Involvement of the p53/p21 complex in p53-dependent gene expression

The p53 tumor suppressor regulates cell functions either by acting as a transcription factor or by interacting with other proteins. Previously, we reported that the non-transcriptional actions of p53 can be facilitated by the binding of p53 to p21. Herein, we investigated whether p53/p21 interaction influences the transcriptional activity of p53. We observed that the expression of the p53 promoter-based reporter gene is dependent on p21 levels. Moreover, using a p21 variant that is unable to bind p53, we showed that p53 promoter activity requires p53/p21 interaction. To investigate the possible role of p21 in regulating the expression of endogenous p53 targets, we analyzed mRNA levels of Puma, Mdm2, and Gadd45a in untreated control and γ-ray-irradiated cells. We observed that while Puma expression is dependent on p53 regardless of γ-irradiation, p53 mediates the expression of Mdm2 and Gadd45a only in irradiated cells. Notably, p53/p21 interaction is required only for the p53-dependent expression of the tested genes and not Mdm2 and Gadd45a in non-irradiated cells. Moreover, chromatin immunoprecipitation assay revealed that p21 is required for the binding of p53 to the promoters of Puma, Mdm2, and Gadd45a. Collectively, our data support the view that the p53/p21 complex is involved in regulating p53-dependent gene expression. These findings provide a new foundation for understanding the transcriptional action of p53.