Transient expression of wild-type or mitochondrially targeted Bcl-2 induces apoptosis, whereas transient expression of endoplasmic reticulum-targeted Bcl-2 is protective against Bax-induced cell death

Bcl-2 protein family members function either to promote or inhibit programmed cell death. Bcl-2, typically an inhibitor of apoptosis, has also been demonstrated to have pro-apoptotic activity (Cheng, E. H., Kirsch, D. G., Clem, R. J., et al. (1997) Science 278, 1966-1968). The pro-apoptotic activity has been attributed to the cleavage of Bcl-2 by caspase-3, which converts Bcl-2 to a proapoptotic molecule. Bcl-2 is a membrane protein that is localized in the endoplasmic reticulum (ER) membrane, the outer mitochondrial membrane, and the nuclear envelope. Here, we demonstrate that transient expression of Bcl-2 at levels comparable to those found in stably transfected cells induces apoptosis in human embryonic kidney 293 cells and in the human breast cell line MDA-MB-468 cells. Furthermore, we have targeted Bcl-2 specifically to either the ER or the outer mitochondrial membrane to test whether induction of apoptosis by Bcl-2 is dependent upon its localization within either of these membranes. Our findings indicate that Bcl-2 specifically targeted to the mitochondria induces cell death, whereas Bcl-2 that is targeted to the ER does not. The expression of Bcl-2 does result in its cleavage to a 20-kDa protein; however, mutation of the caspase-3 cleavage site (D34A) does not inhibit its ability to induce cell death. Additionally, we find that transiently expressed ER-targeted Bcl-2 inhibits cell death induced by Bax overexpression. In conclusion, the ability of Bcl-2 to promote apoptosis is associated with its localization at the mitochondria. Furthermore, the ability of ER-targeted Bcl-2 to protect against Bax-induced apoptosis suggests that the ER localization of Bcl-2 may play an important role in its protective function.