greatest hTERT gene dosage and mRNA level, a recurrent medulloepithelioma. hTERT was amplified in the recurrent tumor but not in the primary lesion, suggesting this locus can be involved in tumor progression. Our data indicate that hTERT gene amplification is relatively common in embryonal brain tumors, and that increased expression of hTERT mRNA may be associated with biologically aggressive tumor behavior.

Biochemical aspects of telomerase function

Arthur Kornberg "never met a dull enzyme" (For the Love of Enzymes: The Odyssey of a Biochemist, Harvard University Press, 1989) and telomerase is no exception. Telomerase is a remarkable polymerase that uses an internal RNA template to reverse-transcribe telomere DNA, one nucleotide at a time, onto telomeric, G-rich single-stranded DNA. In the 17 years since its discovery, the characterization of telomerase enzyme components has uncovered a highly conserved family of telomerase reverse transcriptases that, together with the telomerase RNA, appear to comprise the enzymatic core of telomerase. While not as comprehensively understood as yet, some telomerase-associated proteins also serve crucial roles in telomerase function in vivo, such as telomerase ribonudeoprotein (RNP) assembly, recruitment to the telomere, and the coordination of DNA replication at the telomere. A selected overview of the biochemical properties of this unique enzyme, in vitro and in vivo, will be presented.

Expression profile of a gamma-deletion variant of the human telomerase reverse transcriptase gene

The human telomerase reverse transcriptase (hTERT) is an essential component of the holoenzyme complex that adds telomeric repeats to the ends of chromosomes. The hTERT transcript has been shown to have two deletion type alternative splicing sites. One deletion site induces the alpha-deletion variant, lacking 36 bp from exon 6, and the other induces the beta-deletion variant, lacking 182 bp from exons 7 and 8. Here, we identified a novel deletion variant of the hTERT transcript in hepatocellular carcinoma cell lines. The deleted transcript was characterized by an in-frame deletion of 189 bp, spanning nucleotides 2710 to 2898, corresponding to the complete loss of exon 11 (gamma-deletion). The region lacking in the gamma-deletion lies within RT motifs D and E, suggesting that it is missing conserved residues from the catalytic core of the protein. Both gamma- and alpha-deletion variants were occasionally detected, but the beta-deletion variant was frequently observed. Our results may provide important information for more detailed studies on the regulation of telomerase activity.

hTERT mRNA expression correlates with telomerase activity in human breast cancer

AIMS

Telomerase is a ribonucleoprotein enzyme that synthesises telomeres after cell division and maintains chromosomal length and stability thus leading to cellular immortalisation. hTERT (human telomerase reverse transcriptase) gene seems to be the rate-limiting determinant of telomerase reactivation. hTERT mRNA expression was reported to correlate with telomerase activity in cell lines and some human tumours. However the correlation between telomerase activity and hTERT mRNA expression has not been previously examined in human breast cancer. The present study aims to quantitatively measure the expression of hTERT mRNA and telomerase activity in human breast cancer and examine the relationship between these parameters. Furthermore the associations with other parameters including estrogen receptor (ER) and progesterone receptor (PgR) status, DNA ploidy, and S-phase fraction (SPF) are also examined.

METHODS

RNA was extracted from 18 breast carcinomas and hTERT mRNA expressions were estimated by reverse transcriptase-PCR (RT-PCR) and Taqman methodology. These tumours had already been analysed for ER and PgR status using ligand-binding assays and had had their DNA ploidy and S-phase fractions measured by flow cytometry. Telomerase activity had already been determined by using a modified telomeric repeat and amplification protocol (TRAP) assay.

RESULTS

The expression of hTERT mRNA in the breast tumours ranged between 1.3 and 2.7 x 10(7) copy numbers per micro g of cellular RNA (the median value was 2.7x10(5) and the mean was 3.1 x 10(6)). Telomerase activity was between 0 and 246 units of Total Protein Generated (TPG), where one unit of TPG was equal to 600 molecules, of telomerase substrate primers extended by at least three telomeric repeats. The median level of TPG was 60 units and the mean level was 81 units). Telomerase activity was found to significantly correlate with hTERT expression (r(s)=0.51112, P=0.0302). There was no significant correlation between hTERT and other parameters.

