Enhanced activity of cloned hamster TERT gene promoter in transformed cells

In 7,12-dimethylbenz[a]anthracene-treated hamster pouch epithelial cells, telomerase activity increased within 1 week of treatment and reached a 6-7-fold increase within 3 weeks. To investigate this phenomenon, we have cloned and sequenced the hamster telomerase catalytic subunit (hamTERT) promoter. Transient transfection with different genomic segments upstream of the ATG translation initiation codon linked to the luciferase reporter gene mapped the core promoter within a 250 bp region. Three major transcription initiation sites and several minor sites were found between -42 and -140 bp relative to the ATG site. Like the human and murine TERT promoters, the hamTERT promoter lacks TATA and CAT boxes and all three promoters share similar regulatory factor binding sites. DNase I footprint analysis revealed six protected regions which contain sequences homologous with known transcription factor binding sites. Three protein binding regions (I, II, and III) were essential for the promoter activity. Regions I and III bound to Sp1 and Sp3 transcriptional factors, whereas region II bound to an unknown factor. Transient transfection of a promoter-luciferase plasmid into Drosophila SL2 cells showed that Sp1 and Sp3 regulated the hamster TERT promoter in a concentration-dependent and synergistic manner. Telomerase activity showed a 2-4-fold and 8-10-fold increase in immortalized cells and tumor cells, respectively, but hamTERT expression was only increased 1.7-fold and 2.4-fold, respectively, in the same cells.