Hexokinases inhibit death receptor-dependent apoptosis on the mitochondria.

Death receptor-mediated apoptosis requires the mitochondrial apoptosis pathway in many mammalian cells. In response to death receptor signaling, the truncated BH3-only protein BID can activate the proapoptotic BCL-2 proteins BAX and BAK and trigger the permeabilization of the mitochondria. BAX and BAK are inhibited by prosurvival BCL-2 proteins through retrotranslocation from the mitochondria into the cytosol, but a specific resistance mechanism to truncated BID-dependent apoptosis is unknown. Here, we report that hexokinase 1 and hexokinase 2 inhibit the apoptosis activator truncated BID as well as the effectors BAX and BAK by retrotranslocation from the mitochondria into the cytosol. BCL-2 protein shuttling and protection from TRAIL- and FasL-induced cell death requires mitochondrial hexokinase localization and interactions with the BH3 motifs of BCL-2 proteins but not glucose phosphorylation. Together, our work establishes hexokinase-dependent retrotranslocation of truncated BID as a selective protective mechanism against death receptor-induced apoptosis on the mitochondria.

Identification of a novel Bax-Cdk1 signalling complex that links activation of the mitotic checkpoint to apoptosis.

In eukaryotes, entry into and exit from mitosis is regulated, respectively, by the transient activation and inactivation of Cdk1. Taxol, an anti-microtubule anti-cancer drug, prevents microtubule-kinetochore attachments to induce spindle assembly checkpoint (SAC; also known as the mitotic checkpoint)-activated mitotic arrest. SAC activation causes mitotic arrest by chronically activating Cdk1. One consequence of prolonged Cdk1 activation is cell death. However, the cytoplasmic signal(s) that link SAC activation to the initiation of cell death remain unknown. We show here that activated Cdk1 forms a complex with the pro-apoptotic proteins Bax and Bak (also known as BAK1) during SAC-induced apoptosis. Bax- and Bak-mediated delivery of activated Cdk1 to the mitochondrion is essential for the phosphorylation of the anti-apoptotic proteins Bcl-2 and Bcl-xL (encoded by BCL2L1) and the induction of cell death. The interactions between a key cell cycle control protein and key pro-apoptotic proteins identify the Cdk1-Bax and Cdk1-Bak complexes as the long-sought-after cytoplasmic signal that couples SAC activation to the induction of apoptotic cell death.

BAX Redistribution Induces Apoptosis Resistance and Selective Stress Sensitivity in Human HCC.

Cancer therapies induce differential cell responses, ranging from efficient cell death to complete stress resistance. The BCL-2 proteins BAX and BAK govern the cellular decision between survival and mitochondrial apoptosis. Therefore, the status of BAX/BAK regulation can predict the cellular apoptosis predisposition. Relative BAX/BAK localization was analyzed in tumor and corresponding non-tumor samples from 34 hepatocellular carcinoma (HCC) patients. Key transcriptome changes and gene expression profiles related to the status of BAX regulation were applied to two independent cohorts including over 500 HCC patients. The prediction of apoptotic response was tested using cell lines and polyclonal tumor isolates. Cellular protection from BAX was confirmed by challenging cells with mitochondrial BAX. We discovered a subgroup of HCC with selective protection from BAX-dependent apoptosis. BAX-protected tumors showed enrichment of signaling pathways associated with oxidative stress response and DNA repair as well as increased genetic heterogeneity. Gene expression profiles characteristic to BAX-specific protection are enriched in poorly differentiated HCCs and show significant association to the overall survival of HCC patients. Consistently, addiction to DNA repair of BAX-protected cancer cells caused selective sensitivity to PARP inhibition. Molecular characteristics of BAX-protected HCC were enriched in cells challenged with mitochondrial BAX. Our results demonstrate that predisposition to BAX activation impairs tumor biology in HCC. Selective BAX inhibition or lack thereof delineates distinct subgroups of HCC patients with molecular features and differential response pattern to apoptotic stimuli and inhibition of DNA repair mechanisms.

Assessment of Dynamic BCL-2 Protein Shuttling Between Outer Mitochondrial Membrane and Cytosol.

BCL-2 proteins control stress-dependent commitment to the programmed cell death apoptosis. In nonapoptotic cells the proapoptotic BCL-2 proteins BAX and BAK but also prosurvival family members, like BCL-xL or MCL-1, translocate to the outer mitochondrial membrane (OMM) and retrotranslocate from the mitochondria back into the cytosol. The resulting equilibrium produces a broad range of localization pattern observed for BAX and BAK in human cells and shows correlation between relative BAX and BAK localizations and cellular predisposition to apoptosis. The retrotranslocation of BCL-2 proteins from the OMM can be measured using fluorescence-labeled protein in intact cells or endogenous protein from isolated heavy membrane fractions.

Parkin promotes proteasomal degradation of misregulated BAX.

