title: p53-mediated apoptosis of CLL cells: evidence for a transcription-independent mechanism.
abstract: The p53 protein plays a key role in securing the apoptotic response of chronic lymphocytic leukemia (CLL) cells to genotoxic agents. Transcriptional induction of proapoptotic proteins including Puma are thought to mediate p53-dependent apoptosis. In contrast, recent studies have identified a novel nontranscriptional mechanism, involving direct binding of p53 to antiapoptotic proteins including Bcl-2 at the mitochondrial surface. Here we show that the major fraction of p53 induced in CLL cells by chlorambucil, fludarabine, or nutlin 3a was stably associated with mitochondria, where it binds to Bcl-2. The Puma protein, which was constitutively expressed in a p53-independent manner, was modestly up-regulated following p53 induction. Pifithrin alpha, an inhibitor of p53-mediated transcription, blocked the up-regulation of Puma and also of p21(CIP1). Surprisingly, pifithrin alpha dramatically augmented apoptosis induction by p53-elevating agents and also accelerated the proapoptotic conformation change of the Bax protein. These data suggest that direct interaction of p53 with mitochondrial antiapoptotic proteins including Bcl-2 is the major route for apoptosis induction in CLL cells and that p53's transcriptional targets include proteins that impede this nontranscriptional pathway. Therefore, strategies that block up-regulation of p53-mediated transcription may be of value in enhancing apoptosis induction of CLL cells by p53-elevating drugs.
authors.affiliation: Department of Hematology, Royal Free and University College Medical School, London, United Kingdom.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Steele
authors.forename: Archibald G
authors.initials: AG
authors.lastname: Prentice
authors.forename: A Victor
authors.initials: AV
authors.lastname: Hoffbrand
authors.forename: Birunthini C
authors.initials: BC
authors.lastname: Yogashangary
authors.forename: Stephen M
authors.initials: SM
authors.lastname: Hart
authors.forename: Elisabeth P
authors.initials: EP
authors.lastname: Nacheva
authors.forename: Julie D
authors.initials: JD
authors.lastname: Howard-Reeves
authors.forename: Veronique M
authors.initials: VM
authors.lastname: Duke
authors.forename: Panagiotis D
authors.initials: PD
authors.lastname: Kottaridis
authors.forename: Kate
authors.initials: K
authors.lastname: Cwynarski
authors.forename: Lyubomir T
authors.initials: LT
authors.lastname: Vassilev
authors.forename: R Gitendra
authors.initials: RG
authors.lastname: Wickremasinghe
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D017209:Apoptosis / Q000187:drug effects / Q000235:genetics / Q000502:physiology*; D051017:Apoptosis Regulatory Proteins / Q000378:metabolism; D052160:Benzothiazoles / Q000494:pharmacology; D002699:Chlorambucil / Q000494:pharmacology; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D015451:Leukemia, Lymphocytic, Chronic, B-Cell / Q000235:genetics / Q000378:metabolism* / Q000473:pathology*; D008297:Male; D008875:Middle Aged; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism; D014050:Toluene / Q000031:analogs & derivatives / Q000494:pharmacology; D014158:Transcription, Genetic / Q000187:drug effects; D016159:Tumor Suppressor Protein p53 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014740:Vidarabine / Q000031:analogs & derivatives / Q000494:pharmacology
chemical_list: D051017:Apoptosis Regulatory Proteins; C438378:BBC3 protein, human; D052160:Benzothiazoles; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; C495901:TP53 protein, human; D016159:Tumor Suppressor Protein p53; D002699:Chlorambucil; D014050:Toluene; C121565:pifithrin; D014740:Vidarabine; C024352:fludarabine