SMAC mimetic in combination with HDACi effectively release brakes on cell death of chronic myeloid leukemia K562 cells accelerating ligand-mediated apoptosis

Abstract Enthusiasm for immunotherapy has reached an all-time high within the last year due to remarkable clinical outcomes using strategies directed at activating T cells. The mechanisms by which newly activated T cells execute tumor target cells have not been described; it is assumed that CD4 and CD8 T cells may utilize ligation of TNF receptor family members, Fas, TNF, and TRAIL. However, not all tumors are susceptible to T cell killing by death receptor ligands. Chronic myeloid leukemia K562 cells are notably resistant to pro-apoptotic, ligand-induced cell death; whereas, T cell leukemia Jurkat cells undergo rapid caspase 3/7 activation and cell death (within 4–6 hours) in response to pro-apoptotic death ligands Fas and TRAIL. We hypothesized that K562 cells may be sensitized to respond similarly by targeting the regulatory proteins controlling activation of executioner and/or initiator caspases, thus accelerating cell death. SMAC mimetic compounds (SMC) are a novel class of drugs that target pro-survival signaling molecules. Single agent treatment with SMC enhanced TRAIL- and TNF-induced caspase activation, but had only modest effects on cell death after 24 hours. Addition of histone deacetylase inhibitors (HDACi) in combination with SMC, accelerated TNF- and TRAIL- mediated caspase activation and led to rapid cell death in K562 cells. By relieving multiple levels of inhibition in the tumor targets, we achieved rapid and efficient killing of previously resistant tumor cells using recombinant death receptor ligands. Combination therapy with SMC and HDACi may enhance current immunotherapy approaches designed to 1) activate tumor specific T cells expressing death receptor ligands or 2) administer recombinant ligands such as TRAIL.