Bim Upregulation by Histone Deacetylase Inhibitors Mediates Interactions with the Bcl-2 Antagonist ABT-737: Evidence for Distinct Roles for Bcl-2, Bcl-x_L, and Mcl-1

Abstract

The Bcl-2 antagonist ABT-737 kills transformed cells in association with displacement of Bim from Bcl-2. The histone deactetylase (HDAC) inhibitor suberoyl bis-hydroxamic acid (SBHA) was employed to determine whether and by what mechanism ABT-737 might interact with agents that upregulate Bim. Expression profiling of BH3-only proteins indicated that SBHA increased Bim, Puma, and Noxa expression, while SBHA concentrations that upregulated Bim significantly potentiated ABT-737 lethality. Concordance between SBHA-mediated Bim upregulation and interactions with ABT-737 was observed in various human leukemia and myeloma cells. SBHA-induced Bim was largely sequestered by Bcl-2 and Bcl-x_L, rather than Mcl-1; ABT-737 attenuated these interactions, thereby triggering Bak/Bax activation and mitochondrial outer membrane permeabilization. Knockdown of Bim (but not Puma or Noxa) by shRNA or ectopic overexpression of Bcl-2, Bcl-x_L, or Mcl-1 diminished Bax/Bak activation and apoptosis. Notably, ectopic expression of these antiapoptotic proteins disabled death signaling by sequestering different proapoptotic proteins, i.e., Bim by Bcl-2, both Bim and Bak by Bcl-x_L, and Bak by Mcl-1. Together, these findings indicate that HDAC inhibitor-inducible Bim is primarily neutralized by Bcl-2 and Bcl-x_L, thus providing a mechanistic framework by which Bcl-2 antagonists potentiate the lethality of agents, such as HDAC inhibitors, which upregulate Bim.

ACKNOWLEDGMENTS

This work was supported by grants CA63753, CA93738, and CA100866 from the National Institutes of Health; award 61 81-10 from the Leukemia and Lymphoma Society of America; an award from the Multiple Myeloma Research Foundation; an award from the V Foundation; Lymphoma SPORE award 1P50 CA130805; and RC2CA148431-01 from the National Cancer Institute.