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SummaryResistance to apoptosis is a major obstacle preventing effective therapy for malignancy. Mitochondria localized anti-death proteins of the Bcl-2 family play a central role in inhibiting apoptosis and therefore present valid targets for novel therapy. The peripheral benzodiazepine receptor (PBR) shares a close physical association with the permeability transition pore complex (PTPC), a pivotal regulator of cell death located at mitochondrial contact sites. In this study we investigated the cytotoxicity of the PBR ligand, PK11195, in the micromolar concentration range. PK11195 induced antioxidant inhibitable collapse of the inner mitochondrial membrane potential (∆Ψ m ) and mitochondrial swelling in HL60 human leukaemia cells, but not in SUDHL4 lymphoma cells (which exhibited a higher level of reduced glutathione and relative tolerance to chemotherapy or pro-oxidant induced ∆Ψ m dissipation). PK11195 induced the production of hydrogen peroxide that was not inhibited by Bcl-2 transfection, nor depletion of mitochondrial DNA. ROS production was however blocked by protonophore, implicating a requirement for ∆Ψ m . Our findings suggest that PK11195-induced cytotoxicity relies upon Bcl-2 resistant generation of oxidative stress; a process only observed at concentrations several orders of magnitude higher that required to saturate its receptor. 1397-1404 © 2001 Cancer Research Campaign doi: 10.1054/ bjoc.2001.1788, available online at http://www.idealibrary.com on http://www.bjcancer.com methyl-N-methyl-N-(1-methylpropyl)-isoquinoline carboxamide (PK11195), carbonylcyanide m-chlorophenylhdrazone (CCCP), propidium iodide and all cell culture reagents were purchased from Sigma-Aldrich Ltd, UK. FITC-conjugated mouse antihuman Bcl-2 monoclonal antibody, and the Dako intrastain fixation/permeabilization kit were purchased from Dako, UK. Cell culture and treatmentsHL60, KG1A, K652 cells stably transfected with the Bcl-2 gene (K562 B4) or vector only (K562 N2) (kindly provided by Dr DE Banker and Dr F Applebaum, The Fred Hutchinson Cancer Research Center, USA), and SUDHL4 lymphoma cell lines, were maintained in exponential suspension cultures in RPMI 1640 medium supplemented with 10% fetal calf serum, 5 mM glutamine, 100 µg ml -1 streptomycin and 100 U ml -1 penicillin. Cells were grown in a humidified atmosphere of 5% CO 2 /95% air at 37˚C. PK11195 was dissolved in ethanol at a stock concentration of 8.7 mg ml -1 , and added to cells at a 75 µM final concentration for 4 hours; vehicle alone was also used in medium. To test the effect of an antioxidant on PK11195 activity, cells were treated with 10 mM N-acetylcysteine for 45 minutes prior to treatment with PK11195.Cells were incubated with VP16, or ara-C at 10 µM final concentration for 24 hours, or with the thiol crosslinking prooxidant, diamide, at 100 µM concentration for 24 hours. The protonophore CCCP was used to dissipate ∆Ψ m by treating cells at a concentration of 10 µM for 15 minutes prior to treatment with PK11195. Depletion of mitochondrial DNAKG1A cells ...

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Summary. Bcl-X L , a member of the Bcl-2-related antiapoptosis protein family, antagonizes a diverse range of apoptosis-inducing stimuli by preventing mitochondrial permeability transition, release of apoptogenic factors including cytochrome C, and caspase activation. We have tested the hypothesis that the susceptibility of Bcl-X Lexpressing leukaemic cells to apoptosis induced by VP16 (etoposide) can be enhanced by pharmacological downregulation of Bcl-X L in vivo. Two subcutaneous xenograft models of B-cell leukaemia-employing SEMK-2 and BV173 cell lines were established in severe combined immunodeficient/non-obese diabetic mice followed by 14 d of continuous subcutaneous administration of Bcl-X L -specific second generation oligonucleotides ISIS 16009 or ISIS 15999. Tumours were disaggregated, enabling investigation of Bcl-X L expression and apoptosis susceptibility at single-cell resolution using cytofluorimetry. Marked sequence-specific reduction of Bcl-X L was associated with sequence-specific enhancement of VP16-induced mitochondrial permeability transition, caspase-3 activation and loss of membrane asymmetry. A negative correlation between Bcl-X L expression and apoptosis susceptibility was observed, together with a positive correlation with respect to a reduced redox state. Bcl-X L downregulation reduces the threshold for VP16-induced apoptosis by potentiating mitochondrial dysfunction and its sequelae, and therefore presents a novel therapeutic strategy for reversing chemoresistance.

We conclude that in contrast to normal cell engraftment, engraftment of CML cells in NOD/SCID mice is characterized by a slow but progressive myeloid infiltration, which eventually consists almost entirely of mast cells.