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Bcl-2 blocks apoptosis in cells lacking mitochondrial DNA
M D Jacobson1, J F Burne, M P King
1Department of Biology, University College London, UK.
Abstract:
When the mammalian proto-oncogene bcl-2 is overexpressed it can protect various types of cells both from normal and from experimentally induced apoptosis, but the molecular mechanisms involved are unknown. Although the Bcl-2 protein is membrane-associated, its subcellular location is controversial: two studies have suggested that it is mainly associated with the nuclear envelope and endoplasmic reticulum, whereas another study has suggested that it is mainly located in the inner mitochondrial membrane. The latter study has suggested that Bcl-2 might protect cells from apoptosis by altering mitochondrial function and that mitochondria may be involved in apoptosis. Here we report that human mutant cell lines that lack mitochondrial DNA (mtDNA), and therefore do not have a functional respiratory chain, can still be induced to die by apoptosis, and that they can be protected from apoptosis by the overexpression of bcl-2, suggesting that neither apoptosis nor the protective effect of bcl-2 depends on mitochondrial respiration. We also show that the Bcl-2 protein in overexpressing cells is associated with the nuclear envelope and endoplasmic reticulum, as well as with mitochondria.
Insights
Overexpressing the bcl-2 gene protects cells from apoptosis, even without mitochondrial respiration. The Bcl-2 protein localizes to the nuclear envelope, endoplasmic reticulum, and mitochondria, indicating apoptosis regulation independent of mitochondrial function.
Area of Science:
- Cell Biology
- Molecular Biology
- Oncology
Background:
- The proto-oncogene bcl-2 confers resistance to apoptosis when overexpressed.
- The subcellular localization of Bcl-2 protein, crucial for its function, remains controversial, with proposed roles in the nuclear envelope, endoplasmic reticulum, and inner mitochondrial membrane.
- Mitochondria are implicated in apoptosis, and Bcl-2's potential role in modulating mitochondrial function is under investigation.
Purpose of the Study:
- To investigate the role of mitochondrial respiration in apoptosis and the protective effects of bcl-2.
- To determine the subcellular localization of Bcl-2 protein in cells overexpressing the gene.
- To elucidate the molecular mechanisms underlying bcl-2-mediated protection against apoptosis.
Main Methods:
- Utilized human mutant cell lines lacking mitochondrial DNA (mtDNA) to impair mitochondrial respiration.
- Induced apoptosis in these cell lines under various conditions.
- Assessed the protective effects of bcl-2 overexpression on apoptosis.
- Determined the subcellular localization of Bcl-2 protein using cell fractionation and microscopy.
Main Results:
- Human cell lines lacking mtDNA and functional respiratory chains still undergo apoptosis.
- Overexpression of bcl-2 effectively protected these mtDNA-deficient cells from apoptosis.
- These findings indicate that apoptosis and bcl-2's protective function are independent of mitochondrial respiration.
- Bcl-2 protein was found to associate with the nuclear envelope, endoplasmic reticulum, and mitochondria in overexpressing cells.
Conclusions:
- Apoptosis and the anti-apoptotic function of bcl-2 do not rely on mitochondrial respiration.
- The Bcl-2 protein's localization to multiple cellular compartments, including mitochondria, nuclear envelope, and endoplasmic reticulum, suggests a complex role in regulating cell death pathways.
- These findings challenge the exclusive role of mitochondria in bcl-2-mediated apoptosis protection and highlight its broader cellular involvement.