The E6 oncoprotein from high-risk HPV types activates human telomerase reverse transcriptase (hTERT) transcription in human keratinocytes. Studies on how E6 regulates hTERT have implicated E-box or X-box elements in the hTERT promoter (Veldman et al., Proc Natl Acad Sci USA 2003;100:8211-14; Oh et al., J Virol 2001;75:5559-66; Gewin et al., Genes Dev 2004;18:2269-82), but the mechanism of activation by E6 is still controversial and not well defined. Here, we demonstrate that induction of both hTERT expression and telomerase activity by HPV-16 E6 in early passage keratinocytes is associated with acetylation of histone H3 at the hTERT promoter, is dependent on the E6 associated protein (E6AP) and is not exclusively reliant on E-box or X-box elements. Further increases in histone acetylation of the hTERT promoter and hTERT transcriptional activity in E6 expressing cells that had been passaged extensively in culture were found to occur only with the endogenous promoter and not with an exogenously introduced hTERT promoter construct. Telomerase activity at both early and late passages, however, was dependent on E6AP expression, implying a continued reliance on E6 function for telomerase activity. Our results demonstrate that E6 induces hTERT promoter acetylation, but that further increases in telomerase activity and histone acetylation in later passage E6 expressing cells are independent of E6 activation of the core hTERT promoter. We also provide evidence that the transcription factor p300 is a potential repressor of telomerase activation and histone acetylation in the context of E6 expression. These studies give insight into how immortalization by HPV results in upregulation of hTERT and furthers our understanding of how telomerase is activated during the process of malignant transformation.
Copyright 2006 Wiley-Liss, Inc.