FREE-RADICALS GENERATED BY IONIZING-RADIATION SIGNAL NUCLEAR TRANSLOCATION OF P53

The p53 tumor suppressor is a transcription factor that regulates seve ral pathways, which function collectively to maintain the integrity of the genome. Nuclear localization is critical for wild-type function. However, the signals that regulate subcellular localization of p53 hav e not been identified. Here, we examine the effect of ionizing radiati on on the subcellular localization of p53 in two cell lines in which p 53 is normally sequestered in the cytoplasm and found that ionizing ra diation caused a biphasic translocation response. p53 entered the nucl eus 1-2 h postirradiation (early response), subsequently emerged from the nucleus, and then again entered the nucleus 12-24 h after the cell s had been irradiated (delayed response). These changes in subcellular localization could be completely blocked by the free radical scavenge r, WR1065. By comparison, two DNA-damaging agents that do not generate free radicals, mitomycin C and doxorubicin, caused translocation only after 12-24 h of exposure to the drugs, and this effect could not be inhibited by WR1065. Hence, although all three DNA-damaging agents ind uced relocalization of p53 to the nucleus, only the translocation caus ed by radiation was sensitive to free radical scavenging. We suggest t hat the free radicals generated by ionizing radiation can signal p53 t ranslocation to the nucleus.