Distinct roles of mitochondria- and ER-localized Bcl-xL in apoptosis resistance and Ca2+ homeostasis

Bcl-2 proteins are major regulators of cellular responses to intrinsic and extrinsic apoptotic stimuli. Among them, overexpression of the antiapoptotic protein Bcl-x(L) modulates intracellular Ca2+ homeostasis and organelle-specific apoptotic signaling pathways. However, the specific activities of Bcl-x(L) at mitochondria and the endoplasmic reticulum (ER) have not been fully defined. To further explore this, we generated mouse embryonic fibroblast (MEF) cell lines deficient in Bcl-x(L) expression (Bcl-x-KO). Deficiency in Bcl-x(L) expression did not induce compensatory changes in the expression of other Bcl-2 proteins, and Bcl-x-KO MEF cells showed increased sensitivity to various apoptotic stimuli compared with wild-type MEF cells. Targeting Bcl-x(L) at mitochondria but not at the ER restored apoptosis protection in Bcl-x-KO MEF cells to the degree observed in wild-type MEF cells. However, expression of ER-targeted Bcl-x(L) but not mitochondrially targeted Bcl-x(L) was required to restore Ca2+ homeostasis in Bcl-x-KO MEF cells. Of importance, ER-targeted Bcl-x(L) was able to protect cells against death stimuli in the presence of endogenous Bcl-x(L). These data indicate that mitochondrial Bcl-x(L) can regulate apoptosis independently of ER Bcl-x(L) and that when localized exclusively at the ER, Bcl-x(L) impinges on Ca2+ homeostasis but does not affect apoptosis unless Bcl-x(L) is present in additional cellular compartments.