CONCLUSION

hTERT mRNA expression significantly correlates with telomerase activity in human breast cancer. This is consistent with the hypothesis that hTERT is the catalytic and rate-limiting determinant subunit of the enzyme.

hTERT expression in human breast cancer and non-cancerous breast tissue: correlation with tumour stage and c-Myc expression

Telomerase is a ribonucleoprotein enzyme that synthesises telomeres after cell division and maintains chromosomal length and stability thus leading to cellular immortalisation. hTERT (human telomerase reverse transcriptase) gene is the rate-limiting determinant of telomerase reactivation. The present study aims to quantitatively measure the expression of hTERT mRNA in human breast cancer, adjacent non-cancerous tissue (ANCT) and benign breast lesions, examine the association between hTERT and the clinicopathological characteristics of the cancer specimens and to explore the relationship between c-Myc and hTERT expressions. RNA was extracted from 49 breast carcinomas, 46 matched ANCT, and eight fibroadenomas. hTERT and c-Myc mRNA expressions were estimated by reverse transcriptase-PCR (RT-PCR) and Taqman methodology. hTERT mRNA was present in all of the cancerous and most of ANCT specimens with levels being much higher in the cancerous tissue than in ANCT. The ratio of hTERT mRNA in tumour to that in ANCT was 2011 (95% confidence interval 373-10,853, P < 0.0001). There was no significant association between tumour hTERT expression and patient's age, tumour size, grade, nodal metastasis, estrogen receptor (ER) positivity, lymphovascular (LVI) or c-Myc expression. However, there was a weak but significant negative correlation between hTERT expression and progesterone receptor (PR) status (p = 0.04) in tumours. hTERT mRNA expression was also significantly higher in carcinomas (median = 2.61 x 10(6)) than in fibroadenomas (median = 424).We conclude that hTERT mRNA expression is significantly higher in human breast cancer than in non-cancerous breast tissue suggesting that hTERT has a potential role in breast cancer diagnosis. The hTERT mRNA levels in tumour do not seem to be associated with the patient's age or advanced tumour stage. Furthermore, hTERT mRNA expression does not correlate with c-Myc mRNA expression in breast cancer.

Retinoic acid receptor beta and telomerase catalytic subunit expression in bronchial epithelium of heavy smokers

Expression of retinoic acid receptor beta (RAR beta) and telomerase catalytic subunit (human telomerase reverse transcriptase [hTERT]) was analyzed in 285 bronchial biopsy specimens from 53 heavy cigarette smokers and four former smokers by using in situ hybridization. Of the 191 biopsy specimens that were positive for RAR beta, 69% expressed hTERT, whereas only 54% of the 94 RAR beta negative biopsy specimens expressed hTERT (P =.014). Because hTERT expression in bronchial tissue has been previously associated with increased risk of lung cancer, the association between RAR beta and hTERT expression suggests that RAR beta expression may be an indicator of increased risk of lung cancer in heavy smokers.

Protective effect of HO-1 against oxidative stress in human hepatoma cell line (HepG2) is independent of telomerase enzyme activity

Heme oxygenase-1 (HO-1) is a stress response protein and its induction is associated with protection against oxidative stress. Cell survival during exposure to environmental stresses is associated with elevation of HO-1. Telomerase plays an important role in cell proliferation and immortalization. Our objective was to determine whether the adaptive cellular response to survive exposure to environmental stresses is dependent on expression of HO-1 and telomerase activity in hepatoma cell line (HepG2). Exposure of HepG2 to oxidants, H(2)O(2) (100 microM), as well as HO-1 inducers, heme (10 microM) and stannic chloride (SnCl(2)) (10 microM), resulted in an increased HO-1 mRNA, protein and total HO activity. On the other hand, HO activity was inhibited by addition of stannic mesoporphyrin (SnMP) (10 microM). These effects were brought about without altering endogenous HO-2 protein levels. Telomerase activity was not affected by oxidants, inducers of HO-1 or inhibitors of HO activity. Similarly, the catalytic subunit of telomerase enzyme human telomerase reverse transcriptase (hTERT), which is considered as the major regulator of telomerase activity, was not affected by oxidants, heme and H(2)O(2), or downregulation of HO gene activity by SnMP. This study demonstrates, for the first time, that induction of HO-1 gene mediates protection against oxidants and increases cell survival by a mechanism independent of telomerase enzyme activity. Suppression of HO activity by SnMP decreased cell resistance to oxidant stressors without altering telomerase activity.

