Selective BH3 mimetics synergize with BET inhibition to induce mitochondrial apoptosis in rhabdomyosarcoma cells

BH3 mimetics are promising novel anticancer therapeutics. By selectively inhibiting BCL-2, BCL-x(L), or MCL-1 (i.e. ABT-199, A-1331852, S63845) they shift the balance of pro- and anti-apoptotic proteins in favor of apoptosis. As Bromodomain and Extra Terminal (BET) protein inhibitors promote pro-apoptotic rebalancing, we evaluated the potential of the BET inhibitor JQ1 in combination with ABT-199, A-1331852 or S63845 in rhabdomyosarcoma (RMS) cells. The strongest synergistic interaction was identified for JQ1/A-1331852 and JQ1/S63845 co-treatment, which reduced cell viability and long-term clonogenic survival. Mechanistic studies revealed that JQ1 upregulated BIM and NOXA accompanied by downregulation of BCL-x(L), promoting pro-apoptotic rebalancing of BCL-2 proteins. JQ1/A-1331852 and JQ1/S63845 co-treatment enhanced this pro-apoptotic rebalancing and triggered BAK- and BAX-dependent apoptosis since a) genetic silencing of BIM, BAK or BAX, b) inhibition of caspase activity with zVAD.fmk and c) overexpression of BCL-2 all rescued JQ1/A-1331852- and JQ1/S63845-induced cell death. Interestingly, NOXA played a different role in both treatments, as genetic silencing of NOXA significantly rescued from JQ1/A-1331852-mediated apoptosis but not from JQ1/S63845-mediated apoptosis. In summary, JQ1/A-1331852 and JQ1/S63845 co-treatment represent new promising therapeutic strategies to synergistically trigger mitochondrial apoptosis in RMS.