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.
Subramanian
T
T
Institute for Molecular Virology, Saint Louis University School of Medicine, 3681 Park Ave., St. Louis, Missouri 63110, USA.
Chinnadurai
G
G
D017209:Apoptosis / Q000502:physiology*; D020169:Caspases / Q000502:physiology; D045744:Cell Line, Tumor; D004789:Enzyme Activation; D019254:Genes, bcl-2 / Q000502:physiology*; D006801:Humans; D015151:Immunoblotting; D008565:Membrane Proteins / Q000172:deficiency / Q000502:physiology*; D009154:Mutation; D011518:Proto-Oncogene Proteins / Q000172:deficiency / Q000502:physiology*; D019253:Proto-Oncogene Proteins c-bcl-2; D020506:Untranslated Regions / Q000235:genetics; D050998:bcl-2 Homologous Antagonist-Killer Protein; D051028:bcl-2-Associated X Protein
C494772:BAK1 protein, human; C494826:BAX protein, human; D008565:Membrane Proteins; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D020506:Untranslated Regions; D050998:bcl-2 Homologous Antagonist-Killer Protein; D051028:bcl-2-Associated X Protein; D020169:Caspases