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THE PRODUCTS OF THE PROTO-ON-cogene c-rel viral oncogene are members of a family of transcription factor NF-κB (p50 subunits of transcription factor NF-kB (p50 and p65) and the Dro-sophila maternal [...]

The human TERT (hTERT) gene encodes the telomerase catalytic subunit which plays a role in telomerase regulation. Telomerase is activated in more than 90% of all human malignancies and understanding how telomerase is regulated is necessary for implementation of successful anti-cancer therapies. microRNAs (miRNAs) are important regulators of gene expression in eukaryotic cells but evidence of their role in telomerase regulation has not been documented. To determine whether hTERT activity is regulated by multiple miRNAs, eight miRNAs which have putative binding sites in the hTERT 3′UTR together with miR-138-5p were evaluated in luciferase assays with a reporter containing the hTERT 3′UTR. Six miRNAs (let-7g*, miR-133a, miR-138-5p, miR-342-5p, miR-491-5p, and miR-541-3p) specifically inhibited the expression of the reporter luciferase-driven constructs and let-7g*, miR-133a, miR-138-5p, and miR-491-5p also downregulated endogenous telomerase activity in cells. Moreover, all six miRNAs significantly inhibited cell proliferation. miRNAs (miR-133a, miR-138-5p, 342-5p, 491-5p, 541-3p) also have predicted binding sites within the 3′UTR of three genes involved in Wnt signaling (TCF7, MSI1, and PAX5). These miRNAs inhibited the expression of the luciferase reporter constructs containing 3′UTRs of these genes and downregulated protein expression of the TCF7 transcription factor, which mediates the canonical Wnt pathway. Together, these results suggest the existence of a miRNA regulatory network involving the hTERT and Wnt pathway. [ABSTRACT FROM AUTHOR]

Eight human and six chicken novel alternatively spliced (AS) variants of telomerase reverse transcriptase (TERT) were identified, including a human variant (Δ4-13) containing an in-frame deletion which removed exons 4 through 13, encoding the catalytic domain of telomerase. This variant was expressed in telomerase-negative normal cells and tissues as well as in transformed telomerase-positive cell lines and cells which employ an alternative method to maintain telomere length. The overexpression of the Δ4-13 variant significantly elevated the proliferation rates of several cell types without enhancing telomerase activity, while decreasing the endogenous expression of this variant by use of small interfering RNA (siRNA) technology reduced cell proliferation. The expression of the Δ4-13 variant stimulated Wnt signaling. In chicken cells, AS TERT variants containing internal deletions or insertions that eliminated or reduced telomerase activity also enhanced cell proliferation. This is the first report that naturally occurring AS TERT variants which lack telomerase activity stimulate cell proliferation. [ABSTRACT FROM AUTHOR]

Background: The TERT gene encodes the catalytic subunit of the telomerase complex and is responsible for maintaining telomere length. Vertebrate telomerase has been studied in eutherian mammals, fish, and the chicken, but less attention has been paid to other vertebrates. The platypus occupies an important evolutionary position, providing unique insight into the evolution of mammalian genes. We report the cloning of a platypus TERT (OanTERT) ortholog, and provide a comparison with genes of other vertebrates. Results: The OanTERT encodes a protein with a high sequence similarity to marsupial TERT and avian TERT. Like the TERT of sauropsids and marsupials, as well as that of sharks and echinoderms, OanTERT contains extended variable linkers in the N-terminal region suggesting that they were present already in basal vertebrates and lost independently in ray-finned fish and eutherian mammals. Several alternatively spliced OanTERT variants structurally similar to avian TERT variants were identified. Telomerase activity is expressed in all platypus tissues like that of cold-blooded animals and murine rodents. OanTERT was localized on pseudoautosomal regions of sex chromosomes X3/Y2, expanding the homology between human chromosome 5 and platypus sex chromosomes. Synteny analysis suggests that TERT co-localized with sex-linked genes in the last common mammalian ancestor. Interestingly, female platypuses express higher levels of telomerase in heart and liver tissues than do males. Conclusions: OanTERT shares many features with TERT of the reptilian outgroup, suggesting that OanTERT represents the ancestral mammalian TERT. Features specific to TERT of eutherian mammals have, therefore, evolved more recently after the divergence of monotremes. [ABSTRACT FROM AUTHOR]

