Apoptosis and cell cycle progression in an acidic environment after irradiation

Apoptosis and cell cycle progression in HL60 cells irradiated in an acidic environment were investigated. Apoptosis was determined by TUNEL staining, PARP cleavage, DNA fragmentation, and flow cytometry. The majority of the a poptosis that occurred in HL60 cells after 4 Gy irradiation took place afte r G(2)/M-phase arrest. When irradiated with 12 Gy, a fraction of the cells underwent apoptosis in G(1) and S phases while the rest of the cells underw ent apoptosis in G(2)/M phase. The apoptosis caused by 4 and 12 Gy irradiat ion was transiently suppressed in medium at pH 7.1 or lower. An acidic envi ronment was found to perturb progression of irradiated cells through the ce ll cycle, including progression through G(2)/M phase. Thus it was concluded that the suppression of apoptosis in the cells after 4-12 Gy irradiation i n acidic medium was due at feast in part to a delay in cell cycle progressi on, particularly the prolongation of G(2)/M-phase arrest. Irradiation with 20 Gy indiscriminately caused apoptosis in all cell cycle phases, i.e. G(1) , S and G(2)/M phases, rapidly in neutral pH medium and relatively slowly i n acidic pH medium. The delay in apoptosis in acidic medium after 20 Gy irr adiation appeared to result from mechanisms other than prolonged G(2)/M-pha se arrest. (C) 2000 by Radiation Research Society.