HPV E6 protein interacts physically and functionally with the cellular telomerase complex

Abstract

Telomerase activation is critical for the immortalization of primary human keratinocytes by the high-risk HPV E6 and E7 oncoproteins, and this activation is mediated in part by E6-induction of the hTERT promoter. E6 induces the hTERT promoter via interactions with the cellular ubiquitin ligase, E6AP, and with the c-Myc and NFX-1 proteins, which are resident on the promoter. In the current study we demonstrate that E6 protein interacts directly with the hTERT protein. Correlating with its ability to bind hTERT, E6 also associates with telomeric DNA and with endogenous active telomerase complexes. Most importantly, E6 increases the telomerase activity of human foreskin fibroblasts transduced with the hTERT gene, and this activity is independent of hTERT mRNA expression. Unlike its ability to degrade p53, E6 does not degrade hTERT protein in vitro or in vivo. Our studies of E6/hTERT interactions also reveal that the C-terminal tagged hTERT protein, although incapable of immortalizing fibroblasts, does immortalize keratinocytes in collaboration with the viral E7 protein. Thus, E6 protein mediates telomerase activation by a posttranscriptional mechanism and these findings provide a model for exploring the direct modulation of cell telomerase/telomere function by an oncogenic virus and suggest its potential role in both neoplasia and virus replication.cell immortalization ͉ hTERT ͉ oncoproteins ͉ papillomavirus T he HPV-16 E6 oncoprotein increases cellular telomerase activity, predominantly by inducing transcription of the hTERT gene (1-6). The hTERT protein is the catalytic, ratelimiting subunit of the telomerase enzyme complex and is selectively expressed in a small subset of normal cells (stem cells), tumor tissues, and tumor-derived cell lines (7-10). Interestingly, overexpression of hTERT or c-Myc (which transactivates the hTERT promoter) can substitute for E6 in the immortalization of primary HFKs, indicating that telomerase activation constitutes a major immortalizing function of E6 (11,12). Our previous studies and those of other laboratories indicate that E6-mediated hTERT transactivation is independent of its ability to degrade p53 or interact with PDZ proteins (11-16). However, hTERT transactivation by E6 is dependent upon its binding the cellular ubiquitin ligase, E6AP (13,14,(17)(18)(19)(20). There appear to be two critical targets for E6/E6AP on the hTERT promoter, Myc (12, 15, 16) and NFX-1 (13, 18, 21), which transactivate and transrepress the promoter respectively. The model for E6 regulation of the hTERT promoter is likely to be quite complex and it is anticipated that there might be additional targets for E6 on this promoter.In addition to activating telomerase, E6 interacts with a spectrum of cellular proteins that may contribute to HPV oncogenicity. For example, E6 binds proteins that regulate cell differentiation, adhesion, polarity, proliferation, apoptosis, gene transcription, and chromosomal stability (22,23). E6 interacts with these target proteins via several mechanisms, including two known ...

Characterization of a cluster of oncogenic mutations in E6 of a human papillomavirus 83 variant isolated from a high-grade squamous intraepithelial lesion

“…Transcriptional control of the catalytic subunit of human telomerase reverse transcriptase (hTERT) plays a major role in the regulation of telomerase activity in many human cancers (Ducrest et al, 2002;Horikawa & Barrett, 2003). In normal somatic cells, E6 of HR-HPV induces the hTERT promoter (Liu et al, 2009;Veldman et al, 2001) through degradation of the transcriptional repressor NFX1-91 (Gewin et al, 2004;Katzenellenbogen et al, 2007Katzenellenbogen et al, , 2009 or through its binding to Myc (Liu et al, 2008;Veldman et al, 2003). Indeed, E6 from HR-HPV can increase cell proliferation through the upregulation of telomerase activity (Veldman et al, 2001).…”

Targeting the Human Papillomavirus E6 and E7 Oncogenes through Expression of the Bovine Papillomavirus Type 1 E2 Protein Stimulates Cellular Motility

“…hrHPV E6 is best known for its ability to target p53 for proteasomal degradation via its association with the E6-associated protein (E6-AP), a ubiquitin ligase (52,53,64). Additionally, hrHPV E6 activates telomerase to extend the life span of primary human keratinocytes (28,33,58) and binds to and deregulates several PDZ proteins that are known to regulate cell polarity, adhesion, and proliferation (27,32,41,57). This results in deregulation of tumor-suppressive activity and, in so doing, contributes to carcinogenesis.…”

High-Risk Human Papillomavirus E6 Protein Promotes Reprogramming of Fanconi Anemia Patient Cells through Repression of p53 but Does Not Allow for Sustained Growth of Induced Pluripotent Stem Cells

“…E6 targets p53 for degradation through interaction with the E6AP ubiquitin ligase (29). It can also activate c-MYC and telomerase (30)(31)(32). E7 targets the Rb family pocket proteins for degradation, allowing constitutive activation of the E2F transcription factors (33), and also binds and inhibits p21, which controls the G 1 /S transition downstream of p53 (34,35).…”