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Negative regulation of mitochondrial VDAC channels by C-Raf kinase
Véronique Le Mellay1, Jakob Troppmair, Roland Benz
1Institut für Medizinische Strahlenkunde und Zellforschung (MSZ), Universität Würzburg, 97078 Würzburg, Germany. lemellay@univ-montp2.fr
Background:
Growth of cancer cells results from the disturbance of positive and negative growth control mechanisms and the prolonged survival of these genetically altered cells due to the failure of cellular suicide programs. Genetic and biochemical approaches have identified Raf family serine/threonine kinases B-Raf and C-Raf as major mediators of cell survival. C-Raf cooperates with Bcl-2/Bcl-XL in suppression of apoptosis by a mechanism that involves targeting of C-Raf to the outer mitochondrial membrane and inactivation of the pro-apoptotic protein Bad. However, apoptosis suppression by C-Raf also occurs in cells lacking expression of Bad or Bcl-2.
Results:
Here we show that even in the absence of Bcl-2/Bcl-XL, mitochondria-targeted C-Raf inhibits cytochrome c release and caspase activation induced by growth factor withdrawal. To clarify the mechanism of Bcl-2 independent survival control by C-Raf at the mitochondria a search for novel mitochondrial targets was undertaken that identified voltage-dependent anion channel (VDAC), a mitochondrial protein (porin) involved in exchange of metabolites for oxidative phosphorylation. C-Raf forms a complex with VDAC in vivo and blocks reconstitution of VDAC channels in planar bilayer membranes in vitro.
Conclusion:
We propose that this interaction may be responsible for the Raf-induced inhibition of cytochrome c release from mitochondria in growth factor starved cells. Moreover, C-Raf kinase-induced VDAC inhibition may regulate the metabolic function of mitochondria and mediate the switch to aerobic glycolysis that is common to cancer cells.
Insights
Cancer cells survive by evading apoptosis. C-Raf kinase targets mitochondria, interacting with VDAC to block cell death, even without Bcl-2/Bcl-XL, potentially driving cancer metabolism.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- Cancer cell growth involves dysregulated cell proliferation and survival, often due to failures in apoptosis.
- Raf kinases, including C-Raf, are key mediators of cell survival and can suppress apoptosis.
- C-Raf's anti-apoptotic function involves mitochondrial targeting and interaction with proteins like Bad and Bcl-2/Bcl-XL.
Purpose of the Study:
- To elucidate the mechanism of C-Raf-mediated apoptosis suppression at the mitochondria, independent of Bcl-2/Bcl-XL.
- To identify novel mitochondrial targets of C-Raf involved in cell survival.
- To investigate the functional consequences of C-Raf interaction with mitochondrial proteins.
Main Methods:
- Investigated C-Raf's role in apoptosis suppression using cell models lacking Bcl-2/Bcl-XL.
- Utilized biochemical approaches to identify and characterize novel mitochondrial targets of C-Raf.
- Employed in vivo complex formation assays and in vitro channel reconstitution experiments to study C-Raf-VDAC interaction.
Main Results:
- Mitochondria-targeted C-Raf inhibited cytochrome c release and caspase activation during growth factor withdrawal, independent of Bcl-2/Bcl-XL.
- Voltage-dependent anion channel (VDAC), a key mitochondrial protein, was identified as a novel target of C-Raf.
- C-Raf formed a complex with VDAC in vivo and functionally inhibited VDAC channels in vitro.
Conclusions:
- The interaction between C-Raf and VDAC likely mediates the inhibition of cytochrome c release from mitochondria in growth factor-starved cells.
- C-Raf-induced VDAC inhibition may alter mitochondrial metabolic function.
- This interaction could contribute to the metabolic shift towards aerobic glycolysis observed in cancer cells.