E2F-1 represses transcription of the human telomerase reverse transcriptase gene

The ends of human chromosomes (telomeres) lose up to 200 bp of DNA per cell division. Chromosomal shortening ultimately leads to senescence and death in normal cells. Many human carcinoma lines are immortal in vitro, suggesti ng that these cells have a mechanism for maintaining the ends of their chro mosomes, Telomerase is a ribonucleoprotein complex that synthesizes telomer ic DNA onto chromosomes using its RNA component as template. Telomerase act ivity is found in most tumor cells, but is absent from normal cells. Little is known about how normal human cells repress telomerase (hTERT) gene expr ession. Mice carrying an E2F-1 null mutation develop a variety of malignant tumors, suggesting that this transcription factor has a tumor suppressor f unction. To determine mechanisms by which E2F-1 suppresses tumor formation, we examined the role of this transcription factor in regulation of the hTE RT promoter in human cells. We identified two putative E2F-1-binding sites proximal to the transcriptional start site of the hTERT promoter, Mutation of these sites produced dramatic increases in promoter activity, Overexpres sion of E2F-1 but not a mutant E2F-1 repressed hTERT promoter activity in r eporter gene assays. This repression was abolished by mutation of the E2F-1 -binding sites in the hTERT promoter. Human cancer cell lines stably overex pressing E2F-1 exhibited decreased hTERT mRNA expression and telomerase act ivity, We! conclude that E2F-1 has an atypical function as a transcriptiona l repressor of the hTERT gene in human cells.