The human telomerase catalytic subunit hTERT: organization of the gene and characterization of the promoter

Absence of telomerase activity and telemorase catalytic subunit mRNA in melanocyte cultures

The classic model of activation of telomerase, for which activity has been found in most cancers including cutaneous malignant melanoma (CMM), dictates that enzyme activity is generated by pathological reactivation of telomerase in telomerase-negative somatic cells. However, recent data demonstrated physiological up-regulation in some normal cell types when established as proliferating cultures, indicating that, in some cancer types, telomerase is expressed by the process of up-regulation in telomerase-competent precursor cells. In this study, cultures of epidermal melanocytes, progenitor cells of CMM, were established and harvested in the logarithmic phase of growth. Telomerase activity was looked for using a non-isotopic variant of the telomeric repeat amplification protocol, and transcript expression of the hTERT gene, the rate-limiting catalytic telomerase subunit, was investigated by the reverse transcription polymerase chain reaction. Neither telomerase activity nor hTERT mRNA could be detected in proliferating melanocyte cultures. Our in vitro data argue against the model of telomerase as a common biomarker of cell proliferation. The results further suggest that telomerase is tightly controlled in normal melanocytes, and that telomerase is reactivated rather than up-regulated in melanocytic precursors during melanoma initiation or progression.

Expression profile of telomerase subunits in human pleural mesothelioma

Using the TRAP assay, telomerase activity was previously detected in over 90% of human pleural mesotheliomas (MMs), but not in mesothelial cell cultures (MCCs), suggesting that telomerase re-activation occurs during multi-step mesothelioma carcinogenesis. The present study determined the expression of the telomerase RNA template (hTERC), the telomerase-associated protein (hTEP1), and the telomerase catalytic sub-unit (hTERT), in 16 pleural MMs and 4 MM-derived cell lines, in two pleural solitary fibrous tumours and in six MCCs. Reverse transcription-polymerase chain reaction analysis revealed that hTERT mRNA expression parallels the activity status documented by the TRAP assay, whereas hTERC and hTEP1 mRNA are commonly expressed in all malignant and non-malignant serosal cells and tissues. Three alternatively spliced hTERT transcripts were detected in all telomerase-positive samples, whereas neither variant could be detected in the MCCs. Detection of the hTERT protein with a commercially available antibody was not successful. These results indicate that hTERT expression is rate-limiting for human telomerase activity and that re-activation, rather than up-regulation, of hTERT expression can play a critical role in MM carcinogenesis. While waiting suitable anti-hTERT antibodies, these results provide information for the design of hTERT mRNA-specific in situ probes to study telomerase in archived pre-malignant serosal lesions.

The Wilms' tumor 1 tumor suppressor gene represses transcription of the human telomerase reverse transcriptase gene

Regulation of the human telomerase reverse transcriptase (hTERT) gene is the primary determinant for telomerase enzyme activity, which is found in tumor cells but is largely absent from normal somatic cells. Recent studies have shown that Myc protein can transcriptionally activate the hTERT gene. However, little is known about the repression mechanism of the hTERT gene and telomerase enzyme. Here, we developed an expression cloning strategy to identify cDNAs whose products can repress hTERT promoter activity in telomerase-positive immortal cells. Using this screen, we isolated the Wilms' tumor 1 suppressor gene (WT1). WT1 can repress hTERT promoter activity in 293 kidney cells. The WT1 binding site on the hTERT promoter was identified by deletional analysis. Alteration of the WT1 binding site markedly derepresses transcription from an isolated hTERT promoter by inhibiting interaction of WT1 with DNA. These specific repression effects of WT1 were not observed in HeLa cells, which express no endogenous WT1. Furthermore, we show that WT1 can repress the endogenous hTERT promoter and telomerase enzyme activities. These results suggest that WT1 may be a transcriptional repressor of the hTERT gene, at least in some specific cells.

Studies of the molecular mechanisms in the regulation of telomerase activity

Telomerase, a specialized RNA-directed DNA polymerase that extends telomeres of eukaryotic chromosomes, is repressed in normal human somatic cells but is activated during development and upon neoplasia. Whereas activation is involved in immortalization of neoplastic cells, repression of telomerase permits consecutive shortening of telomeres in a chromosome replication-dependent fashion. This cell cycle-dependent, unidirectional catabolism of telomeres constitutes a mechanism for cells to record the extent of DNA loss and cell division number; when telomeres become critically short, the cells terminate chromosome replication and enter cellular senescence. Although neither the telomere signaling mechanisms nor the mechanisms whereby telomerase is repressed in normal cells and activated in neoplastic cells have been established, inhibition of telomerase has been shown to compromise the growth of cancer cells in culture; conversely, forced expression of the enzyme in senescent human cells extends their life span to one typical of young cells. Thus, to switch telomerase on and off has potentially important implications in anti-aging and anti-cancer therapy. There is abundant evidence that the regulation of telomerase is multifactorial in mammalian cells, involving telomerase gene expression, post-translational protein-protein interactions, and protein phosphorylation. Several proto-oncogenes and tumor suppressor genes have been implicated in the regulation of telomerase activity, both directly and indirectly; these include c-Myc, Bcl-2, p21(WAF1), Rb, p53, PKC, Akt/PKB, and protein phosphatase 2A. These findings are evidence for the complexity of telomerase control mechanisms and constitute a point of departure for piecing together an integrated picture of telomerase structure, function, and regulation in aging and tumor development-Liu, J.-P. Studies of the molecular mechanisms in the regulation of telomerase activity.

