Without MnTE-2-PyP, the p300 binding complex can directly bind to the promoter region of human PAI-1 gene, acetylate the chromatin and promote PAI-1 gene transcription. In the presence of MnTE-2-PyP, with or without radiation, the p300 transcriptional complex no longer binds to the promoter region of human PAI-1 gene, histone acetylation is reduced and, thus, PAI-1 gene expression is down-regulated.