Previous studies have demonstrated that estrogen-responsive human breast cancer cells can be induced to undergo an energy-dependent, genetically programmed series of biochemical changes that result in the active suicide of the cells following estrogen ablation. In contrast, estrogen-independent human breast cancer cells do not activate this programmed cell death pathway following estrogen ablation. This could be due either to the absence of the cellular machinery required for programmed cell

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... th or simply to the inability of estrogen ablation to activate this machinery. To discriminate between these two possibilities, the MDA-MB-468 estrogen-independent human mammary adenocarcinoma cell line was used as a model system to study the mechanism of cell death following cytotoxic drug treatment. Exposure of these cells to the fluorinated pyrimidines, 5-fluoro-2'-deoxyuridine or trifluorothymidine, resulted in growth inhibition and loss of proliferative capacity within 24 h. These changes occurred while cell membrane integrity was intact as measured by either cellular morphology or trypan blue exclusion. After 48 h of drug treatment, loss of cell membrane integrity was followed by cell lysis and a rapid decline in cell number. The addition of 16 microM thymidine prior to drug treatment prevented cell death, but thymidine did not rescue these cells once drug treatment was initiated. Analysis of DNA revealed the characteristic fragmentation into nucleosomal oligomers that is a hallmark of programmed cell death. Associated with this death pathway was a 15-fold induction of transforming growth factor beta 1 gene expression that has been previously observed in a variety of cellular systems undergoing programmed cell death. These results indicate that MDA-MB-468 estrogen-independent human mammary carcinoma cells retain the ability to undergo programmed cell death after treatment with cytotoxic drugs that induce a "thymineless" state.