Prognostic potential of the telomerase subunit human telomerase reverse transcriptase in tumor tissue and nontumorous mucosa from patients with colorectal carcinoma

BACKGROUND

The stabilization of telomere lengths by telomerase activation is an important step in carcinogenesis and cell immortalization. Human telomerase reverse transcriptase (hTERT) is the catalytic subunit of this enzyme. The objective of this study was to evaluate the use of real-time reverse transcriptase-polymerase chain reaction (RT-PCR) analysis for the quantification of hTERT in tumor and nontumorous tissue samples.

METHODS

Matched samples of tumor and adjacent nontumorous mucosa samples from 57 patients with completely resected colorectal carcinoma (International Union Against Cancer Stage I-IV) who underwent complete resection (R0) were quantified for hTERT mRNA expression using real-time RT-PCR. The expression levels were correlated with histopathologic findings and with survival. The median follow-up was 76 months.

RESULTS

hTERT mRNA was expressed in all tumor samples and in all samples of adjacent mucosa. In 12 patients (21%), there was higher hTERT expression in tumor samples compared with nontumorous samples. Compared with tumor samples, the expression of hTERT in samples of nontumorous mucosa decreased with age (P = 0.06). hTERT mRNA expression in both tumor tissue and adjacent mucosa was correlated significantly with the histologic grade of colorectal carcinoma (P < 0.04 and P < 0.05, respectively). Patients with hTERT expression in tumor tissue in relation to the adjacent mucosa of > 0.57 had a significantly poorer overall survival compared with patients with lower hTERT ratios (P < 0.02). In addition to the established prognostic factor lymphatic vessel invasion, the hTERT ratio proved to be of independent prognostic value (P < 0.05).

CONCLUSIONS

The prognostic potential of hTERT in patients with colorectal carcinoma and the correlation of hTERT with tumor grade underlines the role of hTERT as a molecular marker for biologic tumor staging.

Transcription and alternative splicing of telomerase reverse transcriptase in benign and malignant breast tumours and in adjacent mammary glandular tissues: implications for telomerase activity

Telomerase activity was determined in 15 breast cancers, 24 benign breast lesions, and 36 breast tissues adjacent to benign or malignant tumours. A positive TRAP (telomeric repeat amplification protocol) signal was detected in 67% of carcinomas and 29% of benign tumours. In five of ten cases, non-invaded breast tissues adjacent to telomerase-positive carcinomas also displayed telomerase activity. Conversely, in peritumoural specimens adjacent to benign lesions, telomerase activity was never detected. To investigate the regulatory mechanisms of telomerase activity in breast tissues, the expression of telomerase subunits was assessed, as well as the presence of alternatively spliced variants of human telomerase reverse transcriptase (hTERT). The presence of the hTERT full-length transcript appeared necessary for telomerase activity in breast carcinomas. Specifically, all telomerase-positive carcinomas expressed the hTERT full-length message, together with different combinations of alternatively spliced variants, whereas in telomerase-negative cancers, the hTERT full-length transcript was not detectable, or its abundance was markedly lower than that of alternatively spliced variants. Results obtained in benign tumours and normal tissues surrounding carcinomas instead showed that the presence of hTERT full-length transcript was not sufficient to determine telomerase activity. These findings suggest that in non-neoplastic tissues there are other mechanisms that suppress telomerase activity downstream from hTERT transcription and mRNA splicing and that such mechanisms have been lost during neoplastic transformation.