The pro-apoptotic BCL-2 protein BAX commits human cells to apoptosis by permeabilizing the outer mitochondrial membrane. BAX activation has been suggested to require the separation of helix α5 from α6 - the 'latch' from the 'core' domain - among other conformational changes. Here, we show that conformational changes in this region impair BAX translocation to the mitochondria and retrotranslocation back into the cytosol, and therefore BAX inhibition, but not activation. Redirecting misregulated BAX to the mitochondria revealed an alternative mechanism of BAX inhibition. The E3 ligase parkin, which is known to trigger mitochondria-specific autophagy, ubiquitylates BAX K128 and targets the pro-apoptotic BCL-2 protein for proteasomal degradation. Retrotranslocation-deficient BAX is completely degraded in a parkin-dependent manner. Although only a minor pool of endogenous BAX escapes retrotranslocation into the cytosol, parkin-dependent targeting of misregulated BAX on the mitochondria provides substantial protection against BAX apoptotic activity.

Bax transmembrane domain interacts with prosurvival Bcl-2 proteins in biological membranes.

The Bcl-2 (B-cell lymphoma 2) protein Bax (Bcl-2 associated X, apoptosis regulator) can commit cells to apoptosis via outer mitochondrial membrane permeabilization. Bax activity is controlled in healthy cells by prosurvival Bcl-2 proteins. C-terminal Bax transmembrane domain interactions were implicated recently in Bax pore formation. Here, we show that the isolated transmembrane domains of Bax, Bcl-xL (B-cell lymphoma-extra large), and Bcl-2 can mediate interactions between Bax and prosurvival proteins inside the membrane in the absence of apoptotic stimuli. Bcl-2 protein transmembrane domains specifically homooligomerize and heterooligomerize in bacterial and mitochondrial membranes. Their interactions participate in the regulation of Bcl-2 proteins, thus modulating apoptotic activity. Our results suggest that interactions between the transmembrane domains of Bax and antiapoptotic Bcl-2 proteins represent a previously unappreciated level of apoptosis regulation.

The porin VDAC2 is the mitochondrial platform for Bax retrotranslocation.

The pro-apoptotic Bcl-2 protein Bax can permeabilize the outer mitochondrial membrane and therefore commit human cells to apoptosis. Bax is regulated by constant translocation to the mitochondria and retrotranslocation back into the cytosol. Bax retrotranslocation depends on pro-survival Bcl-2 proteins and stabilizes inactive Bax. Here we show that Bax retrotranslocation shuttles membrane-associated and membrane-integral Bax from isolated mitochondria. We further discover the mitochondrial porin voltage-dependent anion channel 2 (VDAC2) as essential component and platform for Bax retrotranslocation. VDAC2 ensures mitochondria-specific membrane association of Bax and in the absence of VDAC2 Bax localizes towards other cell compartments. Bax retrotranslocation is also regulated by nucleotides and calcium ions, suggesting a potential role of the transport of these ions through VDAC2 in Bax retrotranslocation. Together, our results reveal the unanticipated bifunctional role of VDAC2 to target Bax specifically to the mitochondria and ensure Bax inhibition by retrotranslocation into the cytosol.

Differential retrotranslocation of mitochondrial Bax and Bak.

The Bcl-2 proteins Bax and Bak can permeabilize the outer mitochondrial membrane and commit cells to apoptosis. Pro-survival Bcl-2 proteins control Bax by constant retrotranslocation into the cytosol of healthy cells. The stabilization of cytosolic Bax raises the question whether the functionally redundant but largely mitochondrial Bak shares this level of regulation. Here we report that Bak is retrotranslocated from the mitochondria by pro-survival Bcl-2 proteins. Bak is present in the cytosol of human cells and tissues, but low shuttling rates cause predominant mitochondrial Bak localization. Interchanging the membrane anchors of Bax and Bak reverses their subcellular localization compared to the wild-type proteins. Strikingly, the reduction of Bax shuttling to the level of Bak retrotranslocation results in full Bax toxicity even in absence of apoptosis induction. Thus, fast Bax retrotranslocation is required to protect cells from commitment to programmed death.

Real-time magnetic resonance imaging and quantification of lipoprotein metabolism in vivo using nanocrystals.

Semiconductor quantum dots and superparamagnetic iron oxide nanocrystals have physical properties that are well suited for biomedical imaging. Previously, we have shown that iron oxide nanocrystals embedded within the lipid core of micelles show optimized characteristics for quantitative imaging. Here, we embed quantum dots and superparamagnetic iron oxide nanocrystals in the core of lipoproteins--micelles that transport lipids and other hydrophobic substances in the blood--and show that it is possible to image and quantify the kinetics of lipoprotein metabolism in vivo using fluorescence and dynamic magnetic resonance imaging. The lipoproteins were taken up by liver cells in wild-type mice and displayed defective clearance in knock-out mice lacking a lipoprotein receptor or its ligand, indicating that the nanocrystals did not influence the specificity of the metabolic process. Using this strategy it is possible to study the clearance of lipoproteins in metabolic disorders and to improve the contrast in clinical imaging.