Bax-dependent regulation of Bak by voltage-dependent anion channel 2.
Many studies have demonstrated a critical role of Bax in mediating apoptosis, but the role of Bak in regulating cancer cell apoptotic sensitivities in the presence or absence of Bax remains incompletely understood. Using isogenic cells with defined genetic deficiencies, here we show that in response to intrinsic, extrinsic, and endoplasmic reticulum stress stimuli, HCT116 cells show clear-cut apoptotic sensitivities in the order of Bax+/Bak+ > Bax+/Bak- >> Bax-/Bak+ >> Bax-/Bak-. Small interference RNA-mediated knockdown of Bak in Bax-deficient cells renders HCT116 cells completely resistant to apoptosis induction. Surprisingly, however, Bak knockdown in Bax-expressing cells only slightly affects the apoptotic sensitivities. Bak, like Bax, undergoes the N terminus exposure upon apoptotic stimulation in both Bax-expressing and Bax-deficient cells. Gel filtration, chemical cross-linking, and co-immunoprecipitation experiments reveal that different from Bax, which normally exists as monomers in unstimulated cells and is oligomerized by apoptotic stimulation, most Bak in unstimulated HCT116 cells exists in two distinct protein complexes, one of which contains voltage-dependent anion channel (VDAC) 2. During apoptosis, Bak and Bax form both homo- and hetero-oligomeric complexes that still retain some VDAC-2. However, the oligomeric VDAC-2 complexes are diminished, and Bak does not interact with VDAC-2 in Bax-deficient HCT116 cells. These results highlight VDAC-2 as a critical inhibitor of Bak-mediated apoptotic responses.
Chandra
Dhyan
D
Department of Carcinogenesis, The University of Texas M. D. Anderson Cancer Center, Science Park-Research Division, Smithville, Texas 78957, USA.
Choy
Grace
G
Daniel
Peter T
PT
Tang
Dean G
DG
D017209:Apoptosis; D015153:Blotting, Western; D020169:Caspases / Q000378:metabolism; D045744:Cell Line, Tumor; D002850:Chromatography, Gel; D003432:Cross-Linking Reagents / Q000494:pharmacology; D045304:Cytochromes c / Q000378:metabolism; D015536:Down-Regulation; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D047468:Immunoprecipitation; D008565:Membrane Proteins / Q000502:physiology*; D008856:Microscopy, Fluorescence; D008928:Mitochondria / Q000378:metabolism; D018272:Porins / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000502:physiology*; D034741:RNA, Small Interfering / Q000378:metabolism; D013347:Subcellular Fractions / Q000378:metabolism; D013997:Time Factors; D050996:Voltage-Dependent Anion Channel 2; D050994:Voltage-Dependent Anion Channels; D050998:bcl-2 Homologous Antagonist-Killer Protein; D051028:bcl-2-Associated X Protein
C494772:BAK1 protein, human; C494826:BAX protein, human; D003432:Cross-Linking Reagents; D004791:Enzyme Inhibitors; D008565:Membrane Proteins; D018272:Porins; D019253:Proto-Oncogene Proteins c-bcl-2; D034741:RNA, Small Interfering; D050996:Voltage-Dependent Anion Channel 2; D050994:Voltage-Dependent Anion Channels; D050998:bcl-2 Homologous Antagonist-Killer Protein; D051028:bcl-2-Associated X Protein; D045304:Cytochromes c; D020169:Caspases