Previously we showed that the transcription of the p53 gene in breast cancer cells was down regulated by OM. To elucidate the molecular mechanisms underlying the OM effect on p53 transcription, in this report, we dissected the p53 promoter region and analyzed the p53 promoter activity in breast tumor cells. We showed that deletion of the 5'-flanking region of the p53 promoter from -426 to -97 did not affect the OM effect. However, further deletion to MO completely abolished the repressive effect of OM. The p53 promoter region -96 to 41 contains NF-kB and c-myc binding sites, and a newly identified UV-inducible element PE21. Mutations to disrupt NF-kB binding or c-myc binding to the p53 promoter decreased the basal promoter activity without affecting the OM-mediated suppression, whereas mutation at the PE21 motif totally abolished the OM effect. We further demonstrated that insertion of PE21 element upstream of the thymidine kinase minimal promoter generated an OM response analogous to that of the p53 promoter. Finally, we detected the specific binding of a nuclear protein with a molecular mass of 87 kDa to the PE21 motif. Taken together, we demonstrate that OM inhibits the transcription of the p53 gene through the PE21 element. Thus, the PE21 element is functionally involved in p53 transcription regulated by UV-induction and OM suppression.
