A comprehensive study of p53 transcriptional activity in thymus and spleen of γ irradiated mouse:: High sensitivity of genes involved in the two main apoptotic pathways

Purpose: gamma-irradiation leads to activation of p53 tumour suppressor gene and to p53-dependant stimulation of a large panel of cellular genes including proapoptotic genes involved in intrinsic and extrinsic pathways. Most in vivo published data referred to high ( lethal) irradiation doses. The present study was performed to analyse the p53-dependent response to more relevant low irradiation doses. Materials and methods: Mice were whole body exposed to irradiation doses decreasing from 5-0.05 Gy. Gene expression was estimated by real time reverse transcriptase polymerase chain reaction measurements on RNA extracted from thymus and spleen. Apoptosis was evaluated by the percentage of either annexin V positive or sub-G1 cells. Results: A 0.1 Gy irradiation dose already gives a significant stimulation of Puma ( p53 up-regulated modulator of apoptosis), and 0.2 Gy of Bax (Bcl-2-associated X protein) and Killer/DR5 (Death Receptor 5). The expression of genes involved in the two apoptotic pathways was induced as soon as 1 h post-irradiation and reached a maximum at 3 h, the induction level depending on both the gene and the organ. A significant increase in the number of apoptotic cells is already detectable at 0.5 Gy with a maximum of induction at 6 h. Conclusions: Our results reveal the high in vivo sensitivity of p53-dependent transcriptional activation of genes involved in the two main apoptotic pathways, their stimulation preceding the induction of apoptosis.