title: Identification of a novel telomerase repressor that interacts with the human papillomavirus type-16 E6/E6-AP complex.
abstract: The critical immortalizing activity of the human papillomavirus (HPV) type-16 E6 oncoprotein is to induce expression of hTERT, the catalytic and rate-limiting subunit of telomerase. Additionally, E6 binds to a cellular protein called E6-associated protein (E6-AP) to form an E3 ubiquitin ligase that targets p53 for proteasome-dependent degradation. Although telomerase induction and p53 degradation are separable and distinct functions of E6, binding of E6 to E6-AP strongly correlated with the induction of hTERT. Here, we demonstrate using shRNAs to reduce E6-AP expression that E6-AP is required for E6-mediated telomerase induction. A yeast two-hybrid screen to find new targets of the E6/E6-AP E3 ubiquitin ligase complex identified NFX1. Two isoforms of NFX1 were found: NFX1-123, which coactivated with c-Myc at the hTERT promoter, and NFX1-91, which repressed the hTERT promoter. NFX1-91 was highly ubiquitinated and destabilized in epithelial cells expressing E6. Furthermore, knockdown of NFX1-91 by shRNA resulted in derepression of the endogenous hTERT promoter and elevated levels of telomerase activity. We propose that the induction of telomerase by the HPV-16 E6/E6-AP complex involves targeting of NFX1-91, a newly identified repressor of telomerase, for ubiquitination and degradation.
authors.affiliation: Division of Human Biology, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109, USA.
authors.forename: Lindy
authors.initials: L
authors.lastname: Gewin
authors.forename: Hadley
authors.initials: H
authors.lastname: Myers
authors.forename: Tohru
authors.initials: T
authors.lastname: Kiyono
authors.forename: Denise A
authors.initials: DA
authors.lastname: Galloway
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002478:Cells, Cultured; D004268:DNA-Binding Proteins; D004789:Enzyme Activation; D016259:Genes, myc; D006801:Humans; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D009154:Mutation; D009856:Oncogene Proteins, Viral / Q000235:genetics / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D020033:Protein Isoforms; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*; D019098:Telomerase / Q000235:genetics / Q000378:metabolism; D016159:Tumor Suppressor Protein p53 / Q000235:genetics / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases / Q000235:genetics / Q000378:metabolism
chemical_list: D004268:DNA-Binding Proteins; C068651:E6 protein, Human papillomavirus type 16; D046911:Macromolecular Substances; C492666:NFX1 protein, human; D009856:Oncogene Proteins, Viral; D020033:Protein Isoforms; D012097:Repressor Proteins; D016159:Tumor Suppressor Protein p53; C471639:UBE3A protein, human; D044767:Ubiquitin-Protein Ligases; D019098:Telomerase