Molecular switching mechanisms from repression to activation in the transcription of the human telomerase reverse transcriptase gene

Abstract

Maintenance of telomeres is required for sustained cell proliferation, and thus activation of telomerase is critical for cellular immortalization and malignant transformation. In human cells, telomerase activity is tightly regulated by the expression of its catalytic subunit, human telomerase reverse transcriptase (hTERT). hTERT is expressed in many human tumor cells, but not in most normal human somatic cells. Here we present the direct role of Sp1 in these differential activities of the hTERT gene in normal and tumor cells. Mutational analyses have identified regulatory DNA sequences for Sp1 in the hTERT promoter which act as the repressive element in normal cells, but as the activating element in tumor cells. We found that the N-terminal domain of Sp1 specifically interacts with histone deacetylase (HDAC) and recruits HDAC onto the endogenous hTERT promoter in normal human fibroblasts, but not in human tumor cell lines. Consistent with this, the N-terminal domain of Sp1 exhibits opposing intrinsic transcriptional activities, acting as an activator in tumor cells, but as a repressor in normal cells. Furthermore, when overexpressed, the N-terminal domain of Sp1 acts as dominant-negative mutant, relieving the repression of the hTERT promoter in normal human somatic cells, but blocking the de-repression of the hTERT promoter in human tumor cells. Taken together, these results suggest that Sp1 is actively engaged in the transcriptional repression of hTERT in normal somatic cells, and in the activation of hTERT in tumor cells, by its differential interaction with HDAC through its N-terminal domain