p53-Dependent Elevation of p21^Waf1 Expression by UV Light Is Mediated through mRNA Stabilization and Involves a Vanadate-Sensitive Regulatory System

ABSTRACT

Exposure of mammalian cells to adverse stimuli triggers the expression of numerous stress response genes, many of which are presumed to enhance cell survival. In this study, we examined the mechanisms contributing to the induction of p21^Waf1 by stress and its influence on the survival of cells subjected to short-wavelength UVC irradiation. UVC was found to elevate p21^Waf1 mRNA expression in mouse embryonal fibroblasts (MEFs) and human colorectal carcinoma (RKO) cells in a p53-dependent manner. The lack of p21^Waf1 induction in p53-deficient MEFs and RKO cells correlated with diminished cell survival following UVC irradiation. Unexpectedly, UVC treatment was also found to block the induction of p21^Waf1 by various stress-inducing agents such as mimosine in the p53-deficient cells. Additional studies indicated that induction of p21^Waf1 by UVC occurs primarily through enhanced mRNA stability rather than increased transcription; in p53^−/− MEFs, failure to elevate p21^Waf1 after treatment with UVC appears to be due to their inability to stabilize the p21^Waf1 transcripts. Treatment of the p53^−/− MEFs with the protein tyrosine phosphatase inhibitor vanadate reversed the UVC-induced block on p21^Waf1 induction and resulted in their enhanced survival following irradiation. Thus, in cells bearing normal p53, UVC augments p21^Waf1 expression by increasing the half-life of p21^Waf1 mRNA; without p53, p21^Waf1 mRNA remains unstable after UVC, apparently due to a pathway involving tyrosine phosphatase activity.

ACKNOWLEDGMENTS

We thank M. B. Kastan for the RKO neo and RKO E6 cells, T. Jacks for the wt and p53^−/− MEFs, P. Leder for the p21^−/− MEFs, and B Vogelstein for plasmids pCEP4Waf1 and WWT-Luc. We are also grateful to S. Shack, A. Passaniti, K. Z. Guyton, and Y. Liu for helpful discussions and D. L. Longo for critical reading of the manuscript.