Liu Xuefeng, Dakic Aleksandra, Chen Renxiang, Disbrow Gary L, Zhang Yiyu, Dai Yuhai, Schlegel Richard
Department of Pathology, Georgetown University Medical School, Washington, DC 20057, USA.
J Virol. 2008 Dec;82(23):11568-76. doi: 10.1128/JVI.01318-08. Epub 2008 Sep 25.
The high-risk human papillomaviruses (HPVs) are the causative agents of nearly all cervical cancers and are etiologically linked to additional human cancers, including those of anal, oral, and laryngeal origin. The main transforming genes of the high-risk HPVs are E6 and E7. E6, in addition to its role in p53 degradation, induces hTERT mRNA transcription in genital keratinocytes via interactions with Myc protein, thereby increasing cellular telomerase activity. While the HPV type 16 E6 and E7 genes efficiently immortalize human keratinocytes, they appear to only prolong the life span of human fibroblasts. To examine the molecular basis for this cell-type dependency, we examined the correlation between the ability of E6 to transactivate endogenous and exogenous hTERT promoters and to immortalize genital keratinocytes and fibroblasts. Confirming earlier studies, the E6 and E7 genes were incapable of immortalizing human fibroblasts but did delay senescence. Despite the lack of immortalization, E6 was functional in the fibroblasts, mediating p53 degradation and strongly transactivating an exogenous hTERT promoter. However, E6 failed to transactivate the endogenous hTERT promoter. Coordinately with this failure, we observed that Myc protein was not associated with the endogenous hTERT promoter, most likely due to the extremely low level of Myc expression in these cells and/or to differences in chromatin structure, in contrast with hTERT promoters that we found to be activated by E6 (i.e., the endogenous hTERT promoter in primary keratinoctyes and the exogenous hTERT core promoter in fibroblasts), where Myc is associated with the promoter in either a quiescent or an E6-induced state. These findings are consistent with those of our previous studies on mutagenesis and the knockdown of small interfering RNA, which demonstrated a requirement for Myc in the induction of the hTERT promoter by E6 and suggested that occupancy of the promoter by Myc determines the responsiveness of E6 and the downstream induction of telomerase and cell immortalization.
高危型人乳头瘤病毒（HPV）几乎是所有宫颈癌的致病因子，并且在病因上与其他人类癌症相关，包括肛门癌、口腔癌和喉癌。高危型HPV的主要转化基因是E6和E7。E6除了在p53降解中发挥作用外，还通过与Myc蛋白相互作用诱导生殖角质形成细胞中的hTERT mRNA转录，从而增加细胞端粒酶活性。虽然HPV 16型E6和E7基因能有效地使人类角质形成细胞永生化，但它们似乎只能延长人类成纤维细胞的寿命。为了研究这种细胞类型依赖性的分子基础，我们检测了E6激活内源性和外源性hTERT启动子的能力与使生殖角质形成细胞和成纤维细胞永生化之间的相关性。与早期研究一致，E6和E7基因无法使人类成纤维细胞永生化，但确实能延缓衰老。尽管不能永生化，但E6在成纤维细胞中具有功能，介导p53降解并强烈激活外源性hTERT启动子。然而，E6未能激活内源性hTERT启动子。与此失败相协调，我们观察到Myc蛋白与内源性hTERT启动子不相关，这很可能是由于这些细胞中Myc表达水平极低和/或染色质结构差异，这与我们发现被E6激活的hTERT启动子（即原代角质形成细胞中的内源性hTERT启动子和成纤维细胞中的外源性hTERT核心启动子）形成对比，在这些启动子中，Myc以静止或E6诱导状态与启动子相关。这些发现与我们之前关于诱变和小干扰RNA敲低的研究结果一致，这些研究表明E6诱导hTERT启动子需要Myc，并表明Myc对启动子的占据决定了E6的反应性以及端粒酶的下游诱导和细胞永生化。
