Ablation of p21waf1cip1 expression enhances the capacity of p53-deficient human tumor cells to repair UVB-induced DNA damage.

During periods of genotoxic stress, the cyclin-dependent kinase inhibitor p21waf1cip1 (hereafter referred to as p21) is transcriptionally up-regulated by the p53 tumor suppressor and subsequently plays a key role in cellular growth arrest. Investigations have also indicated that p21 may regulate nucleotide excision repair, a critical pathway that removes carcinogenic DNA damage induced by UV light and other mutagens. In this study, we examined whether low levels of endogenous p21 expression can modulate nucleotide excision repair in p53-deficient human tumor cells after UVB exposure. For this purpose, we used the well-characterized p53-/-p21+/+ adenocarcinoma cell strain DLD1 and its isogenic counterpart carrying a homozygous knockout for p21 (p53-/-p21-/- DLD1). Because p53-/-p21+/+ DLD1 expresses very low levels of endogenous p21 protein that are not up-regulated after mutagen exposure, this strain has been considered functionally p21-deficient in the cellular response to DNA damage. Nonetheless, the ligation-mediated PCR technology was used here to demonstrate, at nucleotide resolution, that p53-/-p21+/+ DLD1 excises UVB-induced cyclobutane pyrimidine dimers from the c-jun proto-oncogene at a significantly lower rate than the isogenic p53-/-p21-/- derivative. The higher efficiency of DNA repair in UVB-exposed p53-/-p21-/- DLD1 cells is accompanied by increased clonogenic survival and reduced levels of apoptosis, relative to the p53-/-p21+/+ counterpart. Our results show that ablation of p21 expression can significantly enhance the capacity of p53-deficient human tumor cells to repair UVB-induced DNA damage.