An oxidative stress-mediated death pathway in irradiated human leukemia cells mapped using multilaser flow cytometry.

OCI/AML-2 acute myeloid leukemia cells were found to undergo apoptosis after treatment with γ rays from a ¹³⁷ Cs source. Multilaser flow cytometry techniques using probes for live cell function were used to monitor the biochemical changes that occurred prior to the loss of surface membrane integrity. These showed increases in the generation of reactive oxygen species (ROS) and in the glutathione (GSH) content of irradiated cells. An additional population of cells that showed a further increase in ROS and depletion of GSH was seen in irradiated cells but not in controls. This population showed loss of mitochondrial membrane potential (Δ Ψ _m), indicative of the mitochondrial permeability transition, and exposure of phosphatidylserine on the cell surface. Increases in intracellular calcium were observed in a proportion of these $\text{low-}\Delta \Psi _{{\rm m}}/\text{high-}{\rm ROS}$ cells. Similar findings were seen using the antileukemia drug cytosine arabinoside (ara-C), although cell cycle analysis showed that the loss of Δ Ψ _m occurred mainly in G₁ phase with ara-C treatment, and mainly in G₂ phase with irradiation. Furthermore, the protective effect of overexpression of BCL2 was more pronounced after ara-C treatment than with radiation. Cells of the TP53 (formerly known as p53)-null human AML line OCI M2 showed growth arrest in G₂ phase after radiation treatment, with no loss of Δ Ψ _m or morphological changes indicative of apoptosis. The flavine-dependent oxidoreductase inhibitor diphenylene iodonium failed to inhibit generation of ROS in irradiated OCI/AML-2 cells, indicating that the mechanism is unlikely to involve the TP53-induced gene PIG3. These results show that oxidative stress can occur in irradiated human leukemia "blasts", and may play a direct role in radiation-induced apoptosis.