title: Radiosensitization of oropharyngeal squamous cell carcinoma cells by human papillomavirus 16 oncoprotein E6∗I.
abstract: OBJECTIVE
  Patients with oropharyngeal squamous cell carcinoma (OSCC) whose disease is associated with high-risk human papillomavirus (HPV) infection have a significantly better outcome than those with HPV-negative disease, but the reasons for the better outcome are not known. We postulated that they might relate to an ability of HPV proteins to confer a better response to radiotherapy, a commonly used treatment for OSCC.
  
  
  METHODS
  We stably expressed the specific splicing-derived isoforms, E6∗I and E6∗II, or the entire E6 open reading frame (E6total), which gives rise to both full length and E6∗I isoforms, in OSCC cell lines. Radiation resistance was measured in clonogenicity assays, p53 activity was measured using transfected reporter genes, and flow cytometry was used to analyze cell cycle and apoptosis.
  
  
  RESULTS
  E6∗I and E6total sensitized the OSCC cells to irradiation, E6∗I giving the greatest degree of radiosensitization (approximately eightfold lower surviving cell fraction at 10 Gy), whereas E6∗II had no effect. In contrast to radiosensitivity, E6∗I was a weaker inhibitor than E6total of tumor suppressor p53 transactivator activity in the same cells. Flow cytometric analyses showed that irradiated E6∗I expressing cells had a much higher G2M:G1 ratio than control cells, indicating that, after G2, cells were diverted from the cell cycle to programmed cell death.
  
  
  CONCLUSIONS
  This study supports a role for E6∗I in the enhanced responsiveness of HPV-positive oropharyngeal carcinomas to p53-independent radiation-induced death.
authors.affiliation: Sydney Head and Neck Cancer Institute, Sydney Cancer Centre, Department of Radiation Oncology, Royal Prince Alfred Hospital, Discipline of Dermatology, University of Sydney, NSW, Australia.
authors.forename: Ervinna
authors.initials: E
authors.lastname: Pang
authors.forename: Naomi C
authors.initials: NC
authors.lastname: Delic
authors.forename: Angela
authors.initials: A
authors.lastname: Hong
authors.forename: Mei
authors.initials: M
authors.lastname: Zhang
authors.forename: Barbara R
authors.initials: BR
authors.lastname: Rose
authors.forename: J Guy
authors.initials: JG
authors.lastname: Lyons
mesh_terms: D017209:Apoptosis / Q000502:physiology; D002294:Carcinoma, Squamous Cell / Q000378:metabolism / Q000473:pathology / Q000532:radiotherapy* / Q000821:virology; D045744:Cell Line, Tumor; D005434:Flow Cytometry; D016193:G1 Phase / Q000502:physiology; D016195:G2 Phase / Q000502:physiology; D052162:Human papillomavirus 16 / Q000378:metabolism*; D006801:Humans; D015603:Keratinocytes / Q000821:virology; D009856:Oncogene Proteins, Viral / Q000502:physiology*; D016366:Open Reading Frames; D009959:Oropharyngeal Neoplasms / Q000378:metabolism / Q000473:pathology / Q000532:radiotherapy* / Q000821:virology; D020033:Protein Isoforms / Q000502:physiology; D011836:Radiation Tolerance / Q000502:physiology*; D012097:Repressor Proteins / Q000502:physiology*; D015533:Transcriptional Activation / Q000502:physiology; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism
chemical_list: C068651:E6 protein, Human papillomavirus type 16; D009856:Oncogene Proteins, Viral; D020033:Protein Isoforms; D012097:Repressor Proteins; D016159:Tumor Suppressor Protein p53