BH3-only proteins target BCL-xL/MCL-1, not BAX/BAK, to initiate apoptosis

  • Cell Res. 2019 Nov;29(11):942-952. doi: 10.1038/s41422-019-0231-y.
Kai Huang  #  1  2 ,  Katelyn L O'Neill  #  1 ,  Jian Li  1  2 ,  Wei Zhou  1  3 ,  Na Han  1  4 ,  Xiaming Pang  1 ,  Wei Wu  1  5 ,  Lucas Struble  1 ,  Gloria Borgstahl  1 ,  Zhaorui Liu  1  6 ,  Liqiang Zhang  1 ,  Xu Luo  7  8
Affiliations
  • 1. Eppley Institute for Research in Cancer and Allied Diseases, Fred & Pamela Buffett Cancer Center, University of Nebraska Medical Center, Omaha, NE, 68198-7696, USA.
  • 2. Department of Pathology and Microbiology, University of Nebraska Medical Center, Omaha, NE, 68198-7696, USA.
  • 3. Department of Gastroenterology, Renmin Hospital of Wuhan University, Wuhan, 430060, Hubei, China.
  • 4. Department of Oncology, The Second Affiliated Hospital of Zhengzhou University, Zhengzhou, 450014, Henan, China.
  • 5. Department of Critical Care Medicine, Renmin Hospital of Wuhan University, Wuhan, 430060, Hubei, China.
  • 6. School of Medicine, Shandong University, Jinan, 250012, Shandong, China.
  • 7. Eppley Institute for Research in Cancer and Allied Diseases, Fred & Pamela Buffett Cancer Center, University of Nebraska Medical Center, Omaha, NE, 68198-7696, USA. [email protected].
  • 8. Department of Pathology and Microbiology, University of Nebraska Medical Center, Omaha, NE, 68198-7696, USA. [email protected].
  • # Contributed equally.
PMID: 31551537 DOI: 10.1038/s41422-019-0231-y
Abstract

It has been widely accepted that mitochondria-dependent Apoptosis initiates when select BH3-only proteins (BID, Bim, etc.) directly engage and allosterically activate effector proteins Bax/Bak. Here, through reconstitution of cells lacking all eight pro-apoptotic BH3-only proteins, we demonstrate that all BH3-only proteins primarily target the anti-apoptotic Bcl-2 proteins Bcl-xL/Mcl-1, whose simultaneous suppression enables membrane-mediated spontaneous activation of Bax/Bak. BH3-only proteins' apoptotic activities correlate with affinities for Bcl-xL/Mcl-1 instead of abilities to directly activate Bax/Bak. Further, BID and Bim do not distinguish Bax from Bak or accelerate Bax/Bak activation following inactivation of Bcl-xL/Mcl-1. Remarkably, death ligand-induced Apoptosis in cells lacking BH3-only proteins and Mcl-1 is fully restored by BID mutants capable of neutralizing Bcl-xL, but not direct activation of Bax/Bak. Taken together, our findings provide a "Membrane-mediated Permissive" model, in which the BH3-only proteins only indirectly activate Bax/Bak by neutralizing the anti-apoptotic Bcl-2 proteins, and thus allowing Bax/Bak to undergo unimpeded, spontaneous activation in the mitochondrial outer membrane milieu, leading to Apoptosis initiation.