P53-mediatd Gene Regulation in Response to Low Dose Radiation

Activation of tumor suppressor protein p53 following DNA damages results in different cellular responses depending on the cell type. For example, hematopietic cells were observed to undergo p53-dependent apoptosis more easily than fibroblasts [1]. Fei et al. [2] indicated that the selective regulation of p53 target gene could be a determinant of whether cells undergo cell cycle arrest or apoptosis. We consider that a similar selective regulation of p53 target genes may play an important role also in cellular response to low dose radiation.It has been revealed that low dose radiation causes distinct biological responses, such as adaptive responses, bystander effects and induction of genomic instability, which could not be observed in experiments using lethal dose of radiation. There is evidence for a pivotal role played by p53 in these low dose radiation-distinct responses [3-5]. As p53 is activated as a transcription factor by a wide range of radiation doses, it is considered that the mechanism for low dose radiation-distinct biological responses involve, at least in part, molecular consequences of selective regulation of p53 target genes. The selective regulation of the p53 target genes should be attributed to their promoter structure. We consider that several ways of distinguishing the target gene promoter by p53 are possible. First, transcription factor(s), that can cooperate or modify the p53 function, may be activated by low dose radiation and recognize a specific set of p53 target gene promoters. This is an analogy to the observation that p63 and/or p73 is necessary for regulation of proapoptotic genes, but not cell cycle arrest-related genes, by p53 after DNA damage [6]. In addition, it should be noted that a lot of transcription factors have been demonstrated to associate with p53, modulating its function in regulation of the downstream genes. For examples, transcription factors Sp1 [7, 8], gut-enriched kruppel-like factor (GKLF) [9], Ets1 [10], and interferon regulatory factor-1 (IRF-1) [11] have been shown to be involved in the p53-dependent transcriptional activation after DNA damage or p53-overexpression. It is interesting to examine if some of transcription factors may be activated to facilitate the selective regulation by p53 after low dose radiation. Second, p53 may be post-translationally modified by low dose radiation, so that the binding affinity to a specific set of p53 target gene promoters may be selectively changed. This is thought to be a possible way of selection of the target gene promoter, because the p53 consensus sequence has a remarkable redundancy. There is some evidence that phosphorylation of p53 on serine 46 causes the selective regulation of the p53 target gene by directing p53 towards activation of proapoptotic genes including p53Aip1 after DNA damage [12]. Alternatively, post-translational modification of p53, such as phosphorylation, acetylation, sumolation, ubiquitination, may enhance interaction with nuclear factors that facilitate selective gene regulation. It was revealed that BRCA1 stabilizes p53, causing upregulation of cell cycle arrest genes and DNA repair genes, but not proapoptotic genes [13].We have developed sensitive methods for analyzing the promoter of the p53 target genes, i.e. the comprehensive EMSA [14] and the adenoassociated virus-based reporter vector. The comprehensive EMSA enables us to detect an alteration of DNA/protein interaction in the promoter after irradiation with low dose radiation. And the adenoassociated virus-based reporter vector responds to low dose radiation with a high sensitivity, and can be used to reveal the functional DNA elements by a site-directed mutagenesis analysis. In the Crossover Research Project, we will attempt to identify transcription factors involved in low dose radiation-response of various p53 target genes in various types of the cell by use of these methods. In addition, we will examine whether exchange of p53 recognition elements between separate p53 target genes would influence selective gene regulation. On the basis of the data obtained from these experiments, we aim at providing an important insight into the selective regulation of p53 target genes by low dose radiation.International Workshop on Biological Responses to Low Dose Radiatio