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

Abstract

We have identi®ed the mouse telomerase reverse transcriptase component (mTERT) and demonstrate both substantial sequence homology to the human ortholog (hTERT), and the presence of reverse transcriptase and telomerase speci®c motifs. Furthermore, we show functional interchangeability with hTERT in in vitro telomerase reconstitution experiments, as mTERT produces strong telomerase activity in combination with the human telomerase RNA component hTR. The mouse TERT is widely expressed at low levels in adult tissues, with greatest abundance during embryogenesis and in adult thymus and intestine. The mTERT component mRNA levels were regulated during both di erentiation and proliferation, while mTR levels remained constant throughout both processes. Comparison of mTERT and mTR levels to telomerase activity indicates that mTERT expression is more tightly linked to the regulation of telomerase activity during these processes than is mTR. In contrast to the situation in human cell cultures, mTERT transcript levels are present at readily detectable levels in primary cultured cells and are not upregulated following crisis. The widespread expression of mTERT in primary cells and mouse tissues could explain the increased frequency of spontaneous immortalization of mouse cells in culture and tumorigenesis in vivo.

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

“…(b) Graph of mTERT and c-myc hybridization signals normalized to b-actin and 28S rRNA respectively, and plotted as a function of time of exposure to HMBA. Open circle, Myc56MEL; closed circle, MEL; solid line, mTERT; dotted line, c-myc; quantitation of expression=the relative ratio of TERT or myc signal to actin or 28S, respectively HMBA-exposure of parental MEL cultures was associated with a rapid 3 ± 4-fold decline in both cmyc and TERT mRNA levels by 4 h post-induction as reported previously (Greenberg et al, 1998). In contrast, c-myc and TERT expression in the HMBAtreated Myc56MEL cultures remained at pre-induction levels in two separate experiments.…”

“…Upon hexamethylene bisacetamide (HMBA) induction of MEL cell di erentiation, c-Myc and TERT exhibit highly regulated and strikingly similar patterns of gene expression (Lachman and Skoultchi, 1984;Greenberg et al, 1998). To examine more closely the relationship between c-Myc and TERT gene expression, we measured steady state c-myc and TERT mRNA levels following HMBA treatment in parental MEL cultures and in Myc56MEL cells, a well characterized MEL clonal derivative that constitutively expresses c-Myc (Dmitrovsky et al, 1986).…”

“…In addition, exposure of three di erent human leukemic cell lines to antisense c-myc oligonucleotides results in down-regulation of telomerase activity (Fujimoto and Takahashi, 1997); whereas Nmyc gene ampli®cation in neuroblastoma is associated with high telomerase activity (Hiyama et al, 1995). These observations, together with highly similar expression pro®les for mTERT and c-myc during mouse erythroleukemia (MEL) cell di erentiation and mitogen-stimulated lymphocyte proliferation (Greenberg et al, 1998;Lachman and Skoultchi, 1984;Reed et al, 1986), imply a potential link between increased Myc transactivation activity and up-regulation of TERT gene expression in normal proliferating and transformed cells. Indeed, recent evidence has shown that enforced expression of c-Myc in normal human ®broblasts or normal human mammary epithelial cells leads to increased expression of hTERT (Wang et al, 1998).…”

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

“…Alignment of the first 300 bp of the mTERT and hTERT promoter sequences reveals a number of conserved regions. 52 Relative to the A in the initiation codon, there is an Ebox at position À314 in the hTERT promoter and at À32 in the mouse promoter. In addition, there are two SP1-binding sites located at À168 and À134, which are highly conserved between mouse and human.…”

“…Unlike humans, in which the expression of hTERT is limited to a small number of normal tissues, 25 mTERT expression is widely expressed at low levels in many adult tissues. 52 Despite the difference in expression pattern, adenoviral vectors using the hTERT promoter to drive LacZ or HSV-thymidine kinase genes had undetectable transgene expression in the livers of mice, 28,30,33 suggesting that activity of the hTERT promoter is low in normal mouse liver. This was not attributable to the inability of the hTERT promoter to use the mouse transcriptional machinery, since LacZ expression was seen in the mouse lung carcinoma cell line M109.…”