title: Protein kinase C-dependent activation of the tumor necrosis factor receptor-mediated extrinsic cell death pathway underlies enhanced apoptosis in human myeloid leukemia cells exposed to bryostatin 1 and flavopiridol.
abstract: Interactions between the protein kinase C activator bryostatin 1 and the cyclin-dependent kinase (CDK) inhibitor flavopiridol (FP) have been examined in human myeloid leukemia cells (U937 and HL-60). Previous studies have demonstrated synergistic induction of apoptosis in leukemic cells exposed to the potent differentiation-inducer phorbol 12-myristate 13-acetate (PMA) in conjunction with FP [L. Cartee et al., Cancer Res., 61: 2583-2591, 2001]. Although bryostatin 1 (10 nM) is a very weak inducer of differentiation compared with PMA in these cells, coadministration of a minimally toxic concentration of FP (100 nM) did not promote bryostatin 1-related maturation but instead caused a marked increase in mitochondrial damage (e.g., cytochrome c release; loss of Deltapsi(m)), caspase activation, poly(ADP-ribose) polymerase cleavage, and apoptosis. Bryostatin 1/FP-induced apoptosis was significantly diminished in cells ectopically expressing dominant-negative Fas-associated death domain or by coadministration of tumor necrosis factor (TNF)-alpha soluble receptors, implicating the extrinsic pathway in bryostatin 1/FP actions. Enhanced apoptosis in bryostatin 1/FP-treated cells was accompanied by down-regulation of Mcl-1 and a sustained increase in TNF-alpha release. The selective protein kinase C inhibitor GFX blocked TNF-alpha and cytochrome c release in bryostatin 1/FP-treated cells and attenuated apoptosis. Finally, coadministration of bryostatin 1 (or PMA) with FP induced a marked increase in apoptosis in U937 cells ectopically expressing an NH(2)-terminal phosphorylation loop-deleted Bcl-2 protein, which are otherwise highly resistant to FP-mediated lethality. Taken together, these findings suggest that synergistic induction of apoptosis by bryostatin 1 and FP does not stem from disruption of the leukemic cell maturation process but instead results from enhanced release of TNF-alpha and activation of the extrinsic apoptotic cascade, culminating in cell death.
authors.affiliation: Department of Medicine, Virginia Commonwealth University, Medical College of Virginia, Richmond, Virginia 23298, USA.
authors.forename: Leanne
authors.initials: L
authors.lastname: Cartee
authors.forename: Sonia C
authors.initials: SC
authors.lastname: Maggio
authors.forename: Rebecca
authors.initials: R
authors.lastname: Smith
authors.forename: Heidi M
authors.initials: HM
authors.lastname: Sankala
authors.forename: Paul
authors.initials: P
authors.lastname: Dent
authors.forename: Steven
authors.initials: S
authors.lastname: Grant
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D054713:Bryostatins; D002448:Cell Adhesion / Q000187:drug effects; D016923:Cell Death / Q000187:drug effects / Q000502:physiology*; D004357:Drug Synergism; D005419:Flavonoids / Q000494:pharmacology*; D018922:HL-60 Cells; D006801:Humans; D007425:Intracellular Membranes / Q000187:drug effects / Q000502:physiology; D007700:Kinetics; D007783:Lactones / Q000494:pharmacology*; D018942:Macrolides; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008928:Mitochondria / Q000187:drug effects / Q000502:physiology; D010880:Piperidines / Q000494:pharmacology*; D011493:Protein Kinase C / Q000378:metabolism*; D018124:Receptors, Tumor Necrosis Factor / Q000502:physiology*; D020298:U937 Cells
chemical_list: D000970:Antineoplastic Agents; D054713:Bryostatins; D005419:Flavonoids; D007783:Lactones; D018942:Macrolides; D010880:Piperidines; D018124:Receptors, Tumor Necrosis Factor; C046785:bryostatin 1; C077990:alvocidib; D011493:Protein Kinase C