Sustained activation of p53 in confluent nucleotide excision repair-deficient cells resistant to ultraviolet-induced apoptosis.

p53 activation is one of the main signals after DNA damage, controlling cell cycle arrest, DNA repair and apoptosis. We have previously shown that confluent nucleotide excision repair (NER)-deficient cells are more resistant to apoptosis induced by ultraviolet irradiation (UV). Here, we further investigated the effect of cell confluence on UV-induced apoptosis in normal and NER-deficient (XP-A and XP-C) cells, as well as the effects of treatments with the ATM/ATR inhibitor caffeine, and the patterns of p53 activation. Strong p53 activation was observed in either proliferating or confluent cells. Caffeine increased apoptosis levels and inhibited p53 activation in proliferating cells, suggesting a protective role for p53. However, in confluent NER-deficient cells no effect of caffeine was observed. Transcription recov- ery measurements showed decreased recovery in proliferating XPA-deficient cells, but no recovery was observed in confluent cells. The levels of the cyclin/Cdk inhibitor, p21 Waf1/Cip1 ,