Persistent DNA damage induced by ultraviolet light inhibits p21waf1 and bax expression: implications for DNA repair, UV sensitivity and the induction of apoptosis.

Ultraviolet light (UV) induced DNA lesions efficiently block transcript elongation and induce the p53 response. Although p53 contributes to transcriptional activation of the p21waf1 and bax genes, accumulation of these proteins requires that these genes are free of UV induced pyrimidine dimers. We assessed the level of expression of p53 and the p53 regulated p21waf1 and bax gene products in normal diploid fibroblasts (NDF) and several nucleotide excision repair deficient fibroblasts following UV-irradiation. At low UV fluences, increased expression of p53, p21waf1 and bax was only observed in fibroblasts deficient in transcription coupled repair (TCR). Whereas p53 protein levels increased in all cell types at high UV fluences, p21waf1 levels initially decreased and then recovered in a manner dependent on TCR. At later times, expression of p21waf1 and bax was only elevated in TCR-proficient cells. The lack of TCR strongly correlated with an enhanced induction of apoptosis. Furthermore, we assessed the effect of modulation of the p53/p21waf1/pRb pathway on clonogenic survival following UV irradiation. Expression of E2F-1, E2F-4, and the large tumour antigens of SV40 and Polyomavirus conferred UV sensitivity to NDF whereas p21waf1 protected cells against UV treatment. We propose that the fluence dependent attenuation of protective functions of p53 by blockage of transcription favours apoptosis following UV exposure.