Telomerase activity is downregulated in somatic cells but is upregulated during the activation of cells of the immune system. The mechanism of this reactivation is not well understood. In this study, we demonstrated that interferon regulatory factor 4 (IRF-4) and, to a lesser extent, IRF-8 induce telomerase activity. The suppression of IRF-4 results in decreased levels of TERT (telomerase reverse transcriptase) mRNA and telomerase activity and reduces cell proliferation. The overexpression of TERT compensates for this proliferation defect, suggesting that telomerase contributes to the regulation of cell proliferation by IRF-4. The induction of telomerase by IRF-4 and IRF-8 correlates with the activation of the TERT promoter. IRF-4 binds the interferon response-stimulated element and the gamma interferon-activated sequence composite binding site in the TERT core promoter region in vivo. Additionally, the binding of Sp1, Sp3, USF-1, USF-2, and c-Myc to the TERT promoter is elevated in cells expressing IRF-4. IRF-4, but not IRF-8, synergistically cooperates with Sp1 and Sp3 in the activation of the TERT promoter. Collectively, these results indicate that IRF-4 and IRF-8, two lymphoid cell-specific transcription factors, increase telomerase activity by activating TERT transcription in immune cells. [ABSTRACT FROM AUTHOR]

Previous studies have shown that treatment of avian reticuloendotheliosis virus-transformed RECC-UTC4-1 (C4-1) lymphoblastoid cells with 10 μg/ml (18.8 μM) of RRR-α-tocopheryl succinate (vitamin E succinate, VES) for 3 days induced approximately 50% of the cells to undergo apoptosis. Elevated and prolonged expression of c-jun mRNA and protein was temporally correlated with VES-induced cell death. Data presented in this paper show that the elevated and prolonged expression of c-jun message and protein are not accounted for by enhanced stability, and show the involvement of c-Jun in VES-induced apoptosis in this lymphoblastoid cell type. C4-1 cells infected with a virus carrying a dominant, negatively acting mutant form of c-Jun, supjun-1, exhibited: (i) 71% reduction in VES-induced apoptosis, (ii) a 2.0 – 2.5-fold decrease in wildtype, endogenous c-Jun expression, and (iii) a 2.4 – 2.6-fold reduction in AP-1 binding activity. Additionally, cells co-treated with VES plus RRR-α-tocopherol, exhibited a 70% reduction in apoptosis, a marked reduction in c-Jun expression and a 1.6-fold reduction in AP-1 binding activity. These studies suggest that c-Jun plays a crucial role in VES-induced apoptosis in C4-1 cells, and add to our understanding of mechanisms of action involved in VES-mediated tumor cell growth inhibition. [ABSTRACT FROM AUTHOR]

Telomerase is activated during the transformation of lymphoid cells and fibroblasts by v-Rel, the oncogenic member of the Rel/NF-ΚB family of transcription factors. v-Rel-transformed cell lines have longer telomeres than untransformed chicken lymphoid cells and have high levels of telomerase activity. v-Rel-mediated activation of telomerase is achieved by multiple mechanisms. The expression of the gene encoding the catalytic subunit of telomerase (TERT) was directly upregulated by v-Rel. Moreover, the expression of v-Rel altered the ratio of alternatively spliced and full-length TERT transcripts in favor of the full-length form$. The activation of telomerase by v-Rel in lymphocytes was also accompanied by inactivation of nuclear inhibitors. The inhibition of telomerase activity in v-Rel-transformed cell lines led to apoptosis within 24 h. The expression of v-Rel in a macrophage cell line resulted in elevated levels of reactive oxygen species (ROS), increased telomerase activity, and increased sensitivity to telomerase inhibitors. In contrast, the ectopic expression of TERT decreased the extent of apoptosis induced by ROS. The activation of telomerase by v-Rel may, therefore, partially protect the transformed cells from apoptosis induced by ROS. [ABSTRACT FROM AUTHOR]