title: Distinct roles of mitochondria- and ER-localized Bcl-xL in apoptosis resistance and Ca2+ homeostasis.
abstract: 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 Ca(2+) 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 Ca(2+) 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 Ca(2+) homeostasis but does not affect apoptosis unless Bcl-x(L) is present in additional cellular compartments.
authors.affiliation: Molecular Targets Program, James Graham Brown Cancer Center, University of Louisville, Louisville, KY 40202, USA.
authors.forename: Colins O
authors.initials: CO
authors.lastname: Eno
authors.forename: Emily F
authors.initials: EF
authors.lastname: Eckenrode
authors.forename: Kristen E
authors.initials: KE
authors.lastname: Olberding
authors.forename: Guoping
authors.initials: G
authors.lastname: Zhao
authors.forename: Carl
authors.initials: C
authors.lastname: White
authors.forename: Chi
authors.initials: C
authors.lastname: Li
mesh_terms: D000818:Animals; D017209:Apoptosis; D002118:Calcium / Q000378:metabolism*; D020169:Caspases / Q000378:metabolism; D002470:Cell Survival; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D015870:Gene Expression; D020869:Gene Expression Profiling; D006706:Homeostasis; D006801:Humans; D051379:Mice; D018345:Mice, Knockout; D008928:Mitochondria / Q000378:metabolism*; D020411:Oligonucleotide Array Sequence Analysis; D051560:Proto-Oncogene Proteins c-bcl-6 / Q000235:genetics / Q000378:metabolism; D051020:bcl-X Protein / Q000235:genetics / Q000378:metabolism*
chemical_list: C494812:Bcl2l1 protein, mouse; D051560:Proto-Oncogene Proteins c-bcl-6; D051020:bcl-X Protein; D020169:Caspases; D002118:Calcium