Potent antileukemic interactions between flavopiridol and TRAIL/Apo2L involve flavopiridol-mediated XIAP
R R Rosato1, Y Dai, J A Almenara
1Department of Medicine, Medical College of Virginia, Virginia Commonwealth University, Richmond, VA 23298, USA.
Abstract:
Interactions between the cyclin-dependent kinase inhibitor flavopiridol (FP) and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL/Apo2L), were examined in human leukemia cells (U937 and Jurkat). Coexposure of cells to marginally toxic concentrations of TRAIL and FP (24 h) synergistically increased mitochondrial injury (eg, cytochrome c, AIF, Smac/DIABLO release), cytoplasmic depletion of Bax, activation of Bid as well as caspase-8 and -3, PARP cleavage, and apoptosis. Coadministration of TRAIL markedly increased FP-induced apoptosis in leukemic cells ectopically expressing Bcl-2, Bcl-x(L), or a phosphorylation loop-deleted form of Bcl-2 (DeltaBcl-2), whereas lethality was substantially attenuated in cells ectopically expressing CrmA, dominant-negative-FADD, or dominant-negative-caspase-8. TRAIL/FP induced no discernible changes in FLIP, DR4, DR5, Mcl-1, or survivin expression, modest declines in levels of DcR2 and c-IAP, but resulted in the marked transcriptional downregulation of XIAP. Moreover, cells stably expressing an XIAP-antisense construct exhibited a pronounced increase in TRAIL sensitivity comparable to degrees of apoptosis achieved with TRAIL/FP. Conversely, enforced XIAP expression significantly attenuated caspase activation and TRAIL/FP lethality. Together, these findings suggest that simultaneous activation of the intrinsic and extrinsic apoptotic pathways by TRAIL and FP synergistically induces apoptosis in human leukemia cells through a mechanism that involves FP-mediated XIAP downregulation.
Insights
Flavopiridol (FP) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) synergistically induce apoptosis in leukemia cells. This occurs via combined intrinsic and extrinsic pathway activation, with FP downregulating XIAP expression.
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- Leukemia cells are often resistant to apoptosis.
- Targeting apoptosis pathways is a key strategy in cancer therapy.
- Flavopiridol (FP) and TRAIL are known apoptosis inducers.
Purpose of the Study:
- To investigate the synergistic interaction between FP and TRAIL in human leukemia cells.
- To elucidate the molecular mechanisms underlying this synergy.
Main Methods:
- Coexposure of U937 and Jurkat leukemia cells to FP and TRAIL.
- Analysis of mitochondrial injury markers (cytochrome c, AIF, Smac/DIABLO).
- Assessment of apoptosis-related protein expression and activity (Bax, Bid, caspases, PARP, Bcl-2 family, XIAP).
Main Results:
- Coexposure synergistically increased apoptosis, mitochondrial injury, and caspase activation.
- FP and TRAIL modulated expression of apoptosis regulators, notably downregulating XIAP.
- XIAP downregulation was critical for the synergistic lethality, as shown by antisense and overexpression studies.
Conclusions:
- Simultaneous activation of intrinsic and extrinsic apoptotic pathways by FP and TRAIL synergistically kills leukemia cells.
- FP-mediated transcriptional downregulation of XIAP is a key mechanism driving this synergistic apoptosis.
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