At low concentrations, cells remained arrested in the GM phase, at intermediate concentrations in S phase, and at the highest concentrations in the G phase of the cell cycle. However, when after days of exposure to SN the drug was withdrawn and the cells were recultured in drugfree medium, the levels of p decreased dramatically, which resulted in a gradual decrease in p levels. Cells were seeded at cellsml of medium and left to attach for hbefore SN and VP exposure.Treated LST, A, and MCF cells predominantly arrested in the G phase of the cell cycle, whereas HCA cells underwent apoptosis.DNA DAMAGE INDUCES SENESCENCE IN TUMOR CELLS staining was observed in only a few individual cells, showing that the loss of either p or p abrogates the induction of senescence.On day, the majority of the p and pnull cells were in the early stages of apoptosis, as cells with a subG content and caspase cleavage could be observed, similar to the pattern observed in p mutant cell lines.In contrast to the LST cells, no induction of p could be observed in the HCT cells, most likely because of the hypermethylated p promoter.Tumor sections from patients who had not been treated before surgery were stained as controls.All treated patients had received CAF, and tumors had been resected days after chemotherapy.Although not all tumor cells in positive sections showed staining, of tumors, whereas sections of normal tissue and normal tissue surrounding positive tumors were completely negative. Exposure of LST and HCT colon, MCF breast, and A ovarian carcinoma cells to topoisomerase I and II inhibitors resulted in prolonged cell cycle arrest predominantly in the G phase of the cell cycle.The HCA colon adenocarcinoma cells responded mainly by apoptosis.In HCT cell lines in which either the p or p function was compromised, DNA damageinduced senescence was abrogated, confirming the necessity of these proteins in this process.Cells were counted, and cells were loaded for each sample.MCF and A cells also stained positive for galactosidase activity, although the increase was slower than in LST cells.To further assess the requirement for p, p, and p in the process of DNA damageinduced senescence, we examined the consequences of SN treatment in the HCT colon carcinoma cell line, which expresses wildtype p but has a methylated p promoter, and its isogenic derivatives, which are null for either p or p.At the cellular level, in response to treatment with SN and VP cell morphology became flattened and enlarged, indicating that DNA damage induced senescence in LST, MCF, A, and the parental HCT cells.This was strengthened by the finding that after exposure to SN and VP, all cell lines investigated stained positive for galactosidase activity at pH. DNA DAMAGE INDUCES SENESCENCE IN TUMOR CELLS overexpression of p has been shown to block entry into mitosis. Often the use of cytotoxics in the clinic does not result in regression of the tumor, but rather results in stable disease, which has typically been seen as a failure of treatment.This may not be the case if tumors contain senescent cells in response to treatment and may explain some of the controversy around p as a prognostic marker because nonresponders include both progressive and stable disease.