Effects of cis-platinum(II) diamminedichloride on survival and the rate of DNA synthesis in synchronously growing Chinese hamster V79-379A cells in the absence and presence of caffeine inhibited post-replication repair; evidence for an inducible repair mechanism

Abstract

The mechanism by which cis-Platinum(II)diamminedichloride, (cis-Pt(II)), inhibited DNA synthesis was investigated using synchronously growing Chinese hamster V79-379A cells. The progression of G1 phase treated cells into S phase was not inhibited. However the rate of DNA synthesis throughout the S phase was depressed in a dose dependent manner compared to a control. The peak rate of DNA synthesis was delayed and the duration of the S phase was extended resulting in a dose dependent delay in the timing of mitosis in the first cell cycle after treatment. The delay in the timing of mitosis after the second S phase in treated cells was comparable to the delay observed after the first cycle. Post treatment incubation of cells in the presence of a non toxic concentration of caffeine resulted in an alteration in the pattern of DNA synthesis, leading to a reversal of the dose dependent delay in the timing of the peak rate of DNA synthesis to a position corresponding to the control. Caffeine potentiated the toxicity of cis-Pt(II) only if it was added during the first cell cycle after treatment. The patterns of DNA synthesis in synchronous cultures of Chinese hamster V79-379A cells after cis-Pt(II) treatment appeared to be similar to those previously demonstrated for difunctional alkylating agents. The results were discussed in relation to an existing model in which it was postulated that a DNA replication dependent repair mechanism enables cells to replicate DNA using a cis-Pt(II) damaged DNA template. This mechanism has been shown in Chinese hamster V79-379A cells to be sensitive to non toxic concentrations of caffeine resulting in an inhibition of the rate of ligation of nascent DNA which is followed by an increase in chromosome aberrations and cell death. The results presented support the proposed model and emphasise the importance of this replication repair mechanism in enabling cells to tolerate unexcised damage to their genome. In addition we propose that cis-Pt(II) damage stimulates the induction of a system which is involved in enhancing so called post replication repair in Chinese hamster V79 cells and that this induction might be inhibited by the presence of caffeine.