Enzymatic activity of endogenous telomerase associated with intact nuclei from human leukemia CEM cells

Telomerase, a telomere-specific DNA polymerase and novel target for chemotherapeutic intervention, is found in many types of cancers. Telomerase activity is typically assayed using an exogenous primer and cellular extracts as the source of enzyme. Since the nuclear organization might affect telomerase function, we developed a system in which telomerase in intact nuclei catalyzes primer extension. Telomerase activity in isotonically isolated nuclei from human CEM cells shows low processivity (addition of up to four TTAGGG repeats). In contrast, telomerase activity which leaks into a 500 g postnuclear supernatant and the activity in a CHAPS extract are highly processive. The nucleotide inhibitor, 7-deaza-dGTP, seems to be more inhibitory against the nuclei-associated enzyme compared to telomerase from cytoplasmic extracts. However, 7-deaza-dATP and ddGTP are less inhibitory against nuclei-associated telomerase. The results suggest that the association of telomerase with the nuclear chromatin affects telomerase activity. Examination of telomerase activity in a more natural nuclear environment may shed new light on the telomerase function and provide a useful system for the evaluation of new telomerase inhibitors.

Interferon-alpha repressed telomerase along with G1-accumulation of Daudi cells

The implications of telomerase on senescence and human carcinogenesis are widely accepted, but the changes of telomerase activity along with cell cycle modulation by anticancer treatment still remain obscure. In this paper, we issued whether the telomerase activity fluctuated along with cell cycle of cultured cancer cells using the antiproliferative effect of interferon-alpha (IFN-alpha). Daudi Burkitt lymphoma cells, treated with IFN-alpha, showed proliferation inhibition and cell cycle arrest at G1. The telomerase activity at 72 h was repressed to about 20% of control cells. Furthermore, after 72 h IFN-alpha treatment, the cells in G1 phase showed the marked decrease of telomerase activity, while cells in S and G2/M still possessed it. Among expressions of telomerase-related genes, only the catalytic subunit of telomerase (hTERT) decreased from 48 h, while the template RNA component (hTERC) and telomerase-associated protein 1 (TEP-1) were not affected. The downregulation of c-Myc preceded the change of hTERT. Moreover, the analysis of cells treated with IFN-alpha for 24 h revealed that cells in G1-to-S transition mainly expressed high hTERT, while S and G2/M cells had higher level of telomerase activity than that of G1 cells. These results indicate that (i) the expression of hTERT precedes the telomerase activity which is higher in S and G2/M phases than G1 phase, (ii) IFN-alpha repressed the telomerase activity in a cell cycle-dependent manner with the downregulation of hTERT.

Genomic organization and promoter characterization of the gene encoding the human telomerase reverse transcriptase (hTERT)

The enzyme telomerase plays a crucial role in cellular proliferation and tumorigenesis. By adding hexameric repeats to chromosome ends, it prevents telomeric loss and, thus, entry into senescence. Recent data suggest that expression of the human telomerase reverse transcriptase subunit (hTERT) represents the limiting factor for telomerase activity. To gain an insight into the mechanisms regulating hTERT expression, we have determined the complete genomic organization of the hTERT gene and isolated the 5'- and 3'- flanking region. The hTERT gene encompasses more than 37kb and consists of 16 exons. We show that all hTERT insertion and deletion variants described so far most likely result from the usage of alternative splice consensus sequences in intron or exon regions. Furthermore, we identified a new hTERT splice variant. Analysis of the DNA sequence surrounding the putative transcriptional start region revealed a TATA-less promoter located in a CpG island. A promoter fragment spanning the first 1100bp upstream of the initiating ATG start codon exhibited high-level activity in HEK-293 cells. Several consensus binding sites for the transcription factor Sp1 as well as a c-Myc binding site were identified in this promoter region. Altogether, these results provide the basis for more detailed studies on the regulation of telomerase activity in normal and cancer cells, and may lead to the development of new cancer therapies.

Telomere maintenance mechanisms and cellular immortalization

Immortal cell populations are able to proliferate indefinitely. Immortalization is associated with activation of processes that compensate for the telomeric shortening that accompanies cell division in normal somatic cells. In many immortal cell lines, telomere maintenance is provided by the action of the ribonucleoprotein enzyme complex, telomerase. Some immortal cell lines have undetectable or very low levels of telomerase activity and there is evidence that these cells maintain their telomeres by an alternative mechanism.