PREVENTION OF FLUORODEOXYURIDINE-INDUCED CYTOTOXICITY AND DNA-DAMAGE IN HT29 COLON-CARCINOMA CELLS BY CONDITIONAL EXPRESSION OF WILD-TYPE P53 PHENOTYPE

We have examined the effects of conditionally expressing wildtype p53 activity in HT29 cells on DNA damage and cytotoxicity caused by exposu re to fluorodeoxyuridine (FdUrd). Expression of wild-type p53 phenotyp e for 24 hr before FdUrd treatment provided HT29 cells with virtually complete protection from cytotoxicity caused by this drug. In addition , wild-type p53 expression also prevented FdUrd-induced DNA double-str and breaks and, unexpectedly, single-strand breaks in parental (mature ) DNA. Temporary expression of wild-type p53 activity in the absence o f drug treatment caused some loss of clonogenicity, although the magni tude of this cytotoxic effect was small compared with the level of cel l kill obtained by treatment with cytotoxic drugs for similar periods of time, indicating that HT29 cells are not highly sensitive to induct ion of programmed cell death by wild-type p53. Because these observati ons conflict with previously suggested models for FdUrd-induced damage to parental DNA, we propose an alternative model to explain how incor poration of uracil into nascent DNA might result in single-strand brea ks in the opposite (parental) strand and how these breaks might be con verted to the double-strand breaks that produce cell death.