title: Down regulation of p53 with HPV E6 delays and modifies cell death in oxidant response of human diploid fibroblasts: an apoptosis-like cell death associated with mitosis.
abstract: The tumor suppressor p53 protein is known to play a critical role in apoptosis. In normal human diploid fibroblasts (HDFs), expression of the human papillomaviral (HPV) E6 gene results in a reduction of p53 protein and an inhibition of oxidant induced apoptosis within 24 h. In comparison, expression of the HPV E7 gene causes down-regulation of Rb protein without inhibiting apoptosis. Here we determine whether HDFs expressing E6 undergo cell death with a delayed time course following H2O2 exposure. Appearances of caspase-3 activity, cell detachment, trypan blue uptake and aberrant nuclei were all delayed in E6 cells compared to wild type (wt) or E7 cells. A mutant E6 gene that failed to reduce p53 could not delay cell death. Morphological examination revealed nuclear condensation in dying wt or E7 cells but nuclear fragmentation in E6 cells. Flow cytometry analysis indicated an S phase distribution of dying wt or E7 cells but a G2/M phase distribution of dying E6 cells. An elevation of cyclin B was observed in dying E6 cells but not in apoptotic E7 cells. Dying E6 cells also had elevated levels of cdc-2 protein and histone kinase activity, suggesting that the cells died at mitosis. Electron microscopy studies showed that E6 cells may die at prophase or prometaphase. Overexpression of bcl-2 resulted in an inhibition of both caspase-3 and death of E7 or E6 cells. Inactivating caspases with zVAD-fmk also reduced the death rate of E7 and E6 cells. Our data indicate that expression of HPV E6 causes a delay and morphological modification of cell death induced by oxidants. E6 cells die at mitosis, which can be inhibited by bcl-2 overexpression or caspase inhibition.
authors.affiliation: Department of Pharmacology, University of Arizona, College of Medicine, 1501 N Campbell Avenue, Tucson, Arizona, AZ 85724, USA. qchen@email.arizona.edu
authors.forename: Qin M
authors.initials: QM
authors.lastname: Chen
authors.forename: Jessica B
authors.initials: JB
authors.lastname: Merrett
authors.forename: Tarrah
authors.initials: T
authors.lastname: Dilley
authors.forename: Sally
authors.initials: S
authors.lastname: Purdom
mesh_terms: D017304:Annexin A5 / Q000378:metabolism; D017209:Apoptosis; D015153:Blotting, Western; D053148:Caspase 3; D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism; D004171:Diploidy; D015536:Down-Regulation; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005347:Fibroblasts / Q000187:drug effects* / Q000378:metabolism; D005434:Flow Cytometry; D006801:Humans; D006861:Hydrogen Peroxide / Q000378:metabolism / Q000494:pharmacology*; D008938:Mitosis / Q000502:physiology*; D009856:Oncogene Proteins, Viral / Q000235:genetics / Q000378:metabolism*; D016877:Oxidants / Q000378:metabolism / Q000494:pharmacology*; D018384:Oxidative Stress / Q000187:drug effects; D050725:Papillomavirus E7 Proteins; D011419:Propidium; D006656:Protamine Kinase / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism; D012097:Repressor Proteins; D016196:S Phase / Q000502:physiology*; D016159:Tumor Suppressor Protein p53 / Q000235:genetics / Q000378:metabolism*
chemical_list: D017304:Annexin A5; D061945:Caspase Inhibitors; C068651:E6 protein, Human papillomavirus type 16; D004791:Enzyme Inhibitors; D009856:Oncogene Proteins, Viral; D016877:Oxidants; D050725:Papillomavirus E7 Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D012097:Repressor Proteins; D016159:Tumor Suppressor Protein p53; C059731:oncogene protein E7, Human papillomavirus type 16; D011419:Propidium; D006861:Hydrogen Peroxide; D006656:Protamine Kinase; C505407:CASP3 protein, human; D053148:Caspase 3; D020169:Caspases