Telomerase activity and human telomerase reverse transcriptase mRNA expression are correlated with clinical aggressiveness in soft tissue tumors

BACKGROUND

Telomerase is a ribonucleoprotein enzyme that extends telomere specific repeats on the ends of chromosomes. Telomerase activity has been detected frequently in various types of human tumors and has been associated with cell immortality and oncogenesis. Human telomerase reverse transcriptase (hTERT), a telomerase catalytic subunit, reportedly regulates telomerase activity. Little is known about telomerase activity and hTERT mRNA expression in soft tissue tumors. The objective of this study was to clarify the correlation between these two parameters and clinical aggressiveness in soft tissue tumors.

METHODS

In 41 surgically resected soft tissue tumors, telomerase activity was measured by the fluorescence-based telomeric repeat-amplification protocol and hTERT mRNA expression was determined by real-time polymerase chain reaction.

RESULTS

Telomerase activity was detected in 52% of sarcomas and in none of the benign soft tissue tumors (P < 0.05). Telomerase activity was found in 77% of 13 locally recurrent sarcomas and in 89% of 9 sarcomas with distant metastasis. The frequency of the presence of telomerase activity in those tumors was significantly greater compared with the frequency of telomerase activity in the other sarcomas (P < 0.05 and P < 0.01, respectively). All telomerase positive sarcomas expressed hTERT mRNA. The mean level of hTERT mRNA expression in sarcomas was significantly greater compared with the mean hTERT mRNA expression level in benign tumors (P < 0.05) and in locally recurrent sarcomas compared with primary sarcomas (P < 0.001).

CONCLUSIONS

The results of the current study suggest that the detection of telomerase activity and the level of hTERT mRNA expression are useful markers for evaluating the clinical aggressiveness in soft tissue tumors.

Human telomerase and its regulation

The telomere is a special functional complex at the end of linear eukaryotic chromosomes, consisting of tandem repeat DNA sequences and associated proteins. It is essential for maintaining the integrity and stability of linear eukaryotic genomes. Telomere length regulation and maintenance contribute to normal human cellular aging and human diseases. The synthesis of telomeres is mainly achieved by the cellular reverse transcriptase telomerase, an RNA-dependent DNA polymerase that adds telomeric DNA to telomeres. Expression of telomerase is usually required for cell immortalization and long-term tumor growth. In humans, telomerase activity is tightly regulated during development and oncogenesis. The modulation of telomerase activity may therefore have important implications in antiaging and anticancer therapy. This review describes the currently known components of the telomerase complex and attempts to provide an update on the molecular mechanisms of human telomerase regulation.

Is the telomerase assay useful for screening of endometrial lesions?

Telomerase activation is specifically observed in most cancers but not in normal tissues with some exceptions, such as germ cells or certain tissues with regenerative potential, suggesting a diagnostic opportunity for cancers involving measurement of telomerase activity. Cytologic screening for endometrial cancer has not been well established, due to the complexity of diagnostic criterion. In the present study, we investigated the utility of the telomerase assay for screening endometrial lesions. A total of 100 patients with or without endometrial lesions were examined for telomerase activity by the telomeric repeat amplification protocol (TRAP) assay using endometrial scraping samples and the correlation with cytology was investigated. The TRAP assay revealed frequent telomerase activation in normal endometria at reproductive age, particularly in 67% of proliferative-phase endometria, suggesting that the telomerase assay is not suitable for screening women of reproductive age. However, in postmenopausal women, telomerase activity was rarely detected (8%) in normal endometria, while it was observed in >80% of endometrial cancers or hyperplasias. Interestingly, some cases of endometrial cancer and hyperplasia were misdiagnosed by cytology but correctly detected by the TRAP assay. The sensitivity of the TRAP assay to screen endometrial lesions was 87%, equivalent to that of cytology. Combination of cytology and the TRAP assay increased sensitivity to 100%. We thus concluded that measuring telomerase activity in endometrial scrapings is a useful diagnostic tool for the screening of endometrial lesions in postmenopausal women, particularly when used with cytology to increase screening sensitivity.

