Bcl-2 Overexpression Does Not Prevent but Retards Adriamycin Toxicity in CC531 Colon Carcinoma Cells

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Abstract
Background: The Bcl-2 protein is a critical regulator of susceptibility towards cell death induced by antineoplastic drugs. Reduced growth activity and increased glutathione (GSH) levels protect against adriamycin toxicity. We recently demonstrated statistically significantly reduced growth activity and elevated cellular GSH levels in exponentially growing rat CC531 colon carcinoma cells overexpressing the full-length human Bcl-2 protein (CCbcl2#A3). Methods: To assess the importance of reduced growth activity or increased GSH levels, we determined the mitochondrial function, 24 h after adriamycin treatment, in CCbcl2#A3 cells, parental CC531 cells and cells overexpressing the Bcl-2 protein lacking the N-terminal BH4 domain (CCΔBH4): these latter cells contained elevated cellular GSH levels but were not reduced in growth activity. Results: CCbcl2#A3, but not CCΔBH4, cells were 3-fold less susceptible than parental cells suggestive of a protective role for reduced growth but not for increased GSH levels in bcl-2 transfectants. This was confirmed in several growth-inhibited CC531 transfectants and in slowly proliferating (ca. 100% confluent) cell populations compared to exponentially growing (ca. 50% confluent) cell populations. Reduced growth activity might delay the onset of cell death. Therefore, we tested the effect of adriamycin five days after treatment. In this long-term assay we found no differences between the various cells. Conclusion: Reduction of growth activity, for instance by an overexpression of the Bcl-2 protein, only transiently reduced the susceptibility towards adriamycin treatment.

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