DIFFERENTIAL-EFFECTS OF PHOSPHORYLATION OF RAT P53 ON TRANSACTIVATIONOF PROMOTERS DERIVED FROM DIFFERENT P53 RESPONSIVE GENES

The tumor suppressor protein p53 is phosphorylated at multiple sites i n the amino-terminal transactivation domain and at several sites in th e carboxy-terminal region. Phosphorylation appears to modulate its DNA binding activity. Here we demonstrate that phosphorylation of p53 als o modulates its transcriptional activity. Okadaic acid treatment of ce lls resulted in enhanced phosphorylation of p53 and concomitantly in e nhanced transactivation of an mdm2 promoter-linked luciferase reporter gene. This effect was cell type specific, however, since transactivat ion was enhanced in rat and mouse fibroblasts but reduced in the human Saos-2 cell line. Moreover, the effect was dependent on the promoter. In rat cells transcription from the mdm2, waf1 (cip1) and bar gene pr omoters, and the artificial PG13 promoter was enhanced by okadaic acid treatment whereas that from the cyclin G promoter was reduced. When v arious phosphorylation site mutants of p53 were tested for transactiva tion of these promoters, they behaved differently. Amino-terminal muta nts exhibited reduced transcriptional activities on mdm2, waf1 and cyc lin G promoters but enhanced activities with bar and PG13 promoters. O n the other hand, a mutant at the cdk phosphorylation site, A313, show ed reduced activity with mdm2 and waf1 promoters but enhanced activity with the cyclin G promoter, and finally, mutant A390 exhibited enhanc ed activity on waf1 and bar promoters, but reduced activity on the cyc lin G promoter. These results suggest that phosphorylation of p53 may have different effects on its transcriptional activity, depending on t he cellular environment and the particular response element. Moreover, both, amino- and carboxy-terminal phosphorylation sites seem to be in volved in modulating the DNA-binding and the transactivation activitie s.