p21 Cip1/WAF1 down-regulation is required for efficient PCNA ubiquitination and foci formation after UV irradiation

SORIA G, PODHAJCER O, PRIVES C AND GOTTIFREDI V

Columbia University, New York.USA

Workshop; 13th Annual p53 Workshop.; 2006

Columbia University

P21 Cip1/WAF1 DOWN-REGULATION IS REQUIRED FOR EFFICIENT PCNA UBIQUITINATION AND FOCI FORMATION AFTER UV IRRADIATION Gastón Soria 1, Osvaldo Podhajcer 1, Carol Prives 2 and Vanesa Gottifredi 1,* 1 Fundación Instituto Leloir. CONICET. Buenos Aires. 1405. Argentina 2 Dept of Biological Sciences, Columbia University, New York, N.Y. 10027 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.  Recent reports have identified ubiquitination of PCNA as a relevant feature for PCNA-dependent DNA repair. 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, stable p21 expression caused a significant reduction in UV-induced ubiquitinated PCNA. Furthermore, while stable p21 has no effect on the accumulation of damaged DNA, non-degradable p21 interferes with the aggregation of both PCNA and the permissive polymerase pol h into nuclear foci structures after UV irradiation thus suggesting that p21 might not only have a negative impact on NER but also on translesion DNA synthesis (TLS). Taken together, our data suggest that p21 down-regulation is a key event that promotes PCNA participation in DNA synthesis events linked to DNA repair.