Pro‐apoptotic activity of transiently expressed BCL‐2 occurs independent of BAX and BAK

BCL‐2 suppresses apoptosis induced by a wide variety of stimuli in multiple cell types. Most of the in vitro studies that have examined the activity of BCL‐2 have employed stable cell lines that ectopically express BCL‐2. We have reported that BCL‐2 is expressed at high levels in the absence of the 5′‐ and 3′‐UTRs of the Bcl‐2 gene and transient high level of expression results in potent cell death (Uhlmann et al., [1998]: JBC 278:17926–17932). Expression of BCL‐2 under the transcriptional control of the cognate 5′‐ and 3′‐UTRs express lower levels of BCL‐2 and does not cause cell death. Our present results suggest that in contrast to BCL‐2, transient expression of BCL‐xL does not induce cell death and coexpression of BCL‐xL with the pro‐apoptotic BCL‐2 does not suppress cell death. The pro‐apoptotic activity of BCL‐2 appears to involve activation of the cytochrome c/caspase 9/caspase 3 pathway. Elevated levels of BCL‐2 expression results in N‐terminal cleavage of BCL‐2 at a novel site different from a previously identified caspase cleavage site at Asp 34 by a non‐caspase protease. Transient expression of a BCL‐2 mutant lacking aa 51–85 within the loop region induces efficient cell death and N‐terminal cleavage of BCL‐2 while a different deletion mutant lacking aa 30–91 induces reduced levels of cell death in the absence of BCL‐2 cleavage suggesting that N‐terminal processing of BCL‐2 may be an amplification event in BCL‐2‐mediated cell death. Overexpression of BCL‐2 in a Bax‐null human colon cancer cell line (HCT116Bax−/−) induces efficient cell death. The pro‐apoptotic activity of BCL‐2 is also observed in a Bax‐null cells in which BAK expression is inhibited by stable RNAi expression. Our results suggest that BCL‐2 contains an intrinsic pro‐apoptotic activity and can induce apoptosis independent of BAX and BAK under specific conditions. © 2003 Wiley‐Liss, Inc.