Dose-dependent survival of K562 cells subjected to irradiation, time course of endogenous enzyme activity and protective effect of applied superoxide dismutase.

PURPOSE Irradiation-generated reactive species are proven to affect the cell survival and antioxidant enzyme levels. Radioresistance is a phenomenon which includes many cell mechanisms and signaling pathways. Superoxide dismutase (SOD) acts in and outside of cells after irradiation. The aim of this study was to determine LD50 (lethal dose for 50% of K562 cells), to monitor the effect of a chosen dose and exogenously applied superoxide dismutase (ExSOD) on the cell number and the activity of SOD, glutathione peroxidase (GSH-Px) and catalase (CAT). METHODS The survival of irradiated (20-32.5 Gy) K562 cells was determined using the trypan-blue exclusion. Besides irradiated and non-irradiated cells (controls), another two groups of cells were treated with SOD (10-6 M) which then served as SOD-treated controls or were irradiated (30 Gy) one hour later. The number of cells and the activity of SOD, GSH-Px and CAT (using kinetic methods) were monitored after 1, 24, 48, 72 and 96 hrs in unirradiated, irradiated, SOD-treated and SOD-treated/irradiated experimental groups. RESULTS K562 cells showed dose-dependent survival in the chosen range of doses. A dose of 30 Gy induced 50% cell mortality and increased the activity of all three investigated enzymes after 24 hrs. Pretreatment with SOD preserved the survival of irradiated cells and increased SOD, GSH-Px and CAT activity. ExSOD induced an increase of the activity of all examined enzymes. CONCLUSION A balanced enhancement in endogenous antioxidative activity may be the cause of the increased radioresistance of K562 SOD-pretreated cells.