RELATIONSHIP BETWEEN DNA DAMAGE, DNA-REPAIR, METABOLIC STATE AND CELLLETHALITY

Induction of unrepairable DNA damage, accumulation of misrepaired DNA damage, and generation of imbalances in competing biochemical and/or m etabolic processes have been proposed to explain the relationship betw een radiation-induced DNA damage and cell lethality. Theoretically, th e temperature dependence of the critical DNA repair process(es) should be 1) either independent of or identical to the temperature dependenc e of cell killing if the first two hypotheses are correct, and 2) diff erent if the third hypothesis is correct. To test this, exponentially growing rat 9L brain tumor cells were left at 37-degrees-C or equilibr ated for 3-14 h at 20-degrees-C before irradiation. Cells were irradia ted and allowed to repair at either 20-degrees-C or 37-degrees-C. Alte rnatively, the cells were irradiated at one of these temperatures and immediately shifted to the other temperature for repair. DNA damage wa s assessed by the alkaline elution technique; cell kill was assessed b y a clonogenic assay. 9L cells maintained at 20-degrees-C or 37-degree s-C sustained the same amount of DNA damage as measured by alkaline el ution. DNA repair instantaneously assumed the rate characteristic of t he postirradiation temperature. For 9L cells equilibrated, irradiated, and repaired at 20-degrees-C, the half-time of the fast phase of the DNA repair decreased by a factor of almost-equal-to 2 and the half-tim e of the slow phase decreased by a factor of almost-equal-to 5 over th at measured in cells incubated, irradiated and repaired at 37-degrees- C. Although the rate of DNA repair decreased substantially at 20-degre es-C, the survival of 9L cells that were equilibrated and irradiated a t 20-degrees-C was greater (p < 10(-4)) than those incubated and irrad iated at 37-degrees-C, when assayed by an immediate plating protocol. In addition, the survival of 9L cells equilibrated and irradiated at 2 0-degrees-C and then shifted to 37-degrees-C immediately after irradia tion was greater (p < 10(-2)) than that obtained with any other delaye d plating protocol. Thus, the temperature dependence of the DNA repair processes measured by alkaline elution was different from the tempera ture dependence of cell killing measured either by an immediate or del ayed plating protocol. These data support the hypothesis that many irr adiated 9L tumor cells die because of imbalances in sets of competing biochemical and/or metabolic processes.