Degradation of p21(CDKN1A) after DNA damage is independent of type of lesion, and is not required for DNA repair.

The inhibitor of cyclin-dependent kinases p21(CDKN1A) plays a fundamental role in several pathways involved in the DNA damage response, like checkpoint-mediated cell cycle arrest, transcription, apoptosis, and DNA repair. Although p21 protein level is regulated by proteasomal degradation, the relationship of this process with DNA repair pathways is not yet clear. In addition, the role of protein/protein interaction in regulating turnover of p21 protein, is controversial. Here, we show that in human fibroblasts treated with agents inducing lesions repaired through nucleotide excision repair (NER), or base excision repair (BER), p21 degradation was triggered more by the extent, than by the type of DNA damage, or consequent DNA repair pathway. In fact, lowering the amount of DNA damage resulted in an increased stability of p21 protein. Overexpression of p21 was found to obscure degradation, both for p21(wt) and a p21 mutant unable to bind PCNA (p21(PCNA-)). However, when expressed to lower levels, turnover of p21 protein after DNA damage was greatly influenced by interaction with PCNA, since p21(PCNA-) was more efficiently degraded than wild-type protein. Interestingly, a p21 mutant protein unable to localize in the nucleus because of mutations in the NLS region, was not degraded after DNA damage, thus indicating that nuclear localization is necessary for p21 turnover. Removal of p21 was not required for NER activity, since inhibition of p21 degradation by caffeine did not affect the UV-induced recruitment of repair proteins, such as PCNA and DNA polymerase delta, nor significantly influence DNA repair synthesis, as determined by autoradiography. These results indicate that degradation of p21 is not dependent on a particular DNA repair pathway, and is not required for efficient DNA repair.