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 response gene. The present studies demonstrate that this induction of c-jun expression is temporally related to the appearance of internucleosomal DNA fragmentation. These events were maximal at 6 h and transient after exposure to lethal doses (20 Gy) of ionizing radiation. We also demonstrate that N-acetyl-L-cysteine (NAC), an antioxidant, inhibits X-ray-induced c-jun expression and endonucleolytic DNA cleavage. These findings suggested that both events are mediated at least in part through the formation of reactive oxygen intermediates (ROIs). Since ROIs damage DNA and X-ray-induced DNA damage is associated with activation of poly(ADP-ribose) polymerase (ADPRP), we studied the effects of the ADPRP inhibitors 3-aminobenzamide (3-AB), nicotinamide, and theophylline. 3-AB blocked both X-ray-induced c-jun expression and internucleosomal DNA fragmentation. Similar findings were obtained with nicotinamide and theophylline. In contrast, 3-AB had little if any effect on induction of c-jun transcripts or DNA fragmentation induced by the alkylating 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 throughout the cell cycle. Taken together, these results support the coinduction of c-jun transcription and internucleosomal DNA fragmentation by ionizing radiation. Similar studies were performed with H2O2 since this agent also results in the production of ROIs. While H2O2 induced c-jun expression by an NAC-sensitive mechanism, this event was not affected by 3-AB and was not associated with internucleosomal DNA fragmentation. These findings suggest that while activation of the c-jun gene and endonucleolytic DNA cleavage are coinduced by ionizing radiation, these events are differentially regulated by other ROI-mediated mechanisms.