Cell Proliferation and DNA Breaks Are Involved in Ultraviolet Light-induced Apoptosis in Nucleotide Excision Repair-deficient Chinese Hamster Cells

AUTOR(ES)

Dunkern, Torsten R.

FONTE

The American Society for Cell Biology

RESUMO

UV light targets both membrane receptors and nuclear DNA, thus evoking signals triggering apoptosis. Although receptor-mediated apoptosis has been extensively investigated, the role of DNA damage in apoptosis is less clear. To analyze the importance of DNA damage induced by UV-C light in apoptosis, we compared nucleotide excision repair (NER)-deficient Chinese hamster ovary cells (lines 27-1 and 43-3B mutated for the repair genes ERCC3 and ERCC1, respectively) with the corresponding DNA repair-proficient fibroblasts (CHO-9 and ERCC1 complemented 43-3B cells). NER-deficient cells were hypersensitive as to the induction of apoptosis, indicating that apoptosis induced by UV-C light is due to unrepaired DNA base damage. Unrepaired lesions, however, do not activate the apoptotic pathway directly because apoptosis upon UV-C irradiation requires DNA replication and cell proliferation. It is also shown that in NER-deficient cells unrepaired lesions are converted into DNA double-strand breaks (DSBs) and chromosomal aberrations by a replication-dependent process that precedes apoptosis. We therefore propose that DSBs arising from replication of DNA containing nonrepaired lesions act as an ultimate trigger of UV-C–induced apoptosis. Induction of apoptosis by UV-C light was related to decline in the expression level of Bcl-2 and activation of caspases. Decline of Bcl-2 and subsequent apoptosis might also be caused, at least in part, by UV-C–induced blockage of transcription, which was more pronounced in NER-deficient than in wild-type cells. This is in line with experiments with actinomycin D, which provoked Bcl-2 decline and apoptosis. UV-C–induced apoptosis due to nonrepaired DNA lesions, replication-dependent formation of DSBs, and activation of the mitochondrial damage pathway is independent of functional p53 for which the cells are mutated.

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