p21 Cip1/WAF1 down-regulation is required for efficient PCNA ubiquitination alter UV irradiation

GOTTIFREDI, V., SORIA G, PRIVES C, PODHAJCER O

Orlando, Florida

Conferencia; AACR SPECIAL CONFERENCE in CANCER RESEARCH. Ubiquitin and Cancer: From Molecular Targets and Mechanisms to the Clinic; 2006

American Association for Cancer Research

p21Cip/WAF1 is a known inhibitor of the short-gap filling activity of PCNA during DNA repair. In agreement, p21 degradation after UV irradiation promotes PCNA-dependent repair. In those scenarios p21 interaction with PCNA inhibits the re-synthesis step of the repair process. In vivo however, while some groups have found an inhibitory role of the C terminus of p21 on unscheduled DNA synthesis others have reported a positive or null role of p21 in NER. While the biological significance of the PCNA/p21 interaction is not yet clear, a recent report shows that p21 down-regulation is required for efficient PCNA dependent-UDS after UV irradiation (1) implying that, in cultured cells p21 accumulation might be sufficient to inhibit PCNA dependent DNA repair. Importantly, recent reports have identified ubiquitination of PCNA as a relevant feature for PCNA-dependent DNA repair (3,4,5,6). Here we show that PCNA ubiquitination in human cells is notably augmented after UV irradiation and other genotoxic treatments such as HU, APH and MMS. Intriguingly, those DNA damaging agents also promoted down-regulation of p21. While ubiquitination of PCNA was not affected by deficient nucleotide excision repair (NER) and was observed in both proliferating and arrested cells, stabilization of p21 after UV irradiation by inhibition of its N terminal ubiquitination (2) caused a significant reduction in UV-induced ubiquitinated PCNA. Therefore, increased p21 turnover might represent a significant aspect of the cellular response to UV irradiation by promoting increased PCNA ubiquitination. Surprisingly, the negative regulation of PCNA ubiquitination by p21 does not depend on the direct interaction with PCNA but requires the CDK binding domain of p21 therefore suggesting that CDKs activity could participate in one or more aspects of DNA repair after UV irradiation. Taken together, our data suggest that p21 down-regulation plays a role in efficient PCNA ubiquitination after UV irradiation.