Bcl-2 Slowsin VitroBreast Cancer Growth Despite Its Antiapoptotic Effect

Abstract

Background.Although the Bcl-2 protein promotes tumor cell survival by blocking programmed cell death (apoptosis), Bcl-2 expression has been associated with favorable prognostic indicators in breast cancer. We hypothesize that despite its antiapoptotic effects, Bcl-2 slows tumor cell proliferation.Materials and methods.Bcl-2-negative breast cancer cells (SKBr3) were transfected with thebcl-2gene (Bcl2-1 clone, low expression; Bcl2-2 clone, high expression) or plasmid control (Neo). Cell cycle distribution and kinetics were analyzed using bivariate flow cytometry (PI staining and pulse BrdU uptake). Cells were treated for 72 h with doxorubicin (100 ng/ml) or vehicle (0.01% DMSO) and assayed for cytosolic DNA with diphenylamine to measure apoptosis.Results.Cell counting showed increased doubling time in the Bcl-2-expressing clones Bcl2-1 and Bcl2-2 (Bcl-2(+)) relative to the Bcl-2-nonexpressing lines SKBr3 and Neo (Bcl-2(−)). Cell cycle analysis showed a decreased S phase fraction in Bcl-2(+) cells. Pulse BrdU uptake showed an increased G1/G0fraction in Bcl-2(+) cells. Doxorubicin-induced apoptosis occurred in Bcl-2(−) but not in Bcl-2(+) cell lines.Conclusions.Despite antiapoptotic effects favoring tumor survival, Bcl-2 prolongs cell cycle. Decreased tumor proliferation may account for the association of Bcl-2 expression with a favorable outcome in breast cancer, even though Bcl-2 may mediate chemoresistance in some patients.