G2/M Arrest Sensitizes Chronic Myelogenous Leukemia Cells to TRAIL-Induced Apoptosis

Abstract 4465

The death ligand, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receives great interest as it targets and kills cancerous cells, but not non-transformed cells. While it is in phase I/II clinical trials for a range of solid tumours, the generally low sensitivity of leukemia cells to TRAIL makes it a less attractive therapeutic for these cancers.

We found that doxorubicin and cytarabine, agents that induce DNA damage and impair cell cycle progression, can sensitize CML cells to TRAIL with CI<1 at F_a of ED₂₅ and ED₅₀ (based on median-effect method using the isobologram equation). Inhibition of the cell cycle checkpoint kinases Chk1/2 with UCN-01 did not influence TRAIL-induced apoptosis nor could it abolish the sensitizing effect of doxorubicin. Interestingly, inhibition of Ataxia Telangiectasia Mutated (ATM), a key DNA damage response kinase, with KU-55933 induced a G2/M arrest and enhanced TRAIL-induced apoptosis. Inhibition of ATM alone induced 22±3.1% apoptosis and increased TRAIL-induced apoptosis from 27.2±4.7% to 68±7.2%. Cell cycle analysis revealed that while the proportion of cells in the G0/G1 and S phases slightly increased, the proportion of the cells in the G2/M phase dropped by 31.6±3.2% (p<0.05) indicating that G2/M arrested cells were more sensitive to TRAIL than cells in G0/G1 and S phases. TRAIL-induced CML cell death was also synergistically enhanced by arresting the cells in G2/M using the microtubule disrupting drugs, nocodazole or colcemide. Cells were treated with a concentration of nocodazole or colcemide that induced above 90% G2/M arrest for 16 h (0.3 mM and 0.1 mg/ml, respectively) followed by treatment with 250 ng/ml of TRAIL for 24 h. Nocodazole, colcemide and TRAIL individually induced 19±3.7% 26.3±4.4% and 27.2±4.7% cell death, while combination of nocodazole or colcemide with TRAIL resulted 89±6.8% and 93±5.9% cell death, respectively. In summary, we found that induction of DNA damage sensitizes CML cells to TRAIL and that TRAIL-sensitivity of CML cells is cell cycle-dependent.