Double-Stranded-RNA-Activated Protein Kinase PKR Enhances Transcriptional Activation by Tumor Suppressor p53

Abstract

The tumor suppressor p53 plays a key role in inducing G₁ arrest and apoptosis following DNA damage. The double-stranded-RNA-activated protein PKR is a serine/threonine interferon (IFN)-inducible kinase which plays an important role in regulation of gene expression at both transcriptional and translational levels. Since a cross talk between IFN-inducible proteins and p53 had already been established, we investigated whether and how p53 function was modulated by PKR. We analyzed p53 function in several cell lines derived from PKR^+/+ and PKR^−/− mouse embryonic fibroblasts (MEFs) after transfection with the temperature-sensitive (ts) mutant of mouse p53 [p53(Val135)]. Here we report that transactivation of transcription by p53 and G₀/G₁ arrest were impaired in PKR^−/− cells upon conditions that ts p53 acquired a wild-type conformation. Phosphorylation of mouse p53 on Ser¹⁸ was defective in PKR^−/− cells, consistent with an impaired transcriptional induction of the p53-inducible genes encoding p21^WAF/Cip1 and Mdm2. In addition, Ser¹⁸ phosphorylation and transcriptional activation by mouse p53 were diminished in PKR^−/− cells after DNA damage induced by the anticancer drug adriamycin or γ radiation but not by UV radiation. Furthermore, the specific phosphatidylinositol-3 (PI-3) kinase inhibitor LY294002 inhibited the induction of phosphorylation of Ser¹⁸ of p53 by adriamycin to a higher degree in PKR^+/+ cells than in PKR^−/− cells. These novel findings suggest that PKR enhances p53 transcriptional function and implicate PKR in cell signaling elicited by a specific type of DNA damage that leads to p53 phosphorylation, possibly through a PI-3 kinase pathway.