Bimodal effects of 1R,2R-diaminocyclohexane(transdiacetato)(dichloro)platinum(IV) on cell cycle checkpoints

1R,2R-Diaminocyclohexane(trans-diacetato)(dichloro)-platinum(IV) (DACH-acetato-Pt) is a novel platinum-based agent that is highly effective against cisplatin-resistant ovarian tumor cells. To probe its cellular mechanism, the effects of DACH-acetato-Pt (0-6.4 μM) on cell cycle checkpoints were examined using the ovarian cancer A2780 cell line as the model system. We found that DACH-acetato-Pt at ≥0.2 μm dramatically inhibited cell growth and induced cell death. At concentrations ≤0.6 μm (low effective concentrations), DACH-acetato-Pt specifically induced G₁ phase arrest by selectively inhibiting cyclin-dependent kinase 4 (Cdk4) and Cdk2 activities. The Cdc2 activity, which regulates G₂-M phase progression, was unaffected by the drug at these concentrations. At concentrations >0.6 μM (high effective concentrations), DACH-acetato-Pt first transiently inhibited S-phase progression and then blocked cell cycle progression at both G₁ and G₂ phases. These cell cycle effects were associated with sequential inhibitions of Cdk2/cyclin A activity, Cdk4 and Cdk2 activities, and Cdc2 kinase activity. Following the cell cycle effects, both the low and high effective concentrations of DACH-acetato-Pt induced cell death through apoptosis. These results indicate that DACH-acetato-Pt activates multiple cell cycle checkpoints in a bimodal manner and suggest that the cell cycle effects demonstrated in these studies may be linked to its ability to induce apoptosis.