RESUMEN
Elucidation of the mechanisms governing expression of the human telomerase reverse transcriptase (hTERT) is important for understanding cancer pathogenesis. Approximately 90% of tumors express hTERT, the major catalytic component of telomerase. Activation of telomerase is an early event, and high levels of this activity correlate with poor prognosis. Recent studies have shown that the transcription factors c-Myc and Mad1 activate and repress hTERT, respectively. It is not clear how these transcription factors compete for the same recognition sequence in the hTERT core promoter region. Studies have shown that the combined expression of SV40 large T antigen (T-Ag), hTERT, and H-Ras is able to transform human cells. In this study, we used a distinct human cell type, WI-38 fetal lung fibroblasts used extensively for senescence studies. We transduced cells with amphotropic retroviral constructs containing SV40 T antigen, hTERT, and activated H-ras. Transduced cells exhibited anchorage independence in soft agar and expressed increased levels of c-Myc and endogenous hTERT. These effects were observed by 25 population doublings (PDs) following the establishment of the neoplastic cell line. During the process of transformation, we observed a switch from Mad1/Max to c-Myc/Max binding to oligonucleotide sequences containing the hTERT promoter distal and proximal E-boxes. c-Myc can bind specifically to the hTERT promoter in vitro, indicating that c-Myc expression in tumors may account for the increased expression of hTERT observed in vivo. These findings indicate that the widely used model system of WI-38 fibroblasts can be employed for transformation studies using defined genetic elements and that the endogenous hTERT and c-Myc are induced in these cells during early tumorigenesis. Such studies should have important implications in the mechanisms of hTERT and c-Myc induction in the beginning stages of tumorigenesis and facilitate extension of these studies to novel models of tumorigenesis in cellular senescence.
