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, increased expression of the anti‐apoptotic protein Bcl‐XL is involved in tumorogenesis of human prostate, colorectal, and intestinal cancer cells. Although Bcl‐XL has been shown to modulate intracellular Ca 2+ homeostasis and organelle‐specific apoptotic signaling pathways, its specific activities at mitochondria and the endoplasmic reticulum (ER) have not been fully defined. To explore how endogenous Bcl‐XL functions to regulate apoptosis, we generated mouse embryonic fibroblast (MEF) cell lines deficient in Bcl‐XL expression (Bcl‐XL‐KO). Deficiency in Bcl‐XL expression did not induce compensatory changes in the expression of other Bcl‐2 proteins. Bcl‐X‐KO cells showed increased sensitivity to various apoptotic stimuli compared with wild‐type cells. When expressed at the endogenous Bcl‐XL level, mitochondria‐targeted but not ER‐targeted Bcl‐XL restored apoptosis protection of Bcl‐XL‐KO cells to that observed in wild‐type cells. Despite lacking effect on apoptosis, expression of ER‐targeted Bcl‐XL was required to restore Ca 2+ homeostasis in Bcl‐ XL‐KO cells. These data indicate that Bcl‐XL localized to mitochondria and the ER can serve to regulate apoptosis and ER Ca 2+ homeostasis independently. Overall, our studies improve our understanding of how Bcl‐XL regulates apoptosis, which is crucial to develop new anti‐cancer therapeutic strategies to induce apoptosis in Bcl‐XL‐expressing tumor cells.