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Bcl-2 on the endoplasmic reticulum regulates Bax activity by binding to BH3-only proteins
Michael J Thomenius1, Nancy S Wang, Edmunds Z Reineks
1Department of Medicine, Comprehensive Cancer Center, Case Western Reserve University School of Medicine and University Hospitals of Cleveland, Cleveland, Ohio 44106, USA.
Abstract:
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.
Insights
The anti-apoptotic protein Bcl-2, when localized to the endoplasmic reticulum (ER), prevents cell death by indirectly inhibiting Bax activation through BH3-only proteins like Bad.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Bcl-2 family proteins regulate apoptosis, acting as pro- or anti-apoptotic factors.
- While most Bcl-2 members target mitochondria, Bcl-2 localizes to ER, mitochondria, and nuclear membranes.
Purpose of the Study:
- To investigate the role of Bcl-2 localization in its apoptotic regulatory function.
- To characterize the activity of a Bcl-2 mutant (Bcl-2Cb5) restricted to the ER membrane.
Main Methods:
- Expression of Bcl-2Cb5 in MDA-MB-468 cells.
- Induction of apoptosis using staurosporine.
- Analysis of Bax activation and cytochrome c release.
- Investigation of interactions with BH3-only proteins, specifically Bad.
Main Results:
- Bcl-2Cb5 conferred protection against staurosporine-induced apoptosis.
- Bcl-2Cb5 inhibited Bax activation and subsequent cytochrome c release.
- Bcl-2Cb5 sequestered the BH3-only protein Bad (Bad3A), preventing Bax activation, even when Bax was constitutively localized to mitochondria.
Conclusions:
- Bcl-2's anti-apoptotic function can be mediated from the ER membrane.
- An intermediate, likely a BH3-only protein, facilitates communication between ER-localized Bcl-2 and mitochondrial Bax.
- Bcl-2 indirectly protects mitochondrial integrity by modulating BH3-only protein activity.