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 wild-type p53 activity in HT29 cells on DNA damage and cytotoxicity caused by exposure to fluorodeoxyuridine (FdUrd). Expression of wild-type p53 phenotype 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-strand breaks and, unexpectedly, single-strand breaks in parental (mature) DNA. Temporary expression of wild-type p53 activity in the absence of drug treatment caused some loss of clonogenicity, although the magnitude of this cytotoxic effect was small compared with the level of cell kill obtained by treatment with cytotoxic drugs for similar periods of time, indicating that HT29 cells are not highly sensitive to induction of programmed cell death by wild-type p53. Because these observations conflict with previously suggested models for FdUrd-induced damage to parental DNA, we propose an alternative model to explain how incorporation of uracil into nascent DNA might result in single-strand breaks in the opposite (parental) strand and how these breaks might be converted to the double-strand breaks that produce cell death.

Expression of wild-type p53 stimulates an increase in both Bax and Bcl-xL protein content in HT29 cells.

It has been shown previously that wild-type p53 activity can simultaneously up-regulate Bax, a protein which predisposes cells to programmed cell death (PCD), and down-regulate Bcl-2, a protein which antagonizes PCD. These findings have been interpreted to suggest that correction of the mutant p53 status of some tumor cells may be a means of increasing their sensitivity to chemotherapeutic agents, by increasing their likelihood of undergoing PCD. We show here that when wild-type p53 activity is expressed in HT29 human colon cancer cells by use of a temperature sensitive p53 mutant, Bax levels rise, but so do levels of Bcl-xL protein. These observations indicate that Bcl-2 and Bcl-xL are regulated differently in response to wild-type p53 activity and that, while correction of mutant p53 phenotype may effectively kill cells having Bcl-2 as their major defense against PCD, this is not necessarily the case in cells using Bcl-xL as their primary defense.

Dependence of fluorodeoxyuridine-induced cytotoxicity and megabase DNA fragment formation on S phase progression in HT29 cells.

The relationship between cell cycle progression and induction of DNA double-strand breaks and cytotoxicity by exposure to fluorodeoxyuridine (FdUrd) was studied in HT29 human colon cancer cells. Fractionation of drug-treated populations by centrifugal elutriation yielded subpopulations having widely divergent abilities to progress through S phase in the presence of the drug. One of these subpopulations, which appeared to undergo coordinated growth arrest, was resistant to FdUrd cytotoxicity and DNA damage. In contrast, the subpopulation which was able to progress furthest through S phase in the presence of FdUrd underwent unbalanced growth arrest (i.e., increase in size and mass out of proportion to DNA synthesis), and displayed both DNA double-strand break formation (assayed by pulsed field gel electrophoresis) and loss of clonogenicity. When cells were elutriated prior to drug treatment, producing fractions enriched in cells at various cell cycle stages, no significant differences in sensitivity to FdUrd-induced cytotoxicity were detected among elutriation fractions. These findings support the model that, in HT29 cells, progression into and through S phase during drug treatment is an important determinant of FdUrd-induced DNA damage and cytotoxicity, but that the cell cycle position at the start of drug exposure is not a critical factor for these effects.

Bcl-2 inhibits chemotherapy-induced apoptosis in neuroblastoma.

bcl-2 is the first member of a new class of protooncogenes the products of which inhibit programmed cell death (PCD) or apoptosis. We have previously determined that Bcl-2 is expressed in a significant percentage of untreated primary neuroblastoma (NBL) tumors. In these specimens Bcl-2 expression correlated with other markers of poor prognosis suggesting a role for Bcl-2 in the malignant behavior of NBL tumor cells. To investigate this possibility, a Bcl-2-negative human NBL cell line (Shep-1) was transfected with a bcl-2 expression vector (pSFFVneo-bcl-2). Multiple unique clones were isolated which showed variable levels of Bcl-2 protein by quantitative immunoprecipitation. Vector-transfected controls were generated simultaneously. Clones expressing high levels of Bcl-2 were resistant to cisplatin- and etoposide-induced cytotoxicity in a dose-dependent manner. Analysis of propidium iodide-stained nuclei by flow cytometry after cisplatin or etoposide treatment revealed marked DNA degradation in vector-transfected controls whereas bcl-2 transfectants showed a dose-dependent inhibition of DNA degradation. Analysis by pulsed-field gel electrophoresis revealed relatively large fragment DNA degradation (approximately 50 kilobases) in the absence of internucleosomal degradation in vector-transfected control cells treated with either cisplatin or etoposide. In contrast, Bcl-2-expressing cells showed significantly less DNA degradation at all time points. These single gene transfection experiments have revealed that expression of Bcl-2 renders specific NBL cells resistant to chemotherapy-induced PCD and support the hypothesis that Bcl-2 enhances the malignant phenotype of NBL by promoting tumor resistance to chemotherapy agents.