UV radiation is a transcriptional inducer of p21(Cip1/Waf1) cyclin-kinase inhibitor in a p53-independent manner

p53 target genes p21(Cip1/Waf1) cyclin-kinase inhibitor (p21 CKI), GADD45, bax, and cyclin G and genes affecting the redox state of the cells are impl icated in p53 damage control responses. In order to attribute their functio ns and dependency of p53 in UV-damaged cells we undertook an analysis of UV C responses of fibroblasts derived from p53 knock-out mice. UVC radiation e fficiently and rapidly inhibited DNA replication in both p53 -/- and +/+ ce lls. The arrest was persistent in p53 -/- fibroblasts and cells underwent a poptosis, whereas p53 +/+ cells recovered and reentered the cycle. Protein and mRNA analyses of p21 expression showed that it was induced up to sixfol d with similar kinetics both in the presence and in the absence of p53. How ever, high doses of UV abrogated the p21 response in p53 -/- cells, whereas it was maintained in cells with normal p53. UVC radiation transcriptionall y activated p21 expression as demonstrated by luciferase reporter assays us ing deletion constructs of the p21 promoter. The promoter assays further co nfirmed the independency of p53-binding sites in the activation and linked UV-responsive transcriptional regulation of pal to two Spl consensus bindin g sites within -61 bp of the transcription initiation site. A weaker regula tion was mediated by elements between -1300 to -500 bp relative to the tran scription initiation site. The results suggest that in fibroblasts UVC radi ation is a rapid and efficient inducer of p21 expression also in a p53-inde pendent manner. (C) 1999 Academic Press.