Bcl-2 interferes with the execution phase, but not upstream events, in glucocorticoid-induced leukemia apoptosis

Due to their growth arrest- and apoptosis-inducing ability, glucocorticoids (GC) are widely used in the therapy of various lymphoid malignancies. Cell death is associated with activation of members of the interleukin-1 beta-c onverting enzyme (ICE) protease/caspase family and, is presumably prevented by the anti-apoptotic protein Bcl-2. To further address the role of Bcl-2 in CC-mediated cytotoxicity, we generated subclones of the GC-sensitive hum an T-cell acute lymphoblastic leukemia line CCRF-CEM, in which transgenic B cl-2 expression is regulated by tetracycline. Up to about 48 h, exogenous B cl-2 almost completely protected these cells from apoptosis, digestion of p oly-ADP ribose polymerase (PARP) and generation of Asp-Glu-Val-Asp cleaving (DEVDase) activity. However, when the cells were cultured for another 24 h in the continuous presence of GC, they underwent massive apoptosis that wa s associated with DEVDase activity and PARP cleavage. Bcl-2 did not markedl y affect GC-mediated growth arrest, thereby separating the anti-proliferati ve from the apoptosis-inducing effect of GC. Moreover, Bcl-2 did not preven t the dramatic reduction in the levels of several mRNAs observed during GC treatment, including the transgenic Bcl-2 mRNA. Thus, Bcl-2 can be placed u pstream of effector caspase activation, but downstream of other GC-regulate d events, such as growth arrest and the potentially critical repression of steady state levels of multiple mRNA.