Expression of human telomerase reverse transcriptase in lymphangioleiomyomatosis

Telomerase synthesizes nucleotide hexameric repeats (telomeres) at the ends of chromosomes, replacing base sequences that are lost from these sites during each mitotic cycle and protecting these ends against the action of exonucleases and ligases. Therefore, telomerase is essential for maintaining cellular replication. To evaluate the role of telomerase in the proliferation of abnormal smooth muscle cells (lymphangioleiomyomatosis [LAM] cells) in LAM, we performed immunostaining and in situ hybridization studies to identify telomerase protein and messenger RNA (mRNA), respectively, in pulmonary (n = 18) and extrapulmonary (n = 4) lesions from 22 women with LAM (14 untreated and 8 treated with progesterone or tamoxifen). Immunoreactivity and hybridization signals for telomerase were observed in 5 to 20% of LAM cells, mostly of the spindle-shaped type, in 21 of the 22 patients, and were less intense in the treated group. Other types of cells were unreactive in both groups. Telomerase colocalized in the same cells with alpha-smooth muscle actin, but only rarely with HMB-45 antibody (a marker for epithelioid LAM cells); colocalization with proliferating cell nuclear antigen was incomplete. The telomerase-positive LAM cells may constitute the sources of renewal of LAM cells. Modulation of telomerase may be involved in the control of LAM cell proliferation.

Antisense epidermal growth factor receptor RNA transfection in human glioblastoma cells down-regulates telomerase activity and telomere length

Epidermal growth factor receptor is overexpressed and/or amplified in up to 50% of glioblastomas, suggesting an important role of this gene in glial tumorigenesis and progression. In the present study we demonstrated that epidermal growth factor receptor is involved in regulation of telomerase activity in glioblastoma. Antisense-epidermal growth factor receptor approach was used to inhibit epidermal growth factor receptor expression of glioblastoma U87MG cells. Telomerase activity in antisense-epidermal growth factor receptor cells decreased by up to 54 folds compared with control cells. Moreover, the telomere lengths of antisense-epidermal growth factor receptor cells were shortened. In addition, the tumorigenicity of antisense-epidermal growth factor receptor cells was significantly inhibited. Taken together, there were strong correlations between tumorigenicity and epidermal growth factor receptor expression levels, and between tumorigenicity and telomerase activity. These results provide evidence that epidermal growth factor receptor plays an important role in the regulation of telomerase activity of glioma cells. Our findings provide new insights into both the biological functions of epidermal growth factor receptor and the regulation of telomerase activity. The inhibition of telomerase activity triggered by antisense-epidermal growth factor receptor treatment may reflect yet another mechanism of antisense-epidermal growth factor receptor approach in tumour suppression.

Telomerase activity and hTERT mRNA expression can be heterogeneous and does not correlate with telomere length in soft tissue sarcomas

In a previous study, we showed that telomerase activity (TA) and human telomerase reverse transcriptase (hTERT) mRNA expression were undetectable in benign mesenchymal lesions and low-grade soft tissue sarcomas (STSs), but detectable in about 50% of intermediate-/high-grade STSs. We wondered if this lack of TA or hTERT mRNA expression could be related to the tumor sample examined and if there was a relationship between the former 2 parameters and telomere length. Two separate tumor samples from 37 STSs were examined for telomerase activity, using the telomerase repeat amplification protocol (TRAP) assay and for hTERT mRNA expression, using RT-PCR. Telomere length was determined in each tumor sample, using the terminal restriction fragments (TRF) technique. Significant variations in telomere length, TA and hTERT mRNA expression between 2 samples of the same tumor were observed in 27%, 11% and 27% of STSs, respectively. Telomere length did not correlate with TA or hTERT mRNA expression. Despite great intratumoral heterogeneity in telomere length, short and long telomeres were more often seen in the low/intermediate-grade and high-grade STS categories, respectively. Few STSs that showed a TRF pattern suggestive of alternative lengthening of telomeres (ALT) may contain ALT subpopulations.

Telomerase activity in human hepatocellular carcinoma: parallel correlation with human telomerase reverse transcriptase (hTERT) mRNA isoform expression but not with cell cycle modulators or c-Myc expression

BACKGROUND

To explore the possible regulatory mechanisms of telomerase, we examined the telomerase activity (TA), expression of human telomerase RNA (hTR), human telomerase reverse transcriptase (hTERT) mRNA isoforms and cell cycle modulators in human hepatocellular carcinoma (HCC) cell lines (J5, J7) and a normal human immortalized hepatic epithelial cell line (Chang-liver).

