James Michael A, Lee John H, Klingelhutz Aloysius J
Department of Microbiology and Holden Comprehensive Cancer Center, University of Iowa, Iowa City, IA 52242, USA.
Int J Cancer. 2006 Oct 15;119(8):1878-85. doi: 10.1002/ijc.22064.
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.
高危型人乳头瘤病毒（HPV）的E6癌蛋白可在人角质形成细胞中激活人端粒酶逆转录酶（hTERT）转录。关于E6如何调节hTERT的研究表明，hTERT启动子中的E盒或X盒元件与之有关（费尔德曼等人，《美国国家科学院院刊》2003年；100:8211 - 14；吴等人，《病毒学杂志》2001年；75:5559 - 66；格温等人，《基因与发育》2004年；18:2269 - 82），但E6激活的机制仍存在争议且尚未明确界定。在此，我们证明，HPV - 16 E6在早期传代角质形成细胞中诱导hTERT表达和端粒酶活性，与hTERT启动子处组蛋白H3的乙酰化有关，依赖于E6相关蛋白（E6AP），且并非完全依赖于E盒或X盒元件。在培养中广泛传代的表达E6的细胞中，hTERT启动子组蛋白乙酰化和hTERT转录活性的进一步增加仅在内源启动子中出现，而在外源引入的hTERT启动子构建体中未出现。然而，早期和晚期传代时的端粒酶活性均依赖于E6AP表达，这意味着端粒酶活性持续依赖于E6功能。我们的结果表明，E6诱导hTERT启动子乙酰化，但在晚期传代的表达E6的细胞中端粒酶活性和组蛋白乙酰化的进一步增加与E6对核心hTERT启动子的激活无关。我们还提供证据表明，转录因子p300在E6表达的情况下是端粒酶激活和组蛋白乙酰化的潜在抑制因子。这些研究深入了解了HPV导致的永生化如何导致hTERT上调，并进一步加深了我们对恶性转化过程中端粒酶如何被激活的理解。
