Targeting BET bromodomain proteins utilizing small molecules in an emerging anti-cancer strategy with clinical evaluation of at least six inhibitors now underway. While MYC downregulation was initially proposed as a key mechanistic property of BET inhibitors, recent evidence suggests that additional anti-tumor activities are important. Using the Eμ-Myc model of B-cell lymphoma we demonstrate that BET inhibition with JQ1 is a potent inducer of p53-independent apoptosis that occurs in the absence of effects on Myc gene expression. JQ1 skews the expression of pro-apoptotic (Bim) and anti-apoptotic (BCL-2/BCL-xL) BCL-2 family members to directly engage the mitochondrial apoptotic pathway. Consistent with this, Bim knockout or Bcl-2 overexpression inhibited apoptosis induction by JQ1. We identified lymphomas that were either intrinsically resistant to JQ1-mediated death or acquired resistance following in vivo exposure. Strikingly, in both instances BCL-2 was strongly upregulated and was concomitant with activation of RAS pathways. Eμ-Myc lymphomas engineered to express activated Nras upregulated BCL-2 and acquired a JQ1-resistance phenotype. These studies provide important information on mechanisms apoptosis induction and resistance to BET-inhibition, while providing further rationale for the translation of BET inhibitors in aggressive B-cell lymphomas.
- Received November 15, 2015.
- Revision received June 6, 2016.
- Accepted June 22, 2016.
- Copyright ©2016, American Association for Cancer Research.