We have addressed the correlation between sequence-specific DNA binding by the tumor suppressor p53 and transactivation of various target genes, in the context of UV irradiation responses. In A549 cells (p53WT), p53 occupancy at the p21, mdm2, and puma promoters increased significantly after UV irradiation. In contrast, p21 mRNA levels did not change, mdm2 mRNA decreased and both p21 and mdm2 proteins were downregulated shortly after UV. At later times, higher p53 occupancy correlated with enhanced expression of these two genes both at mRNA and protein levels. In the p53 mutant cell lines LX1 (R273H) and SKMes1 (R280K), no significant p53-binding was detected at the gene targets analyzed. Accordingly, p21 and mdm2 proteins were not upregulated after UV irradiation. The kinetics of histone acetylation did not strictly correlate with gene expression. In fact, high levels of acetylated H3 (AcH3) and, particularly, acetylated H4 (AcH4) histones were found shortly after UV irradiation on p21 and mdm2 promoters. At the later time point, when transactivation was detected, acetylation levels decreased significantly although remaining higher than basal levels. Our results indicate that p53 transcription-dependent and -independent responses are activated with different kinetics after UV, possibly relating to the repair of UV-induced DNA damage. Based on the histone acetylation pattern we hypothesize that the DNA repair function of p53, associated to global genome repair and foci of DNA damage, may be relevant for all p53-binding sites, including those where occupancy by p53 is also associated to transcriptional modulation.
c 2006 Wiley-Liss, Inc.