Loss of the Bcl-2 phosphorylation loop domain is required to protect human myeloid leukemia cells from flavopiridol-mediated mitochondrial damage and apoptosis.
Responses to the CDK inhibitor flavopiridol (FP) have been examined in U937 leukemia cells ectopically expressing full-length Bcl-2 or an N-terminal phosphorylation loop-deleted protein (Bcl-2). A 3-fold increase in full-length Bcl-2 protein conferred substantial resistance to ara-C-associated lethality, but not to FP-mediated cytochrome c release, caspase-3 and-9 activation and apoptosis. In a second clonal line, a 6-fold increase in Bcl-2 expression delayed but did not ultimately prevent FP-associated apoptosis. In marked contrast, cells ectopically expressing the Bcl-2 loop-deleted protein (32-80) were highly resistant to FP-mediated cytochrome c release, caspase-3, -8, and -9 activation, Bid and PARP cleavage, and apoptosis despite relatively low levels of protein expression. The loop-deleted Bcl-2, but not full-length Bcl-2 protein also protected clonogenic cells from FP-mediated lethality. Finally, in Bcl-2-overexpressing cells, FP lethality was not attenuated by the caspase-8 inhibitor IETD-FMK, arguing against a role for the extrinsic, receptor-mediated pathway in circumventing Bcl-2-associated resistance. Collectively, these findings indicate that FP induces cytochrome c release in leukemic cells despite overexpression of Bcl-2, and suggest that this event may be modulated by negative regulatory factors residing within the N-terminal phosphorylation loop region.
Decker
Roy H
RH
Department of Medicine, Biochemistry, Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia 23298, USA.
Wang
Shujie
S
Dai
Yun
Y
Dent
Paul
P
Grant
Steven
S
D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D020169:Caspases / Q000502:physiology; D003561:Cytarabine / Q000494:pharmacology; D003574:Cytochrome c Group / Q000378:metabolism; D004249:DNA Damage; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005419:Flavonoids / Q000494:pharmacology*; D006801:Humans; D007951:Leukemia, Myeloid / Q000473:pathology*; D008928:Mitochondria / Q000187:drug effects*; D010766:Phosphorylation; D010880:Piperidines / Q000494:pharmacology*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000737:chemistry / Q000378:metabolism*; D020298:U937 Cells
D000970:Antineoplastic Agents; D003574:Cytochrome c Group; D004791:Enzyme Inhibitors; D005419:Flavonoids; D010880:Piperidines; D019253:Proto-Oncogene Proteins c-bcl-2; D003561:Cytarabine; C077990:alvocidib; D020169:Caspases