COINDUCTION OF C-JUN GENE-EXPRESSION AND INTERNUCLEOSOMAL DNA FRAGMENTATION BY IONIZING-RADIATION

Previous work has demonstrated that the cellular response to ionizing radiation includes transcriptional activation of the c-jun early respo nse gene. The present studies demonstrate that this induction of c-jun expression is temporally related to the appearance of internucleosoma l DNA fragmentation. These events were maximal at 6 h and transient af ter exposure to lethal doses (20 Gy) of ionizing radiation. We also de monstrate that N-acetyl-L-cysteine (NAC), an antioxidant, inhibits X-r ay-induced c-jun expression and endonucleolytic DNA cleavage. These fi ndings suggested that both events are mediated at least in part throug h the formation of reactive oxygen intermediates (ROIs). Since ROIs da mage DNA and X-ray-induced DNA damage is associated with activation of poly(ADP-ribose) polymerase (ADPRP), we studied the effects of the AD PRP inhibitors 3-aminobenzamide (3-AB), nicotinamide, and theophylline . 3-AB blocked both X-ray-induced c-jun expression and internucleosoma l DNA fragmentation. Similar findings were obtained with nicotinamide and theophylline. In contrast, 3-AB had little if any effect on induct ion of c-jun transcripts or DNA fragmentation induced by the alkylatin g agent mitomycin C. While c-jun expression is restricted to cells in G1 and G1/S phases, we have found that X-ray-induced c-jun transcripts are detectable throughout all phases of the cell cycle. The induction of internucleosomal DNA fragmentation by X-rays was also detectable t hroughout the cell cycle. Taken together, these results support the co induction of c-jun transcription and internucleosomal DNA fragmentatio n by ionizing radiation. Similar studies were performed with H2O2 sinc e this agent also results in the production of ROIs. While H2O2 induce d c-jun expression by an NAC-sensitive mechanism, this event was not a ffected by 3-AB and was not associated with internucleosomal DNA fragm entation. These findings suggest that while activation of the c-jun ge ne and endonucleolytic DNA cleavage are coinduced by ionizing radiatio n, these events are differentially regulated by other ROI-mediated mec hanisms.