Modulation of Radiation-Induced Apoptosis and G₂/ M Block in Murine T-Lymphoma Cells

Radiation-induced apoptosis in lymphocyte-derived cell lines is characterized by endonucleolytic cleavage of cellular DNA within hours after radiation exposure. We have studied this phenomenon qualitatively (DNA gel electrophoresis) and quantitatively (diphenylamine reagent assay) in murine EL4 T-lymphoma cells exposed to¹³⁷ Cs γ irradiation. Fragmentation was discernible within 18-24 h after exposure. It increased with time and dose and reached a plateau after 8 Gy of γ radiation. We studied the effect of several pharmacological agents on the radiation-induced G₂/ M block and DNA fragmentation. The agents which reduced the radiation-induced <tex-math>${\rm G}_{2}/{\rm M}\text{-phase}$</tex-math> arrest (caffeine, theobromine, theophylline and 2-aminopurine) enhanced the degree of DNA fragmentation at 24 h. In contrast, the agents which sustained the radiation-induced <tex-math>${\rm G}_{2}/{\rm M}\text{-phase}$</tex-math> arrest (TPA, DBcAMP, IBMX and 3-aminobenzamide) inhibited the DNA fragmentation at 24 h. These studies on EL4 lymphoma cells are consistent with the hypothesis that cells with radiation-induced genetic damage are eliminated by apoptosis subsequent to a G₂/ M block. Furthermore, it may be possible to modulate the process of radiation-induced apoptosis in lymphoma cells with pharmacological agents that modify the radiation-induced G₂/ M block, and to use this effect in the treatment of patients with malignant disease.