Flavopiridol and histone deacetylase inhibitors promote mitochondrial injury and cell death in human leukemia cells that overexpress Bcl-2.
Interactions between the cyclin-dependent kinase (CDK) inhibitor flavopiridol and histone deacetylase (HDAC) inhibitors (suberoylanilide hydroxamide and sodium butyrate) were examined in human leukemia cells (U937 and HL-60) ectopically expressing Bcl-2/Bcl-x(L) and in primary AML cells. Coadministration of flavopiridol with HDAC inhibitors synergistically potentiated mitochondrial damage (cytochrome c, second mitochondria-derived activator of caspases/direct IAP binding protein with low pI, and apoptosis-inducing factor release), caspase activation, poly(ADP-ribose) polymerase degradation, and cell death in both wild type and Bcl-2- or Bcl-x(L)-overexpressing cells and induced a pronounced loss of clonogenicity. In contrast, Bcl-2 and Bcl-x(L) largely blocked these events in cells exposed to the cytotoxic agent 1-beta-d-arabinofuranosylcytosine (ara-C). Enforced expression of dominant-negative Fas-associated death domain failed to protect cells from the flavopiridol/histone deacetylase inhibitor (HDACI) regimen, arguing against the involvement of the receptor pathway in lethality. Ectopic expression of a phosphorylation loop-deleted Bcl-2 or Bcl-2 lacking the serine(70) phosphorylation site, which dramatically protected cells from ara-C lethality, delayed but did not prevent flavopiridol/HDAC inhibitor-induced mitochondrial injury, cell death, or loss of clonogenicity. Ectopic expression of Bcl-2 or Bcl-x(L) was also unable to prevent the flavopiridol/HDACI regimen from inducing a conformational change in and mitochondrial translocation of Bax, and it did not attenuate Bax dimerization. As a whole, these findings indicate that in contrast to certain conventional cytotoxic agents such as ara-C, overexpression of Bcl-2 or Bcl-x(L) are largely ineffective in preventing perturbations in Bax, mitochondrial injury, and cell death in human leukemia cells subjected to simultaneous CDK and HDAC inhibition. They also raise the possibility that a strategy combining CDK and HDAC inhibitors may be effective against drug-resistant leukemia cells overexpressing Bcl-2 or Bcl-x(L).
Dasmahapatra
Girija
G
Department of Medicine, Medical College of Virginia/Virginia Commonwealth University, Richmond, Virginia, USA.
Almenara
Jorge A
JA
Grant
Steven
S
D015153:Blotting, Western; D016923:Cell Death / Q000187:drug effects*; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005419:Flavonoids / Q000494:pharmacology*; D056572:Histone Deacetylase Inhibitors; D006801:Humans; D007938:Leukemia / Q000378:metabolism* / Q000473:pathology; D008928:Mitochondria / Q000187:drug effects*; D010880:Piperidines / Q000494:pharmacology*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D020298:U937 Cells
D004791:Enzyme Inhibitors; D005419:Flavonoids; D056572:Histone Deacetylase Inhibitors; D010880:Piperidines; D019253:Proto-Oncogene Proteins c-bcl-2; C077990:alvocidib