Comprehensive search for X-ray-responsive elements and binding factors in the regulatory region of the GADD45a gene

The growth arrest and DNA damage-inducible protein 45alpha (GADD45a) gene is responsive to a variety of DNA-damaging agents. It is known that induction of the GADD45a gene is regulated in a p53-dependent manner after ionizing irradiation. Our previous study showed that X-ray irradiation increased the transcription rate of the GADD45a gene much earlier than the maximum accumulation of stabilized p53 protein in human myeloblastic leukemia ML-1 cells. We hypothesized that some transcription factor(s) may cooperate with p53 in regulating the GADD45a gene early after the irradiation of ML-1 cells. This idea is supported by recent studies showing that the p53-dependent activation of several genes in human and mouse cells requires some additional transcription factors, such as Sp1, GKLF, Ets1, and IRF-1. To examine the possible involvement of cooperating factors in transcriptional regulation of the GADD45a gene by ionizing radiation, we comprehensively searched for the X-ray-inducible binding locus of the nuclear factor throughout the upstream region (-2244 bp/+89 bp) and the third intron (+1389 bp/+2488 bp) of the GADD45a gene by EMSA using 136 probes. The X-ray-responsive binding of nuclear factors was detected at eight loci. Oct, NF-kappaB, HNF, NF-AT, and KLF family transcription factors were identified by a competition assay. It is possible that some of these factors cooperate with p53 to mediate transcriptional regulation of the GADD45a gene after ionizing irradiation.