METHODS

The cell lines were chemically synchronized in either G1, G1/S, G2/M or M phases. TA was measured by polymerase chain reaction (PCR)-based telomerase repeat amplification protocol assay. The hTR and hTERT mRNA levels were analyzed by reverse transcriptase-polymerase chain reaction. Western blotting and immunocytochemistry were used to assay the cell cycle modulators.

RESULTS

The TA of J5, J7 and Chang-liver cell lines tested was highest in M phase. The expression level of hTERT mRNA associated with the highest TA detected in the M phase of HCC cell lines. Chang-liver expressed markedly less TA and hTERT mRNA than J5 or J7. The elevated TA and expression of hTERT mRNA isoforms in M phase of HCC cell lines did not significantly correlate with that of the cell cycle modulators and c-Myc.

CONCLUSIONS

The results implicate that regulation of TA is related to hTERT mRNA isoform expression, and that regulation is different between the cell immortalization and tumorigenesis.

Dual regulation of telomerase activity through c-Myc-dependent inhibition and alternative splicing of hTERT

Telomerase is believed to be induced upon proliferation and inhibited when cells differentiate. Thus, regulation of telomerase activity could be an important mechanism to limit growth of normal and cancer cells. Using transforming growth factor-beta 1 (TGF-β1), which is known to control proliferation in epithelial cells, we now demonstrate that in the human HaCaT skin keratinocytes, TGF-β1 downregulated c-Myc, and this blocked proliferation. This also caused a decrease in hTERT expression, which in turn inhibited telomerase activity. Overexpressing hTERT recovered telomerase activity but not proliferation, whereas constitutive expression of c-Myc recovered proliferation and hTERT expression. Nevertheless, telomerase remained inhibited, thus dissociating proliferation and telomerase activity. In addition, we show that TGF-β1 inhibited telomerase activity despite ongoing hTERT transcription by inducing loss of the full-length hTERT transcript (mediating telomerase activity) and retaining high expression of the inactive β variant. These changes in the splicing pattern reversed upon TGF-β1 removal, as did inhibition of telomerase activity, suggesting that alternative splicing may represent a novel mechanism of telomerase regulation by TGF-β1. In addition, we show that destruction of tissue integrity (in a model for epidermal blistering) resulted in a rapid induction of the inactive β variant, whereas tissue regeneration (formation of a stratified epithelium) correlated with a shift to the active full-length transcript, which is the dominant form in intact epidermis. Thus alternative splicing may not be restricted to TGF-β1 but may add a more general mechanism of hTERT regulation in epidermal cells.

hTERT gene dosage correlates with telomerase activity in human lung cancer cell lines

Maintenance of telomeres, most often by telomerase, is a necessary prerequisite for immortality of eukaryotic cells. To better understand the mechanisms of telomerase up-regulation during tumorigenesis, we analysed the gene dosage of hTERT on chromosome 5p15, a region known to be overrepresented in a variety of malignancies, in 20 lung cancer cell lines by Southern blotting, fluorescence in-situ hybridization, and comparative genomic hybridization. We found a significant correlation between hTERT gene dosage, hTERT mRNA expression and telomerase activity. Imbalances of chromosome 5p may exert functionally relevant hTERT gene dosage effects in human lung cancer.

The human telomerase gene: complete genomic sequence and analysis of tandem repeat polymorphisms in intronic regions

In this work, the full-length hTERT gene was isolated and the sequence of the previously unknown region in intron 6 as well as that of upstream and downstream hTERT regions was determined. We have shown that intron 6 includes a variable number of tandem repeats (VNTR) of a 38 bp sequence, (hTERT-VNTR 6-1). Eight alleles of hTERT-VNTR 6-1 were identified among 103 unrelated individuals, ranging from 27 to 47 repeats. hTERT-VNTR 2-2 is another new 61 bp minisatellite repeat found in intron 2 of hTERT. At least four alleles of hTERT-VNTR 2-2 can be distinguished. Previous studies have described polymorphisms for minisatellites hTERT-VNTR 2-1, a 42 bp repeat in intron 2, and hTERT-VNTR 6-2, a 36 bp repeat in intron 6. These, together with another minisatellite found in intron 12, add up to five such structures within the hTERT gene. The segregation of hTERT minisatellites was analysed in families, revealing that the VNTRs are transmitted through meiosis following a Mendelian inheritance. Minisatellites in hTERT were also analysed in matching normal and cancer tissues from patients with tumors; in one patient with a kidney tumor, the two VNTRs in intron 6 had undergone concomitant rearrangements. This observation suggests that chromosomal rearrangements implicating these VNTRs may be associated with the activation of telomerase expression in cancer cells.

