Abstract 2636: Mechanisms of therapy-induced senescence with ABT-737
Authors: Song Jin H. , Kraft Andrew S. 
Published: June 2012

Abstract
Aberrant expression of the antiapoptotic Bcl-2 family of proteins in human cancer is correlated with poor outcomes after standard chemotherapy. ABT-737 is a cell permeable Bcl-2 family antagonist that is capable of binding to Bcl-2, Bcl-xL, and Bcl-w. ABT-737 functions by displacing BH3 proteins, such as Bim, from these proteins activating Bax and Bak to induce apoptotic cell death. Resistance to ABT-737 appears to be mediated by the elevated expression of Mcl-1 or Bfl-1/A1 which are not capable of binding this compound. Since renal, prostate and lung carcinomas, exhibit elevated levels of endogenous Mcl-1 many of these tumor cell lines are resistant to killing by this agent.
Gene chip analysis in order to understand anticancer action of ABT-737 in human cancer cells reveals an intrigue data that ABT-737 regulates key genes involving cytokines and chemokines associated with senescence phenotype. Because senescence is considered to be a key mechanism that protects against cancer development, we investigated the mechanism of action for ABT-737 in human cancer cells where this compound is incapable of inducing apoptotic cell death, inhibit growth of tumor cells by the induction of cell senescence. Here, we identify a novel pathway involving the induction of DNA damage response by caspase-3 cleavage that normally induces apoptosis, which instead leads to promotion of cellular senescence.
Treatment of renal, lung and prostate cancer cell lines with ABT-737, but not an inactive enantiomer, caused the induction of senescence-associated β-galactosidase, and additional changes associated with the senescence phenotype including increases in specific gene transcription, secretion of IL-6 and IL-8, and stimulation of the activity of C/EBPβ. ABT-737 treatment of these cell lines induced activation of the DNA damage response pathway as demonstrated by increases in γ-H2AX and phosphorylation of both the ATM and Chk2 protein kinases. Decreasing ATM levels with shRNA blocked the senescent phenotype suggesting that this pathway was critical for the induction of ABT-737-mediated senescence. DNA damage induced by ABT-737 treatment was caused by low level activation of the caspase cascade, which was blocked by decreasing caspase-3 levels with shRNA. ABT-737-induced senescence appeared to be p53-dependent, since the overexpression of dominant-negative p53 inhibited the ABT-737-induced increases in p21 protein levels, β-galactosidase, and IL-6 transcription.
These findings suggest that in multiple cancer cell types where ABT-737 is incapable of inducing sufficient DNA damage to activate apoptosis, it instead inhibits cancer cell growth by inducing senescence, a phenotype that is associated with major changes in gene transcription and protein secretion.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2636. doi:10.1158/1538-7445.AM2011-2636