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Leukemia. 1994 Nov;8(11):1960-9.

High levels of p26BCL-2 oncoprotein retard taxol-induced apoptosis in human pre-B leukemia cells.

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Department of Medicine, Medical University of South Carolina, Charleston 29425.


In human leukemic cells clinically relevant concentrations of taxol have been demonstrated to induce the biochemical and morphologic hallmarks of apoptosis (Leukemia 1993;7:563-568). Since overexpression of the bcl-2 gene has been reported to retard apoptosis due to a variety of anticancer agents, we examined and compared taxol-induced intracellular microtubular bundling and apoptosis in pre-B human leukemia 697 cells and their counterparts which have been transfected with and overexpress cDNA derived from the bcl-2 gene. Treatment with 0.1 or 1.0 mumol/l taxol for 24 h resulted in internucleosomal DNA fragmentation and morphologic features of apoptosis in 697 cells, but not in 697/BCL-2 cells. However, indirect immunofluorescent staining with anti-tubulin antibody revealed that taxol treatment produces stable microtubule bundles resistant to calcium-mediated disassembly in 697, as well as 697/BCL-2 cells. In addition, taxol-induced microtubule bundling was associated with a marked accumulation of the two cell types in the G2/M phase of the cell cycle. Following exposure to taxol, when 697 cells were washed and kept in drug-free medium, they showed rapid onset of apoptosis followed by loss of cell viability and a decline in cell numbers. In contrast, identically treated 697/BCL-2 cells kept in drug-free medium remained in a growth arrested state, but showed little evidence of apoptosis for up to 4 days. They eventually demonstrated features of apoptotic cell death and loss of viability between 5 and 7 days. This was not accompanied by a decrease in p26BCL-2 levels. Anti-phosphotyrosine or anti-MAP kinase immunoblot analyses of proteins isolated from taxol-treated 697 and 697/BCL-2 cells failed to show any difference in tyrosine phosphorylation of cellular proteins. Therefore, our findings indicate that in 697/BCL-2 cells, high levels of p26BCL-2 significantly delay taxol-induced endonucleolytic internucleosomal DNA fragmentation and apoptosis, but do not affect taxol-induced microtubule bundling or cell cycle growth arrest. The delayed onset of taxol-induced DNA fragmentation and apoptosis in 697/BCL-2 cells without down-regulation of p26BCL-2 levels suggests that an alternative mechanism of taxol-mediated apoptosis might be triggered which is unimpeded by high p26BCL-2 levels, or taxol-induced prolongation of mitotic arrest may lead to the inactivation or inhibition of that mechanism by which p26BCL-2 is able to block apoptosis.

[Indexed for MEDLINE]

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