Complex regulatory mechanisms of telomerase activity in normal and cancer cells: how can we apply them for cancer therapy?

Telomerase activation is observed in almost 90% of human cancers but not in normal tissues of somatic origin and thus is a critical step for multistep carcinogenesis. A more thorough understanding of telomerase regulation may provide not only a molecular basis of cancer progression but also as a way to manipulate telomerase activity as a potential therapeutic modality. Recent progress in studies on telomerase regulation has shown that telomerase activation is achieved at various steps, including transcriptional and post-transcriptional levels of the telomerase reverse transcriptase (hTERT) gene. Although a number of potentially important mechanisms of telomerase activation have been proposed, none of the current models can fully explain tumor-specific activation of telomerase, suggesting a need for further extensive analysis. This review includes a summary of recent works on telomerase regulation and a discussion of how we can overcome this situation.

Retinoic acid inhibits telomerase activity and downregulates expression but does not affect splicing of hTERT: correlation with cell growth rate inhibition in an in vitro cervical carcinogenesis/multidrug-resistance model

Telomerase, a ribonucleoprotein complex of hTERT, hTR, and TP1, has been reported to be associated with carcinogenesis and multidrug resistance (MDR). This study used our in vitro human cervical multistep carcinogenesis/MDR model system in which normal human ectocervical and endocervical (HEN) cells were immortalized by HPV18 or 16, respectively, and subsequently transformed. The first evidence was found that immortalization and telomerase activation were correlated with increased expression specifically of two of the hTERT alternatively spliced mRNAs, one encoding wild-type protein containing the full-length functional reverse transcriptase (RT) region and one encoding a defective RT protein. Expression of neither hTERT mRNA containing full-length functional or defective RT motif was affected by transformation/MDR. All-trans-retinoic acid (ATRA) treatment of HPV-immortalized HEN-16-2 cells and transformed/MDR HEN-16-2/CDDP cells inhibited telomerase activity and downregulated expression of hTERT mRNAs containing full-length functional and a defective RT motif, but there were no changes in hTR and TP1 expression. Moreover, ATRA inhibited cell growth rate of HEN-16-2 and HEN-16-2/CDDP cells equally. These results provided the first evidence that ATRA equally in both immortalized and transformed/MDR cell lines inhibits telomerase activity and downregulates expression, but not splicing, of hTERT, and this is correlated with cell growth rate inhibition; the potential is implicated for applying ATRA to hTERT-targeted treatment of cervical cell carcinogenesis/MDR.

mRNA-based electrotransfection of human dendritic cells and induction of cytotoxic T lymphocyte responses against the telomerase catalytic subunit (hTERT)

Dendritic cells (DCs) are recognised as the most potent antigen-presenting cells for induction of cellular immune responses, and vaccination with DCs pulsed with antigens has emerged as a promising strategy for generating protective immunity in mammals. We have developed a transfection method that uses in vitro synthesised mRNA and square-wave electroporation for transient expression in DCs and other cell types. The method is highly efficient and produces almost complete transfection of cells in culture. When using mRNA encoding the enhanced green fluorescence protein (EGFP), highest expression in DCs occurred on the second day after transfection and produced a 76-fold increase in mean fluorescence above background. High levels of expression were maintained for at least 5 days post-transfection. In comparison, square-wave electroporation of DCs with EGFP plasmid DNA yielded 15% transfected cells and a 28-fold increase of mean fluorescence. DCs transfected with mRNA encoding the telomerase catalytic subunit (hTERT) acquired strong telomerase activity and were capable of eliciting a hTERT-specific cytotoxic T lymphocyte (CTL) response in vitro.