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BCL-2, BCL-X(L) sequester BH3 domain-only molecules preventing BAX- and BAK-mediated mitochondrial apoptosis
1Howard Hughes Medical Institute, Harvard Medical School, Dana-Farber Cancer Institute, Boston, MA 02115, USA.
Abstract:
Critical issues in apoptosis include the importance of caspases versus organelle dysfunction, dominance of anti- versus proapoptotic BCL-2 members, and whether commitment occurs upstream or downstream of mitochondria. Here, we show cells deficient for the downstream effectors Apaf-1, Caspase-9, or Caspase-3 display only transient protection from "BH3 domain-only" molecules and die a caspase-independent death by mitochondrial dysfunction. Cells with an upstream defect, lacking "multidomain" BAX, BAK demonstrate long-term resistance to all BH3 domain-only members, including BAD, BIM, and NOXA. Comparison of wild-type versus mutant BCL-2, BCL-X(L) indicates these antiapoptotics sequester BH3 domain-only molecules in stable mitochondrial complexes, preventing the activation of BAX, BAK. Thus, in mammals, BH3 domain-only molecules activate multidomain proapoptotic members to trigger a mitochondrial pathway, which both releases cytochrome c to activate caspases and initiates caspase-independent mitochondrial dysfunction.
Insights
Cellular apoptosis critical issues involve caspases versus organelle dysfunction. Our study reveals BH3 domain-only molecules activate BAX and BAK, initiating a mitochondrial pathway crucial for cell death.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Apoptosis regulation involves complex interplay between caspases, organelle dysfunction, and BCL-2 family members.
- Key questions remain regarding the relative importance of caspases versus mitochondrial dysfunction and the timing of commitment to cell death.
Purpose of the Study:
- To elucidate the roles of downstream caspases and upstream BCL-2 family members in initiating apoptosis.
- To determine the mechanism by which BH3 domain-only molecules trigger cell death pathways.
Main Methods:
- Analysis of cells deficient in downstream apoptotic effectors (Apaf-1, Caspase-9, Caspase-3).
- Assessment of resistance in cells lacking upstream multidomain BAX and BAK to various BH3 domain-only molecules.
- Investigation of BCL-2 and BCL-X(L) interactions with BH3 domain-only molecules in mitochondrial complexes.
Main Results:
- Cells lacking downstream caspases showed transient protection and caspase-independent death via mitochondrial dysfunction.
- Cells lacking upstream BAX and BAK exhibited long-term resistance to all BH3 domain-only molecules.
- Antiapoptotic BCL-2 proteins sequester BH3 domain-only molecules, inhibiting BAX and BAK activation.
Conclusions:
- BH3 domain-only molecules activate multidomain proapoptotic proteins (BAX, BAK) to initiate a mitochondrial cell death pathway.
- This pathway results in both cytochrome c release for caspase activation and caspase-independent mitochondrial dysfunction.
- The findings clarify the central role of the mitochondrial pathway in mammalian apoptosis.