Cellular responses to etoposide: cell death despite cell cycle arrest and repair of DNA damage

The topoisomerase IIα inhibitor etoposide is a ‘broad spectrum’ anticancer agent and a potent inducer of DNA double strand breaks. DNA damage response of mammalian cells usually involves cell cycle arrest and DNA repair or, if unsuccessful, cell death. We investigated these processes in the human colon cancer cell line HT-29 treated with three different etoposide regimens mimicking clinically relevant plasma concentrations of cancer patients. Each involved a period of drug-free incubation following etoposide exposure to imitate the decline of plasma levels between the cycles of chemotherapy. We found a massive induction of double strand breaks that were rapidly and nearly completely fixed long before the majority of cells underwent apoptosis or necrosis. An even greater percentage of cells lost clonogenicity. The occurrence of double strand breaks was accompanied by a decrease in the levels of Ku70, Ku86 and DNA-PKcs as well as an increase in the level of Rad51 protein. Twenty-four hours after the first contact with etoposide we found a pronounced G2/M arrest, regardless of the duration of drug exposure, the level of double strand breaks and the extent of their repair. During the subsequent drug-free incubation period, the loss of clonogenicity correlated well with the preceding G2/M arrest as well as with the amount of cell death found several days after exposure. However, it correlated neither with early apoptosis or necrosis nor with any of the other investigated parameters. These results suggest that the G2/M arrest is an important determinant in the cytostatic action of etoposide and that the removal of DNA double strand breaks is not sufficient to ensure cell survival.