UV-radiation induces dose-dependent regulation of p53 response and modulates p53-HDM2 interaction in human fibroblasts

We address here the effects of increasing fluencies of UV-radiation on stab ility, modifications, activity and HDM2-interactions of endogenous p53 tumo r suppressor and on cellular damage response of human diploid fibroblasts. Low amounts of UVB/C-radiation induced a transient cell cycle arrest of the cells which correlated with rapid but transient increase in p53 levels. In contrast, high UV-fluency caused cell apoptosis and a slower but sustained increase in p53. Regulation of p53 target genes was highly dependent on th e radiation dose used. Whereas low doses induced p21/Cip1/Waf1 and HDM2, hi gh doses induced only GADD45 and BAX increasing the BAX:BCL-2 ratio. The le vels of HDM2, a negative regulator of p53, increased only by the low dose o f UVC and p53-HDM2 association was promoted. In the absence of HDM2-inducti on after the high dose of UV-radiation p53-HDM2-interaction was promoted, b ut HDM2 failed to downregulate p53. p53 site-specific modifications (Ser15, Ser33, Ser37, Lys382) varied kinetically and were dependent on the fluency of the radiation used. Maximal phosphorylation of p53 on Ser15 and Ser33 c orrelated with increased levels of HDM2-free p53. The results suggest that regulation of p53 and HDM2 by UV-radiation is highly dose-dependent and con tributes to the outcome of the cellular response.