THE ROLE OF CELL-CYCLE PROGRESSION IN CISPLATIN-INDUCED APOPTOSIS IN CHINESE-HAMSTER OVARY CELLS

Many anticancer drugs arrest cells in G(2) of the cell cycle and subsequently induce cell death by apoptosis. The current experiments establish a detailed sequence of events that occur in Chinese hamster ovary CHO/UV41 cells following incubation with cisplatin. Synchronized CHO/UV41 cells were damaged with cisplatin in early S. The cells progressed at a normal rate through S but arrested in G(2). The arrested cells exhibited normal levels of the mitosis-promoting kinase p34(cdc2) in its fully phosphorylated, inactive form. After a protracted arrest, the cells dephosphorylated p34(cdc2) and underwent an aberrant mitosis and cytokinesis in which the chromosomes segregated unequally due to the formation of multipolar mitotic spindles. These cells subsequently lost contact with the extracellular matrix, and only then digested their DNA in a manner characteristic of apoptosis. This sequence of events could be dramatically accelerated by the addition of caffeine to G(2)-arrested cells, which induced dephosphorylation of p34(cdc2) and passage through an aberrant mitosis. It has previously been suggested that protein synthesis is required for both caffeine-induced premature mitosis and apoptosis. However, when added in G(2), cycloheximide could inhibit neither the caffeine-induced mitosis nor apoptosis. Inhibition was only seen if cycloheximide was added during S before complete synthesis of the proteins required for mitosis. These results demonstrate that, in this model, the proteins thought to be involved in apoptosis are those required for normal cell cycle progression. It is hypothesized that the DNA digestion results from loss of signal transduction originating from the extracellular matrix but that earlier events leading to loss of cell adhesion are critical for the induction of apoptosis.