IMPAIRMENT OF NUCLEOTIDE EXCISION-REPAIR BY APOPTOSIS IN UV-IRRADIATED MOUSE CELLS

We investigated the relationship between nucleotide excision repair (N ER) activity and apoptosis in UV-irradiated cells. Mouse erythroleukem ia (MEL) and lymphoma (GRSL) cells exhibited enhanced sensitivity to t he cytotoxic effects of UV radiation compared to hamster cell Lines, a lthough normal UV-induced hprt mutation frequencies were found, Determ ination of UV-induced repair replication revealed a limited capacity o f MEL and GRSL cells to perform NER consistent with poor removal of cy clobutane pyrimidine dimers and pyrimidine 6-4 pyrimidone photoproduct s from transcriptionally active genes during the first 8 h after UV ex posure, However, both cyclobutane pyrimidine dimers and pyrimidine 6-4 pyrimidone photoproducts appeared to be processed to almost normal le vel 24 h after UV treatment. In parallel, we observed that the UV-irra diated MEL and GRSL cells suffered from severe DNA fragmentation parti cularly 24 h after UV exposure. Taken together, these data indicate a reduced repair of UV-induced photolesions in apoptotic cells, already established at the early onset of apoptosis, To test whether inhibitio n of repair in cells was due to inactivation of NER or to apoptosis-in duced chromatin degradation, we performed in vitro excision assays usi ng extracts from UV-irradiated MEL cells. These experiments showed tha t the NER capacity during early apoptosis was intact, indicating that slow removal of UV-induced photolesions in apoptotic cells is due to s ubstrate modification (presumably degradation of chromatin) rather tha n direct inhibition of factors involved in NER.