PERSISTENT DNA-DAMAGE INHIBITS S-PHASE AND G(2) PROGRESSION, AND RESULTS IN APOPTOSIS

We used genetically related Chinese hamster ovary cell lines proficien t or deficient in DNA repair to determine the direct role of UV-induce d DNA photoproducts in inhibition of DNA replication and in induction of G(2) arrest and apoptosis. UV irradiation of S-phase-synchronized c ells causes delays in completion of the S-phase sometimes followed by an extended G, arrest and apoptosis. The effects of UV irradiation dur ing the S-phase on subsequent cell cycle progression are magnified in repair-deficient cells, indicating that these effects are initiated by persistent DNA damage and not by direct UV activation of signal trans duction pathways. Moreover, among the lesions introduced by UV irradia tion, persistence of (6-4) photoproducts inhibits DNA synthesis much m ore than persistence of cyclobutane pyrimidine dimers (which appear to be efficiently bypassed by the DNA replication apparatus). Apoptosis begins approximately 24 h after UV irradiation of S-phase-synchronized cells, occurs to a greater extent in repair-deficient cells, and corr elates well with the inability to escape from an extended late S-phase -G(2) arrest. We also find that nucleotide excision repair activity (i ncluding its coupling to transcription) is similar in the S-phase to w hat we have previously measured in G(1) and G(2).