BCL-2 selective inhibitor ABT-199 primes rhabdomyosarcoma cells to histone deacetylase inhibitor-induced apoptosis

BH3 mimetics are emerging novel anticancer therapeutics that potently and specifically inhibit antiapoptotic BCL-2 proteins and thereby induce cell death in many cancer entities. Previously, we demonstrated that JNJ-26481585 (JNJ), a second-generation histone deacetylase inhibitor (HDACI), engages mitochondrial apoptosis via upregulation of several BH3-only proteins. In the present study, we describe synergistic interactions of JNJ with BH3 mimetics (i.e. ABT-737, ABT-199) in rhabdomyosarcoma (RMS) cells. Importantly, JNJ synergizes with ABT-199 to trigger apoptosis in primary-derived RMS cells isolated from tumor samples, underlining the translational importance of combining these compounds and their potential to improve cancer therapy. Importantly, JNJ/ABT-199 cotreatment also significantly inhibits long-term survival of RMS cells. Mechanistically, JNJ increases expression levels of the BH3-only protein BIM, while exposure to ABT-199 displaces BIM from BCL-2 and shuttles BIM to MCL-1, which also constitutively sequesters NOXA. Both BIM and NOXA contribute to JNJ/ABT-199-mediated cell death, as individual knockdown of NOXA or BIM significantly prevents cell death. Further, JNJ and ABT-199 act in concert to activate BAK and BAX, resulting in loss of the mitochondrial membrane potential (MMP) and caspase activation. These events are required for JNJ/ABT-199-mediated apoptosis, since BAK or BAX silencing or inhibition of caspases significantly protects from JNJ/ABT-199-induced cell death. Rescue experiments demonstrate that overexpression of MCL-1, but not overexpression of BCL-2, blocks JNJ/ABT-199-induced apoptosis. In conclusion, this study provides the first demonstration of ABT-199-induced priming, which sensitizes RMS cells to HDACI, such as JNJ, by engaging mitochondrial apoptosis, highlighting that BH3 mimetics show great promise for the treatment of RMS.