Transcriptional Activation of hTERT, the Human Telomerase Reverse Transcriptase, by Nuclear Factor of Activated T Cells

Abstract

Telomerase is essential for telomere maintenance, and its activation is thought to be a critical step in cellular immortalization and tumorigenesis. Human telomerase reverse transcriptase (hTERT) is a major component of telomerase activity. We show here that hTERT is expressed soon after lymphocyte activation and that its expression is inhibited by rapamycin, wortmannin, and FK506, which was the most potent inhibitor. These results suggest a potential role for the transcription factor nuclear factor of activated T cells (NFAT) in the regulation of hTERT expression. Five putative NFAT-binding sites were identified in the hTERT promoter. In luciferase assays, the hTERT promoter was activated by overexpressed NFAT1. Moreover, serial deletions revealed that the promoter activation was mainly due to a ؊40 NFAT1-binding site flanked by two SP1-binding sites. Mutation of the ؊40 NFAT-binding site caused a 53% reduction in the transcriptional activity of hTERT promoter. Simultaneous mutations of the ؊40 NFAT-responsive element together with one or both SP1-binding sites led to a more dramatic decrease in luciferase activity than single mutations, suggesting a functional synergy between NFAT1 and SP1 in hTERT transcriptional regulation. NFAT1 overexpression in MCF7 and Jurkat cell lines induced an increase in endogenous hTERT mRNA expression. Inversely, its down-regulation was induced by NFAT1 silencing. Furthermore, chromatin immunoprecipitation assay demonstrated that NFAT1 directly binds to two sites (؊40 and ؊775) in the endogenous hTERT promoter. Thus, we show for the first time the direct involvement of NFAT1 in the transcriptional regulation of hTERT.Telomeres are specialized structures located at the ends of linear mammalian chromosomes (1). The erosion of human telomeres at each cycle of cellular division is compensated for by de novo synthesis catalyzed by human telomerase reverse transcriptase (hTERT), 3 the catalytic subunit of a ribonucleoprotein complex called telomerase (2). Telomerase maintains telomeres by protecting them from exonucleases and ligases and by preventing illegitimate recombination (3). However, hTERT is also implicated in cell immortalization and tumorigenesis (4) through its telomere-lengthening activity, as well as by a mechanism independent of telomere length (5).Most normal human somatic tissues do not express hTERT, but germinal cells, several types of normally activated or proliferating cells, and tumor cells do (6 -13). In particular, lymphocytes exhibit telomerase activity in response to stimulation (14). Regulation of telomerase expression in these cells is likely to occur in the G 1 phase of the cell cycle as telomerase is inhibited by rapamycin, a compound that affects the mammalian target of rapamycin (mTOR) but is not inhibited by aphidicolin or hydroxyurea, substances that inhibit DNA synthesis (14 -16). Phosphorylation of hTERT and the resulting effects on its nuclear translocation and telomerase activity have been well described (17, 18). These post-translational modificati...

Sister chromatid telomere fusions, but not NHEJ-mediated inter-chromosomal telomere fusions, occur independently of DNA ligases 3 and 4

“…), and NFAT (P = 2.82 × 10 −9 ) transcription factor components of TERT gene regulatory networks (Supplemental Table 3C; Kyo et al 2000;Chebel et al 2009;Hoffmeyer et al 2012), revealing further potential mechanisms by which distal chromosomal locations could be juxtaposed and recombined with consequences for telomere length stabilization and malignant transformation. Repetitive DNA sequences (Cooper et al 2011), including fragile sites (Minca and Kowalski 2011;Thys et al 2015) and Alu (Gu et al 2015) have well-documented associations with genome instability via replication fork-stalling (Ozeri-Galai et al 2011) and stimulation of homology-based recombination processes (Mizuno et al 2013).…”

Telomere‐dependent replicative senescence of B and T cells from patients with type 1a common variable immunodeficiency

“…Moreover, Ca 21 influx from extracellular space positively regulates NFAT, which in turn controls cell growth and proliferation [26]. Interestingly, NFAT positively regulates the transcription of the telomerase reverse transcriptase (TERT) gene, and NFAT silencing down-regulates TERT mRNA expression [28]. A defect of extracellular Ca 21 influx can, therefore, result in NFAT silencing, reduced TERT expression and defective maintenance of telomeres [29].…”

Transcription Factor NFAT1 Activates the mdm2 Oncogene Independent of p53

“…NFAT1 promotes breast cancer cell invasion through the induction of cyclooxygenase-2 and the synthesis of prostaglandins (26,27). NFAT1 also induces human telomerase reverse transcriptase mRNA expression in activated peripheral blood lymphocytes (28). In addition, the ectopic activation of NFAT1 activates c-Myc, which is a critical mechanism for pancreatic cancer cell growth in vitro and in vivo (29,30).…”