Bcl-2 on the endoplasmic reticulum regulates Bax activity by binding to BH3-only proteins.
Bcl-2 family members have been shown to be key mediators of apoptosis as either pro- or anti-apoptotic factors. It is thought that both classes of Bcl-2 family members act at the level of the mitochondria to regulate apoptosis, although the founding anti-apoptotic family member, Bcl-2 is localized to the endoplasmic reticulum (ER), mitochondrial, and nuclear membranes. In order to better understand the effect of Bcl-2 localization on its activity, we have utilized a Bcl-2 mutant that localizes only to the ER membrane, designated Bcl-2Cb5. Bcl-2Cb5 was expressed in MDA-MB-468 cells, which protected against apoptosis induced by the kinase inhibitor, staurosporine. Data presented here show that Bcl-2Cb5 inhibits this process by blocking Bax activation and cytochrome c release. Furthermore, we show that Bcl-2Cb5 can inhibit the activation of a constitutively mitochondrial mutant of Bax, indicating that an intermediate between Bcl-2 on the ER and Bax on the mitochondria must exist. We demonstrate that this intermediate is likely a BH3-only subfamily member. Data presented here show that Bcl-2Cb5 can sequester a constitutively active form of Bad (Bad3A) from the mitochondria and prevent it from activating Bax. These data suggest that Bcl-2 indirectly protects mitochondrial membranes from Bax, via BH3-only proteins.
Thomenius
Michael J
MJ
Department of Medicine, Comprehensive Cancer Center, Case Western Reserve University School of Medicine and University Hospitals of Cleveland, Cleveland, Ohio 44106, USA.
Wang
Nancy S
NS
Reineks
Edmunds Z
EZ
Wang
Zhengqi
Z
Distelhorst
Clark W
CW
D017209:Apoptosis; D001665:Binding Sites; D001943:Breast Neoplasms; D004721:Endoplasmic Reticulum / Q000502:physiology*; D005260:Female; D019254:Genes, bcl-2; D006801:Humans; D008928:Mitochondria / Q000502:physiology*; D009685:Nuclear Envelope / Q000502:physiology; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D014162:Transfection; D014407:Tumor Cells, Cultured; D051028:bcl-2-Associated X Protein
C494826:BAX protein, human; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D011994:Recombinant Proteins; D051028:bcl-2-Associated X Protein