Inhibition of Ionizing Radiation Recovery Processes in Polyamine-depleted Chinese Hamster Cells

Polyamines are involved in many cellular processes, including DNA structure and function. Since DNA, or some DNA-containing structure, is known to be the target for cell killing induced by ionizing radiation and a number of chemotherapeutic agents, we investigated the effects of polyamine depletion on cytotoxic responses of Chinese hamster cells to X-irradiation. Colony forming ability after single, acute radiation exposures of cells growing under oxic conditions was minimally affected by endogenous putrescine and spermidine depletion, achieved after treatment with α-difluoromethyloraithine. Survival of cells rendered hypoxic and then irradiated was unaffected by a-difluoromethylornithine treatment. However, cellular recovery processes were nearly completely suppressed in polyamine-depleted cells, including sublethal damage recovery, as evidenced by split-dose irradiations in log phase cultures, and potentially lethal damage recovery, observed when growth-inhibited cultures were allowed time to repair radiation damage prior to being plated for colony formation. Both these recovery processes were restored by exogenous putrescine treatment. Reaccumulation of intracellular spermidine content closely correlated with restoration of potentially lethal damage recovery. Depletion of putrescine and spermidine pools had little effect on either single or double strand DNA break production or rejoining. These data demonstrate that both sublethal and potentially lethal damage recovery are polyamine-dependent processes in Chinese hamster cells, and imply that the mechanisms by which hamster cells recover from these types of radiation damage are unrelated to their ability to rejoin DNA strand breaks, at least during the first hour after irradiation. Finally, these results suggest that the depletion of tumor polyamine content may be an effective method of enhancing the sensitivity of human tumors to fractionated radiotherapy.