Influence of cis-Diamminedichloroplatinum(II) on DNA Synthesis and Cell Cycle Progression in Excision Repair Proficient and Deficient Chinese Hamster Ovary Cells

Abstract

DNA has been implicated as the critical intracellular target for cis-diamminedichloroplatinum(II) (cis-DDP) action. Inhibition of DNA synthesis is a consequence of platination and has become accepted as the critical step in cis-DDP-induced toxicity. We have previously demonstrated that, following incubation with cis-DDP, murine leukemia L1210 cells progress through synthesis only to arrest in the G₂ phase of the cell cycle. The G₂ arrest was transient at low drug concentrations and was persistent at higher concentrations with a concomitant loss of viability. Chinese hamster ovary cell lines both proficient and deficient for DNA excision repair have been used to analyze the relationship between inhibition of DNA synthesis and toxicity and to determine whether DNA repair is necessary for cell cycle progression. Two repair-deficient cell lines were hypersensitive to cis-DDP and demonstrated a marked arrest in the G₂ phase. The arrest was transient over only a small range of concentrations. At higher concentrations, the arrest was persistent and the cells subsequently died. Incorporation of [³H]thymidine into macromolecules demonstrated no inhibition of DNA synthesis while these cells progressed through the S phase. In contrast, at higher, but nontoxic, concentrations of cis-DDP, the repair-proficient cells exhibited inhibition of DNA synthesis while in S. At toxic concentrations, these cells also arrested in G₂. Therefore, direct inhibition of DNA synthesis correlated only with the concentration of drug and not with the different sensitivities of the cell lines. Arrest of cells in G₂ did correlate with toxicity. In every cell line, the appearance of G₂-arrested cells preceded cell disintegration. It is proposed that the G₂-arrested cells preceded cell disintegration. It is proposed that the G₂ arrest results from the inability of the cells to transcribe genes required for passage into mitosis. Cells proficient in DNA repair can circumvent this arrest by repairing the damaged DNA and permitting transcription to proceed. These results support the hypothesis that inhibition of DNA synthesis is not the critical step in cis-DDP-induced cytotoxicity.