Telomerase activation by hTRT in human normal fibroblasts and hepatocellular carcinomas

Abstract

Telomerase is a specialized type of reverse transcriptase which catalyzes the synthesis and extension of telomeric DNA (for review, see ref.1). This enzyme is highly active in most cancer cells, but is inactive in most somatic cells. This striking observation led to the suggestion that telomerase might be important for the continued growth or progression of cancer cells. However, little is known about the molecular mechanism of telomerase activation in cancer cells. Human telomerase reverse transcriptase (hTRT) has recently been identified as a putative human telomerase catalytic subunit. We transfected the gene encoding hTRT into telomerase-negative human normal fibroblast cells and demonstrated that expression of wild-type hTRT induces telomerase activity, whereas hTRT mutants containing mutations in regions conserved among other reverse transcriptases did not. Hepatocellular carcinoma (20 samples) and non-cancerous liver tissues (19 samples) were examined for telomerase activity and expression of hTRT, the human telomerase RNA component (hTR; encoded by TERC) and the human telomerase-associated protein (hTLP1; encoded by TEP1). A significant correlation between hTRT expression and telomerase activity was observed. These results indicate that the hTRT protein is the catalytic subunit of human telomerase, and that it plays a key role in the activation of telomerase in cancer cells.

Telomerase reverse transcriptase gene is a direct target of c-Myc but is not functionally equivalent in cellular transformation

“…This notion is supported by the observation that TERT transcripts are low or undetectable in most human primary somatic cells and tissues, but readily observable in the same cell or tissue type following neoplastic transformation (Nakamura et al, 1997;Meyerson et al, 1997). Moreover, in approximately 90% of all tumors, telomerase activity (Kim et al, 1994) as well as TERT expression (Nakamura et al, 1997;Meyerson et al, 1997;Harrington et al, 1997;Kilian et al, 1997;Nakayama et al, 1998) are detected. Together, these ®ndings are consistent with the hypothesis that either activation of oncogene or loss of tumor suppressor function serves to override the strict repression of TERT in primary somatic cells.…”

“…Growth beyond the replicative senescence checkpoint (Hay¯ick limit) correlates well with genetic lesions that interfere with one or more key cellular mortality pathways, most prominently Myc, Rb and/or p53 Wright and Shay, 1995). Studies addressing the relationship between telomere length, telomerase regulation and replicative capacity have established a critical role for the telomerase catalytic protein component or TERT (for Telomerase Reverse Transcriptase) (Bodnar et al, 1998;Vaziri and Benchimol, 1998;Nakamura et al, 1997;Meyerson et al, 1997;Harrington et al, 1997;Kilian et al, 1997;Nakayama et al, 1998) in immortalization. Speci®cally, normal human somatic cells can acquire the ability to maintain telomeres and replicate well beyond the Hay¯ick limit by stable enforced expression of TERT (Bodnar et al, 1998;Vaziri and Benchimol, 1998).…”

Expression of mouse telomerase reverse transcriptase during development, differentiation and proliferation

“…Telomeres consist of lengthy G-rich simple repeat sequences that are synthesized de novo by a specialized reverse transcriptase known as telomerase (Greider and Blackburn, 1985;Yu et al, 1990;Singer and Gottschling, 1994). The telomerase core enzyme consists of an RNA component and a polypeptide that bears classical structural motifs and functional activities of a reverse transcriptase Nakamura et al, 1997Nakamura et al, , 1998Meyerson et al, 1997;Harrington et al, 1997;Kilian et al, 1997;. As conventional DNA-dependent DNA polymerases fail to fully replicate the ends of chromosomes, telomerase has evolved to serve as the primary means by which eukaryotic cells maintain telomere length with each cell division cycle.…”

“…The human telomerase reverse transcriptase protein (previously hTRT, now known as hTERT) has recently been cloned by Nakamura et al (1997) and by others under di erent names (hEST2, hTLP2, hTCS1, hTRT) (Meyerson et al, 1997;Harrington et al, 1997;Kilian et al, 1997;Nakayama et al, 1998). These studies, using primary and immortalized human cell lines, have established that an additional level of telomerase activity regulation is achieved through mechanisms governing expression of hTERT Nakamura et al, 1997Nakamura et al, , 1998Meyerson et al, 1997;Harrington et al, 1997;Kilian et al, 1997). In this study, we have isolated and characterized the mouse ortholog of hTERT with the goals of understanding the regulation of telomerase activity in normal and neoplastic processes in the mouse, providing insights into the immortalization behavior of mouse and human cells, and characterizing the catalytic activities of the mouse protein.…”

Antitumor efficacy and tumor-selective replication with a single intravenous injection of OAS403, an oncolytic adenovirus dependent on two prevalent alterations in human cancer

“…4 The hTERT gene has been previously shown to be selectively expressed in the majority of human cancers. [23][24][25][26][27][28] Furthermore, the hTERT promoter has been shown to confer tumor-selective expression of the linked gene in plasmids and Ad constructs. [28][29][30][31][32][33] Oncolytic adenoviruses utilizing the hTERT promoter to control Ad early region genes have also been previously described.…”