Regulation and function of BAX in apoptosis

Abstract: The BCL-2 proteins BAX and BAK are key regulators of mitochondrial apoptosis, which is induced by intracellular stress. In type II mammalian cells, mitochondrial apoptosis occurs also in response to extracellular stimuli, such as ligand-receptor binding, triggering tBID-mediated BAX and BAK activation. Upon activation, BAX and BAK undergo conformational changes and oligomerize at the mitochondria, causing outer mitochondrial membrane permeabilization. Membrane permeabilization releases intermembrane space proteins with apoptotic activity, such as cytochrome c and SMAC, which is followed by caspase activation and subsequently the destruction of the cell. BAX oligomerization was studied by confocal microscopy, which revealed BAX assembly into enormous mitochondria-associated clusters. The size of the BAX clusters and localization are contradictive to its function in OMM permeabilization and raises the question for the purpose of large BAX cluster formation. This work provides insight into the assembly of BAX scaffolds and the potential function of large BAX clusters.

BN-PAGE analysis of BAX variants revealed distinct small and undistinguishable large BAX complexes. Small BAX complexes contribute to OMM permeabilization and promote cytochrome c release for apoptosome-dependent caspase activation, whereas large BAX scaffolds seem to activate caspases independent of OMM permeabilization. The formation of caspase-9 containing BAX complexes during apoptosis suggests that BAX scaffolds provide a recruitment platform for caspase activation. Voltage-dependent anion channel (VDAC) seems to be involved in BAX cluster assembly. BAX scaffold formation may explain caspase activity in an apoptosome deficient system, such as in APAF-1 KO cells. The existence of several mechanisms to activate caspases minimalizes the potential of partial caspase activation and cell damage by apoptosis evasion.

In addition to activational machineries to stimulate caspase activation, there are inhibitory mechanisms to prevent membrane permeabilization and downstream events. For instance, prosurvival protein-mediated BAX retrotranslocation has been discovered to prevent the accumulation of BAX/BAK at the mitochondrial outer membrane and therefore cell death. However, additional factors have been proposed to control BAX/BAK retrotranslocation independent of the BCL-2 protein family.

In this study, a transient interaction-detecting approach uncovered the glycolytic enzymes hexokinases 1 and 2 as BAX binding partners. Confocal microscopy using Fluorescence Loss in Photobleaching identified hexokinase-dependent BAX/BAK and truncated BID (tBID) shuttling from the mitochondria into the cytosol, which seems to occur independent of the hexokinase glycolytic activity. Hexokinases inhibit death-receptor mediated cell death, induced by receptor ligands FasL or TRAIL, but fail to prevent intrinsic apoptosis. A shift of hexokinases towards the cytosol increases receptor-mediated caspase activity, highlighting the importance of the hexokinase mitochondrial localization for its anti-apoptotic function. This work identified a novel BCL-2 protein retrotranslocation mechanism, to prevent receptor-mediated apoptosis by hexokinases. This strategy might be applied by tumor cells to evade the immune system

Object Details

• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Universität Freiburg, Dissertation, 2022

Classification and Topics

• Keyword: Apoptosis ; Proteine

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2021
• Creator: Oelgeklaus, Aline
• Contributor: Edlich, Frank ; Albert-Ludwigs-Universität Freiburg. Fakultät für Biologie

Further information

• DOI: 10.6094/UNIFR/194190
• URN: urn:nbn:de:bsz:25-freidok-1941903
• Rights: Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Last update: 25.03.2025, 1:46 PM CET